Structures_Test - Ironwarrior.org

shawlaskewvilleUrban and Civil

Nov 29, 2013 (3 years and 11 months ago)

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1

Structures Test Review Notes:


Selection of Structural Systems


Standard Structural Systems



Wood



WOOD MEMBERS:



plentiful,



inexpensive,



relativel y strong in compression and tension,



easy to work with



easy to fasten to



used primarily in ONE
-
WAY structural systems where the load is
transmitted through structural members in one direction at a time.




JOISTS are a common use of wood



Light, closely spaced members that span between beams or bearing
walls



Typical sizes are 2x6, 2x8, 2x10, and 2x12



Typical spacings are 12”, 16”, and 24” on center



Typical maximum normal span is about 20 feet, but spa
ns up to 25 feet
are often used.



Plywood or particle board usually span the top of joists as sheathing



Joists must be laterally supported due to their slenderness



Avoids twisting or lateral displacement



Bridging supports the bottom edge of the jois
ts while the sheathing
supports the top



Maximum intervals of not more than 8 feet are recommended for
bridging



Either solid or cross bridging may be used.




WOOD BEAMS are not available in the sizes they once were.



Solid wood beams for longer spans
have generally been replaced with

2

glue
-
laminated construction.



Most common use of solid wood beams is with plank
-
and
-
beam
framing



Members are 4 to 6 inches wide nominal



Spaced at 4, 6, or 8 feet on center



Maximum normal span is about 10 to 20 feet



Wood decking is typically used as sheathing




GLUED LAMINATED construction is a popular method of wood
construction



These structural members are made up of individual pieces lumber 3/4”
or 1
-
1/2” thick glued together in a factory



Standard widths are

3
-
1/8”, 5
-
1/8”, 6
-
3/4”, and 8
-
3/4”



Typical spans range from 15 feet to 60 feet



Aesthetically pleasing structure



Can be manufactured in:



Tapered beams



Tapered and curved beams



Various styles of arches




LIGHT WEIGHT I
-
SHAPED JOISTS



Con
sist of a top and bottom chord of solid or laminated wood separated
by a plywood web



Used in residential and light commercial construction



Allows longer spans than are possible with a wood joist system



This product is stronger and stiffer than a wood

joist




PARALAM WOOD BEAM is made up of many veneers glued together to
form a beam



Used primarily as a header over openings



Has a higher modulus of elasticity than a standard wood joist




Has an allowable stress in bending about twice that of a Dou
glas Fir

3

wood joist




WOOD TRUSS is made up of standard wood members connected with
metal plates



Typical spans range from 24 to 40 feet



Typical depths of members range from 12 to 36 inches



Typical member spacing is 24 inches on center



Useful for residential and light commercial construction



Allow for passage of mechanical systems between webs of joists





SITE FABRICATED STRUCTURAL MEMBERS are used infrequently
due to difficulty in fabrication



BOX BEAM is made of plywood panels glu
ed and nailed to solid wood
members



Solid wood members are usually 2x4s



Typically used where depth of members is not critical



Used where other types of manufactured beams cannot be brought
to the building site



STRESSED SKIN PANELS same construction

as a box beam but used
for floor and roof structures



Solid wood members are usually 2x4s




Steel



Steel is one of the most used structural materials



High strength



Availability



Ability to adapt to a wide variety of structural conditions



Ductile material



Can undergo some deformation and return to its original shape



It will bend before it breaks


giving notice prior to failure



Well suited for multifloor construction



High strength


4



Structural continuity



Two of the most common str
uctural systems are:



BEAM AND GIRDER



Larger members span between vertical supports



Smaller beams frame into larger members



Girders span the shorter distance



Beams span the longer distance



Typical spans range from 25 to 40 feet



Typical beam sp
acing from 8 to 10 feet on center



Steel framing is typically covered with metal decking which spans
between the beams



A concrete floor may be poured over the decking to form a floor




OPEN WEB STEEL JOISTS



Span between beams or bearing walls



Typic
al spans range from 60 to 96 feet



Typical depths range from 8 to 30 inches in 2 inch increments



Deep long
-
span joists can span up to 144 feet



Deep long span joist depths range from 18 to 72 inches



Typical floor joist spacing is 2 to 4 feet on center



Typical roof joist spacing is 4 to 6 feet on center



Efficient structural members



Commonly used for low
-
rise commercial construction



Can span long distances



Are non
-
combustible



Mechanical sys
tems can be run through the webs




Concrete



CAST IN PLACE



Require formwork and are constructed in the field



Generally take longer to build than precast



Can conform to an almost unlimited variety of shapes, sizes, design

5

intentions, and structural
requirements



Majority of cast
-
in
-
place systems utilize only mild steel reinforcing



Some times post
-
tensioning steel is used



LIFT SLAB construction is sometimes used.



Two general types of cast
-
in
-
place concrete structures




One
-
way system



Slabs and beams are designed to transfer loads in one direction
only



A common one
-
way system is the beam
-
and
-
girder system
which functions similar to a steel system in which the slab is
supported by intermediate beams which are carried by larger
girders.



Typical spans range from 15 feet to 30 feet



Advantages:



Economical for most applications



Relatively easy to form



Allows penetrations and openings to be made in the slab



Concrete Joist System



Comprised of members spaced 26 or 36 inches apart

running
in one direction, which frame into larger beams



Typical spans range from 20 to 30 feet



Joist depths range from 12 inches to 24 inches



This system is easy to form since prefabricated metal pan
forms are used.



This system is good for light o
r medium loads where
moderate distances must be spanned.




Two
-
way system



There are three principal two
-
way concrete systems



Flat plate



Flat slab



Waffle slab



All are designed for use in rectangular bays where the distance
between columns is the same (or close to the same) in both

6

directions



Flat Plate



Simplest of the three



Slab is designed and reinforced to span in both directions
directly into the column
s.



Because loads increase near the columns and there is no
provision to increase the thickness of the concrete or the
reinforcing at the columns this system is limited to light loads
and short spans.



Typical spans up to 25 feet



Typical slab thickness range from 6 to 12 inches



Very useful in situations where floor
-
to
-
floor height must be
kept to a minimum or an un
-
cluttered underfloor appearance
is desired.



Flat Slab



When spans of the flat plate are large or the loads heavie
r

flat plates will require drop panels



Drop panels are thickened slabs at the columns



Drop panels provide greater resistance against punching
shear failures



Truncated pyramids or cones are often called column capitals
and are used to handle punching

shear as well as large
bending moments.



This system can accommodate fairly heavy loads



Typical spans are up to 30 feet



Waffle slab system



Supports heavier loads than flat plate and flat slab



Support longer spans than the flat slab system



Typic
al spans up to 40 feet are possible within economical
reason



Typically formed of prefabricated, reusable metal or
fiberglass forms



Typically faster construction due to standardized formwork



Often left unexposed with lighting integrated into the coffe
rs



7



PRECAST



Usually formed in a plant under strictly controlled conditions



Quality control is better



Erection proceeds quickly



Especially if structure is composed of a limited number of repetitive
members



Majority of precast systems are prestressed



Some times mild steel reinforcing is used



Some times precast is done on the site



Typically limited to wall panels (referred to as tilt
-
up panels)



Precast structural members



Come in a variety of forms:



Rectangular beam



Inverted tee beam



L
-
Shaped beam



Single tee



Double tee



Hollow core slab



Pre
-
cast column



Can be used as structural members or wall panels



Precast concrete members are connected in the field using welding
plates cast into mem
bers at the plant




Structural Precast Concrete Members



Precast concrete is typically prestressed



High strength steel cables are stretched in the precasting forms
before the concrete is poured



After the concrete cures to a desired level, the cables are released
and they transfer compressive stresses to the concrete.



When the members are completely cured, the concrete member has
a built
-
in compressive stress which resists the tension forces cau
sed
by the member’s own weight plus the live loads acting on the
member.



8



Single Tee or Double Tee beams



Simultaneously serve as structural supports as well as floor or roof
decking



Are easy and fat to erect



Topping concrete (usually about 2 inches

thick) is placed over the
tees to provide a uniform, smooth floor surface



Topping also provides increased strength when the tees are designed
to ac as composite beams.



Unloaded beams from the plant have camber built into them due to
the prestressing c
aused by the reinforcement



Camber is to disappear when live and dead loads are placed on the
member




Post Tensioned Concrete



Post tensioning steel


often called tendons


is stressed after the
concrete has been poured and cured



Tendons are:



Smal
l high strength wires



Seven
-
wire strands



Solid bars



Tendons are stressed with hydraulic jacks pulling on one or both
ends



Hydraulic jack pressures:



100 to 250 psi for slabs



200 to 500 psi for beams



post tensioned concrete structural systems are useful where high
strength is required and where it may be too difficult to transsport
precast to the job site




Masonry



Masonry as a structural system is generally limited to bearing walls



Masonry has hi
gh compressive strength



Masonry’s unitized nature makes it inherently weak in tension and bending



There are three basic types of masonry bearing wall construction:


9



Single wythe



Have no provisions for reinforcing or grouting



Double wythe



Both la
yers may be of the same material or different



May be grouted and reinforced or un
-
grouted



Cavity



May be grouted and reinforced or un
-
grouted




Advantages of Masonry Bearing Walls:



Strength



Design flexibility



Appearance



Resistance to weathering



Fire resistance



Sound insulation



Their mass makes them ideal for may passive solar energy applications




Joints in masonry must be reinforced horizontally at regular intervals



Strengthens the wall



Controls shrinkage crac
ks



Ties multi
-
wythe walls together



Provides a way to anchor veneers to the back
-
up structure



Comes in a variety of forms and is generally placed at 16 inche o.c.




Vertical Reinforcement



Standard reinforcing bars sized and spaced in accordance wi
th the
structural requirements of the wall.



Horizontal bars are also used and are tied to the vertical bars



Entire assembly is set in grout



In single
-
wythe construction only vertical reinforcing is used with fully
grouted wall cavities




Thickness o
f walls determines three important properties:


10



Slenderness ratio



Ratio of the wall unsupported height to its thickness



Indication of the walls ability to resist buckling due to compressive
loads being applied to it



Flexural strength



Important property when the wall is subjected to lateral forces such as
wind



Fire resistance



Depends on the material of the wall and its thickness




Composite Construction



Any structural system consisting of two or more materials designed to act
tog
ether to resist loads



Composite construction is employed to utilize the best charachteristics of
each of the individual materials



Reinforced concrete construction is the most typical of composite
construction



Other types of systems:



Composite steel

deck and concrete



Steel deck is designed with deformations or wires welded to the deck to
transfer load between steel and concrete



Concrete slab and steel beam systems



Headed stud anchors are used to transfer load between concrete and
steel


making

them act as one unit



Open web steel joists with wood chords



Provide a nailable surface for the floor and ceiling while using the high
strength
-
to
-
weight ratio of steel for the web members



Less frequently used composite systems:



Trusses with wood for compression members and steel rods for tension
members



Concrete filled steel tube sections




Walls and the Building Envelope



Non load bearing walls are not considered part of the structural system of a
building


11



There are 2 important structural considerations that non
-
load bearing walls do
have to withstand



Weight of the envelope that will need to be supported by the structure



Exterior loads placed on the envelope such as wind that will need to be
transferred

to the structure



Typical Attachment by system



Panels and curtain wall systems are attached with clips on the mullions at
the structural frame



Stone and masonry facings are attached with clip angles, continuous
angles, or special fastenings and are a
ttached to the structural framing at
the floor lines



Lightweight facings such as wood siding, shingles, and stucco need to be
applied over continuous sheathing firmly secured to the structural wall
framing



One of the most important considerations is to

allow for movement such as
expansion and contraction



Movement can be provided for



Clip angles with slotted joints



Slip joints



Flexible sealants



Movement considerations:



Steel framed building do not present many problems with movement of
the st
ructural frame itself



Concrete structures are subject to creep over time and need special
connections to exterior envelope systems



Wood structures also deform over time due to wood shrinkage and long
term deflection


Complex Structural Systems



Trusses



Trusses are structures comprised of straight members forming a number of
triangles with the connections arranged so that the stresses in the members are
either in tension or compression



Trusses can be used:



Horizontally


12



Vertically



Diagona
lly



Trusses are primarily tension/compression structural systems



Some amount of bending is present in many of the members



This bending is due to loads applied between the connections and secondary
bending and shear stresses at the connections themsel
ves caused by minor
eccentric loading



Trusses can be field fabricated or assembled in the factory



Such as with open web steel joists and wood trussed rafters



Primary limiing factor is the ability to transport them from factory to the
jobsite




Arche
s



Arches may have hinged or fixed supports



Two
-
hinged and fixed arches are staically indeterminate



Hinged arches are primarily subjected to compressive forces



Allows arch to remain flexible and avoid high bending stresses under:



Live loading



Loading due to temperature changes



Foundation settlement



To resist loads in only compression the arch has to have a funicular
shape



Funicular shape can be found by suspending the anticipate loads from a
cable and then turning the shape of the cable u
pside down.



Antonio Gaudi used this method of analysis in may of his structural
studies



For a hinged arch supporting a uniform load across its span the
resultant shape is a parabola



There are two reactions at the supports of a hinged arch



Vertical
reactions



Horizontal reactions or thrust



Thrust is inversely proportional to the rise or height of the arch



If rise is reduced by one
-
half the thrust will be doubled



Sometimes arches have hinges at their apex


13



This is called a three
-
hinged arch



Makes the structure statically determinate



Arches can be constructed of : (each with inherent limitations)



Steel


span 50 to 500 feet



Concrete

span 20 to 320 feet



Wood


span 50 to 240 feet



Stone



Variety of shapes:



Classic half round
arch of the romans



Pointed gothic arches



Decorate arabic arches



Functional parabolic shapes




Rigid Frames



A rigid frame is constructed so that the vertical and horizontal members work
as a single structural unit.



This is a more efficient struct
ure because all three members resist vertical
and lateral loads together



There are forces and reactions in a rigid frame unlike post
-
and
-
beam due to
the members being rigidly attached to one
-
another



Columns are subjected to both compressive and bending

forces



Columns are subjected to thrust similar to an arch



Attachement of columns to foundation may be hinged or rigid



This connection will result in slightly different loads on the columns



Fixed frame is stiffer than the hinged frame



Thrust in t
he fixed frame is greater then in the hinged frame



When a horizontal beam is not required


the rigid frame takes on the
appearance of a gable frame



This shape decreases the bending stresses in the two inclined members



This shape increases the compression in the two inclined members



Because rigid frames develop a high moment at the connections between
the horizontal and vertical members


the amount of material is often
increased near these points.



14



Space Frames



A space frame is a structural system consisting of trusses in two directions
rigidly connected at their intersections



More common type of space frame is a triangulated space frame


where the
bottom chord is offset from the top chord



Space frames are v
ery efficient structures for enclosing large rectangular
areas



Typical spans up to 350 feet



Span
-
to
-
depth ratios range from 20:1 to 30:1



Light weight



Repetitive nature reduces fabrication and erection time



A computer is needed for analysis and de
sign




Folded Plates



Loads are transferred in two directions



First in the transverse direction



Second in the longitudinal direction



Plates act a beams between supports



There are compressive stresses above the neutral axis



There are tensile str
esses below the neutral axis



Folded plates are usually constructed of reinforced concrete



3 to 6 inches thick



typical spans of 30 to 100 feet (longer spans possible with reinforced
concrete)




Thin Shell Structures



Curved surface that resists loads through tension, compression, and shear in
the plane of the shell only



Theoretically, there are no bending or moment stresses in a thin shell strucure



Material is practically always reinforced concrete 3 to 6 inches th
ick



Forms can be:



Domes



Parabolas



Barrel vaults


15



Saddle shaped hyperbolic paraboloid



Typical spans from 40 feet to over 200 feet



Hyperbolic paraboloids span 30 to 160 feet




Stressed Skin Structures



Comprised of sheathing sandwiching interm
ediate members



Typically made of wood



Typical spans of 12 to 35 feet




Suspension Systems



Most commonly seen on bridge design



Some large stadiums have utilized suspension systems to suspend roofs



Federal Reserve Bank in Minneapolis is a suspende
d building



Cable suspension systems are similar to arches



Loads they support must be resisted by both vertical reactions and
horizontal thrust reactions



In suspension systems vertical reactions are up and horizontal thrust
reactions are outward since

the sag tends to pull the ends together



Shallow sags result in high reactions



Deep sags result in lower reactions



Suspension systems can only resist loads with tension



The shape of the cable will change as the load changes



No bending stresses are possible



Disadvanage: instability due to wind and other types of loading



Suspension systems must be stabilized or stiffened with a heavy infill material




Inflatable Structures



Similar to suspension systems

can only resist l
oads in tension



Held in place with constant air pressure which is greater than the outside air
pressure



Double
-
skin inflatable structure



Created by inflation of a series of voids (like an air mattress)



This type of system elliminates the need for a
n “air
-
lock” for entry and
exists


16



Inflatable structures are inherently unstable in the wind and cannot support
concentrated loads



Used for temporary enclosures and for large, single space buildings such as
sports arenas


Structural System Selection Cri
teria



Resistance to loads



Primary consideration



Anticipated loads



Weight of the structure (dead load)



External forces such as wind, snow, earthquakes



Loads caused by use of the building such as people, furniture, and
equipment (live loads)



Calculated from known weights of equipment and materials and
building code requirements



Un
-
anticipated loads



Difficult to plan for



Include changes in use of the facility



Overloading caused by extra people or equipment



Unusual snow loads



Ponding

of water on the roof



Degradation of the structure itself



Very unusual loads will be the primary determinate of the structural
system




Building Use and Function



Type of occupancy



Example: office building works well with spans in the 30 to 40 foot

range



In a location where building height is limited, a client may want to squeeze as
many floors into a multi
-
story building as possible




Integration with other building systems



Mechanical electrical and plumbing systems and how they integrate wit
h the
structure


17



Exterior cladding systems




Cost Influences



Selecting materials and systems that are most appropriate for the



loads proposed



spans required



style desired



integration needed



fire resistance called for



all other factors



refining the selected sstem so that the most economical arrangement and use
of materials is selected




Fire Resistance



Building codes dictate the fire resistance of materials based on building use



Ratings range from 1 to 4 hours



Range is the indicat
ion of how long a member can withstand a standard
fire test before it becomes dangerously weakened



Two considerations



Combustibility of the member itself



Loss of strength anticipated due to heat




Construction Limitations



Time, material, and labor

availability




Style



International Style


steel post and beam construction



Consider fire proofing requirements




Social and Cultural Influences



Look at surrounding buildings and materials used



Chapter 2


Loads on Buildings


18




General:



Probable magnitudes of building loads have been determined over a long
period of time based on successful experience and the statistical probability
that a particular situation will result in a given load.



Building loads are also based on the worst case
situation.



Loads are defined by building codes and by common practice



Codes provide:



Live load requirements



Wind values



Earthquake values



Standard published tables provide accepted weights of building materials for
dead load calculations.



Mos
t loads on buildings are static



Loads determined to be dynamic, such as wind loads, are considered to be
static so calculations are easier to perform.



Gravity Loads



Dead Loads are vertical loads due to the weight of the buildin and any
permenant equi
pment



Dead loads include:



Beams



Exterior walls



Interior walls



Floors



Mechanical equipment



Weight of structure must be assumed to

make a preliminary calculation of
the size of the structural member; then the actual weight can be used for
checking the calculation.



Most dead loads are easily calculated from published lists of weights of
building materials found in standard referenc
e sources.



UBC requires that floors in office buildings and other buildings where
partition locations are subject to change be designed to support an extra 20
psf of dead load



Live Loads


19



Live loads are those imposed on the building by its particular use and
occupancy



Live loads are generally considered movable or temporary



Examples:



People



Furniture



Movable equipment



Snow



Does not include



Wind loadig



Earthquake loading



Live loads are established by building codes for different occupancies



The code also requires that floors be designed to support concentrated
loads if the specified load on an otherwise unloaded floor would produce
stresses greater than those caused by the uniform load.



The concentrated load is assumed to be located on any
space 2.5 feet
square.



Live Load reductions



When a structural member supports more than 150 square feet
{R=r(A
-
150)}



Except floors in places of public assembly



Excpet for live loads greater than 100 psf



Where there is snow load greater than 20 ps
f on any pitched roof over
20 degrees



Loads on sloped surfaces are assumed to act vertically on a horizontal plane
projected from the slope



Combination Loads



It is generally agreed that when calculating all the loads acting on a
building, all of them

will probably not act at once.



Acceptable combination of loads are as follows:



Dead load plus floor live load plus roof live load (snow)



Deal load plus floor live load plus wind (or seismic)



Dead load plus floor live load plus wind plus 1/2 snow


20



Dead load plus floor live load plus snow plus 1/2 wind



Dead load plus floor live load plus snow plus seismic



Lateral Loads



Wind



Wind loading is a dynamic process



Pressure, direction, and timing are constantly changing



To aid in calculations, wind is considered a static force



Variable that effect wind loading



Wind velocity



Pressure on a building varies as the square of the velocity



P=0.00256V2



Height of the wind above the ground



Slower near the ground and increases with height



Wind values are taken at 33 feet above the ground



Surroundings



Prarie vs. city



Trees



Topography



Size of building



Shape of building



Surface texture of building



Pressures



Positive pressure
on the windward side



Negatve pressure on leeward side and roof



Greater pressure at



Building corners



Overhangs



Parapets



Other projections



Special concerns:



Building shapes


21



Design features



Closely spaced buildings



Small openings at ground level



Wind is fluid



Building drift



The distance a building moves from side to side in the wind



A building should be designed stiff enough so that the maximum
drift does not exceed 1/500 of the height of the building.



Eart
hquake



Earthquake produces dynamic loads



Ground moves both vertically and horizontally (laterally)



Lateral movement is considered most significant



Some tall buildings or structures require a dynamic structural analysis
(requires a computer to calcu
late)



Building codes allow a static analysis of the loads produced by an
earthquake to simplify structural design



With static analysis, the total horizontal shear at the base of the building
is calculated according to a standard formula



This total fo
rce is distributed to the various floors of the building so
the designer knows what force the structure must resist.



Miscellaneous Loads



Dynamic loads



Dynamic loads are loads that are applied suddenly or changes rapidly



When a force is only applied suddenly it is often called an impact load



Examples of dynamic loads:



Moving automobiles



Elevators travelling in a shaft



Helicopter landing on the roof of the building



Dynamic loads do not occur on every building



UB
C lists minimum requirements for many of these types of loads



In many cases, dynamic loads are static loads multiplied by an impact
factor


22



A uniques type of dynamic load is a resonant load



Rhythmic application of a force to a structure with the same
fundamental period as the structure itself.



Fundamental period is the time it takes the structure to complete one
full oscillation



Resonant loads start small and slowly build up over time



A tuned dynamic damper can be placed at the top of tall buildi
ngs to
dampen the effects of wind sway



Temperature induced loads



Expansion and contraction of materials



Measured in the coefficient of expansion in/foot/degree F



Soil Loads



Retaining walls required to resist the lateral pressure of retained
materials



Code allows walls retaining drained earth to be designed for pressure
equal to that exerted by a fluid weighing 30 pcf and having a depth
equal to that of the retained earth



This is in addition to surcharge of vertical loads on the surface and any
other lateral loads



Must resist sliding by at least 1.5 times the lateral force



Must resist overturning by at least 1.5 times the overturning moment



Water



Water tanks, swimm
ing pools, retaining walls that are undrained will
have water loads



Water weighs 62 pcf



Force exerted by water is called hydrostatic pressure


Chapter 3
-

Structural Fundamentals




Statics and Forces



Statics deals with bodies in a state of equilibri
um.



Equilibrium is said to exist when the resultant of any number of forces
acting on a body equal zero.



Three fundamental principals of equilibrium apply to buildings:


23



The sum of all vertical forces acting on a body must equal zero



The sum of all the horizontal forces acting on a body must equal zero



The sum of all the moments acting on a body must equal zero



Forces are actions applied to an object.



External forces are called loads



The internal structure of a building materia
l must resist external loads with
internal forces that are equal in magnitude and opposite in sign (equal and
opposite).



Internal forces are called stresses



The structural design of buildings is primarily concerned with selecting the
size, co
nfiguration, and material components to resist, with a reasonable
margin of safety, external forces acting on them.



A force has both direction and magnitude and is called a vector quantity.



Line of action of a force is a line concurrent with the force
vector.



The principal of transmissibility says you can consider a force acting
anywhere along the line of action as long as the direction and magnitude
don’t change.



There are several types of forces:



Colinear Forces


vectors lie along the same stra
ight line



Concurrent Forces


lines of action meet at a common point



Non
-
concurrent Forces


lines of action do not pass through a common
point



Coplanar Forces


lines of action all lie within the same plane



Non
-
coplanar Forces


lines of action do

not lie within the same plane



Structural forces in buildings can be any combination of these types
(above).



The simplest combination of forces are colinear forces.



Concurrent and non
-
concurrent forces, the effect of the direction of the
force must b
e taken into account.



Stresses



Stress is the internal resistance to an external force.



There are three basic types of stress



Tension



Compression


24



Shear



Tension


stress in which the particles of the member tend to pull apart
under load



Compression


stress in which the particle of the member are pushed
together and the member tends to shorten



Shear


stress in which the particles in a member slide past eachother



Tension and Compression


force acts perpindicular



Shear


force acts
parallel



Other types of stresses



Torsion


is a type of shear in which the member is twisted



Bending


is a combination of tension and compression



Combined


can occur in many situations



Thermal Stresses



Changes in temperature, an increase in e
xpansion when heat is added or,
contraction when it is cooled.



Coefficients of linear expansion is the dimension an object will expand or
contract per degree change



If the material is restrained at both ends, a change in temperature causes an
internal
thermal stress.



Unit stress is independent of the cross
-
sectional area of the member IF
there are no other loads being applied to the member while it is undergoing
thermal stress.



Strain and Deformation



Strain is the deformation of a material caused
by external forces.



As a frorce is applied to a material:



The strain (deformation) is directly proportional to the stress up to a
certain point.



This si known as Hooke’s Law after mathmetician Robert Hooke



At this stage, the material will return to its original size if the force is
removed



At the elastic limit the material will begin to change length at a faster
ration than the applied force.



At this stage, the material will be permanently deformed, even

if the
force is removed.


25



At the yield point, the material continues to deform with very little
increase in load



At ultimate strength this is the point prior to rupture or failure.



The most important portion from a practical standpoint is where the s
tress
and strain are directly proportional up to the elastic limit.



Sound engineering practices establish working stresses to be used in
calculations at some point below the yield point.



The modulus of elasticity is the ratio of stress to strain for a
material.



This is the materials resistance to deformation or its stiffness = E



Actual values of E to be used in calculations should be derived from
building codes or accepted tables of value.



Moment



A moment is the tendancy of a force to cause rota
tion about a point.



It is the product of the force times the distance to the point about which it is
acting.



If the rotation is clockwise in direction


it is considered positive



If the rotation is counter
-
clockwise in direction


it is considered ne
gative



Properties of Sections



The most common properties of sections:



Area



Centroid



Statical moment



Moment of inertia



Section modulus



Radius of gyration



Centroid


in all bodies there is a point at which the mass of the body can be
considered concentrated


Center of Gravity



The point on a plane surface that corresponds to the center of gravity is
called the centroid.



Statical Moment


used to find the centroid
in unsymetrical areas.



Th statical moment of a plane area with respect to an axis is the product of
the area times the perpindicular distance from the centroid of the area to
the axis.



The statial moment of the entire area is equal to the sum of the st
atical

26

moments of the parts.



If there is a hole in the figure, treat the statical moment of the hole as a
negative number.



Moment if Inertia


is a measure of the bending stiffness of a structural
member’s cross
-
sectional shape, similar to how the modu
lus of elasticity is
the measure of the stiffness of the material of a structural member.



The moment of inertia about a certain axis of the section is the summation
of all the infinitely small areas of the section multiplied by the square of
the distance

from the axis to each of these areas.



Common designaton “I” and its units are inches to the fourth power.



The axis passing through the centroid is the most commonly used for this
calculation and is otherwise known as the neutral axis.



In order to fi
nd the moment of inertia for composite areas, you must
transfer the moment of inertia of each section about its centroid to a new
axis.



The moment of inertia is dependant on the area of a section and the
distance o the area rom the neutral axis



The mom
ent of inertia is the summation of the areas times the square of the
distances of those areas from the neutral axis.



The depth of a beam has a greater bearing on its resistance to bending than
its width or total area.



Structural Analysis



Resultant Forces



The resulatant force is the combination of two or more concurrent forces
into one force that produces the same effect as the individual forces.



If the forces are colinear


the resultant is just the sum of the forces.



If the forces are concurrent, both the magnitude and direction must be
taken into account.



For magnitude

use the law of cosines



For direction


use the law of sines



Components of Force



The reactions of the force would be equal in magnitude but opp
osite in
direction to the vertical and horizontal components of the force.



Three or more forces can be resolved by resolving each one into their
horizontal and vertical components, summing these components, then
finding the resultant of the horizontal an
d vertical components with
pythagorean theorem.


27



Free
-
Body Diagrams



Used in analyizing structures



Allows a portion of the structure to be studied using principals of
equilibrium.


Chapter 4


Beams and Columns




Beams


Basic Principles



At the neut
ral axis, or centroid, which is in the geometric center of the beam if
it is rectangular or symmetrical, the beam does not change length so no
compressive or tension stresses are developed.



If the beam is in equilibrium, then all the moment forces must c
ancel out;
those acting in a clockwise rotation must equal those acting in a
counterclockwise rotation.



R x d = (C x c) + (T x t)


this formula represents the basic theory of bending,
that the internal resisting moments at any point in a beam must equal

the
bending moments produced by the external loads on the beam.



The moment increases as the distance from the reaction increases or as the
distance from the neutral axis increases.



In the case of a simply supported beam, the maximum moment occur at the
center of the span and the beam is subjected to its highest bending stresses at
the extreme top and bottom fibers.



In order for a beam to support loads, the material, size, and shape

of the beam
must be selected to sustain the resisting moments at the point on the beam
where the moment is greatest.



The section modulus is the ratio of the beams moment of inertia to the
distance from the neutral axis to the outermost part of the secti
on (extreme
fiber).



Another fundamental type of stress in beams is shear.



Shear is the tendancy of two adjacent portions of the beam to slide past
eachother in a vertical direction.



Horizontal shear


two adjacent portions of a beam to slide past eac
h other
in the direction parallel to the length of the beam.



Usually horizontal shear is not a problem except in wood beams where the
horizontal fibers of the wood make an ideal place for the beam to split and
shear in this direction.



Another importan
t aspect of the behavior of beams is their tendency to

28

deflect under the action of external loads.



Types of Beams



There are several types of beams:



Simply supported



Overhanging beam



Continuous beam



Cantilever beam



Fixed end beam



Simply supp
orted, overhanging, and continuous beam all have ends that
are free to rotate as the load is applied



Cantilever and fixed end beams have one or both sides restrained against
rotation



Continuous beam is one that is held up by more than two supports



Tw
o typical kinds of loads on building structures:



Concentrated loads



Uniformly distributed loads



The resultant of unifomly distributed loads is at the center of the
loads



Simply supported, overhanging, and cantilever beams are statically
d
eterminate which means that the reactions can be found using the
equations of equilibrium



Continuous and fixed end beams are statically indeterminate and
require more complex calculation methods to find the reactions.



Basic requirements for the structural design of a beam is to determine
what the stresses due to bending moment and vertical shear will be that
are caused by the particular loading conditions.



The reactions at the supports must be calculated first.



Defl
ection is the change in vertical position of a beam due to a load



The amount of deflection depends on :



Load



Beam length



Moment of inertia



Modulus of elasticity


29



The amount of allowable deflection is limited by building code
requirements or prac
tical requirements such as how much a beam can
deflect before ceiling surfaces begin to crack or before the spring of the
floor becomes annoying to occupants.



The deflection due to live load is limited to L/360



The deflection due to total load (dead pl
us live) is limited to L/240



Columns


Basic Principles



Consideration


the tendency of a long slender column to buckle under a
load.



Consideration


combined loading (compressive plus lateral loads)



Flexural stress caused by eccentricity is given
by the flexural formula.



Radius of Gyration



Ability of a column to withstand a load is dependant on its length,
cross
-
sectional area, and its moment of inertia.



Radius of gyration is the combination of area and moment of inertia.



In non
-
symmetric sections there are two radii of gyration, one for each
axis.



Most interest in column design is the least radius of gyration


in this axis
the column will fail.



Trusses



Basic Principles



A truss is a structure generally formed of st
raight members to form a number
of triangles with the connections arranged so that the stresses in the members
are either tension or compression.



Typical depth to span ratios range from 1:10 to 1:20



Flat trusses require less overall depth than pitched
trusses



Spans generally range from 40 to 200 feet.



Roof loads on a truss are transferred from the decking to purlins, which are
attached to the truss at the panel points to avoid putting any bending stresses
in the top chord of the truss.



If concentr
ated loads are placed between panel points, or uniform loads are
applied directly to the top chords, the member must be designed for the axial
loading as well as for bending.



Trusses act much like beams in that there is usually compression in the top

30

cho
rds and tension in the bottom chords, with the web members being either
in compression or tension, depending on the loading and type of truss used.



The forces in a parallel chord truss increase toward the center.



Individual truss members are designed a
s columns if they are in compression.



If truss members are in tension, they must have adequate net area to resist the
unit tensile stress allowed by the material being used.



If concentrated loads or uniform loads are placed on any chord member
between
the panel points, the member must also be designed to resist bending
stresses.



The effective length of chord members in compression is important



Kl in which K is determined by the restraint of the ends of the members



For steel trusses, K is usually t
aken as 1.0 so the effective length is the
same as the actual length



The ratio of length to least radius of gyration, l/r, should not exceed 120 for
main members and 200 for secondary and bracing members.
\



Designing steel trusses with double angles


by knowing the compressive
load, the length of the member, and strength of steel, you can determine the
size and thickness of a double angle combination.



For members in tension, the net area must be determined. This is the
actual area of the member le
ss the area of bolt holes which is taken to be
1/8 inch larger than the diameter of the bolt.



Truss members should be designed so they are concentric; so the member
is symmetric on both sides of the centroid axis in the plane of the truss.



The centroi
dal axes of all intersecting members must also meet at a point
to avoid eccentric loading.



Standard practice for steel joists made up of angles is to have gage lines
rather then centroidal axes meet at a common point.



The gage line is a standard dimension from the corner edge of an angle to
the centerline of the bolt hole or holes.



Gage line value depends on the size of the angle.



Truss Analysis



First step in designing a truss is to determine the loads in the vari
ous
members



The following are general guidelines for truss analysis



The sum of the vertical forces at any point equal zero



The sum of the horizontal forces at any point equal zero


31



The sum of the moments about any point equal zero



Forces in each m
ember are shown going away from the joint if in
tension and toward the joint if in compression



Forces acting up or to the right are positive



Forces acting down or to the left are negative



For analysis, trusses are assumed to have pivoting or rolling
supports to
avoid other stresses at these points



Three methods that can be used to determine the forces in truss members:



Method of joints



Method of sections



Graphic method



Method of Joints



Each joint is considered separately as a free body diagram to which the
equations of equilibrium are applied.



Method of Sections



A portion of the truss is cut through three members, one member of
which is under analysis.



Cut section is drawn as a fre
e body diagram and forces are foud by
using sum of moments.



Graphic Method



Drawing a stress diagram to scale showing all the force polygons for
each joint on one drawing.



Not very accurate



Note:



Since the truss is in equilibrium, the force polygo
n of each joint
must close



When developing the force polygon for a joint, work in a clockwise
direction around the joint. Do this consistently or every joint.



To determine if a member is in compressions or tension, trace the
rays of the force polygon.

Imagine that the ray was transposed onto
the truss diagram. If the direction of the ray is toward the joint, the
member is in compression. If it is away from the joint, the member
is in tension.



There will be as many sides to each force polygon as th
ere are truss
members entering a joint.


32




Soil and Foundations



The foundation is the part of the building that transmits all the gravity and
lateral loads to the underlying soil.



Selection and design of foundations depends on two primary elements:



The required strength of the foundation to transmit the loads on it



The ability of the soil to sustain the loads without excessive total
settlement or differential settlement among different parts of the
foundation.



Soil Properties



Soil is t
he material that supports the building



Soil is classified into four groups



Sands and gravels



Granular material that is non
-
plastic



Clays



Small particles that have some cohesion, or tensile strength, and are
plastic in their behavior



Silts



Are

intermediate size between clays and sands and behave as
granular material but are sometimes slightly plastic in their behavior



Organics



Are materials of vegetable or other organic masterials



Solid Rock is another type and has the highest bearing cap
acity of all soil
types.



Subsurface Exploration



Two most common methods of subsurface exploration are:



Borings



Core borings, undistrubed samples of the soil are removed at
reagular intervals



Type of material recovered is recorded in a boring log



Shows the material



The depth at which it was encountered



Standard soil designation


33



Moisture content



Density



Other tests’ results observed on site



Most common bore hole test is the Standard Penetration Test
(SPT)



Measure of the density of gr
anular soils and consistancy of
some clays



2” diameter sampler is driven into the bottom of the bore hole
by a 140 pound hammer falling 30 inches.



The number of blows N required to drive the cylinder 12
inches is recorded



Laboratory test performed on

recovered borings:



Strength tests of bearing capacity



Resistance to lateral pressure



Measure slope stability



Compressibility



Grain size



Specific gravity



Density tests



Usually, a minimum of four borings are taken, one near each
corner of t
he proposed building.



Test pits



Trenches dug at the jobsite that allow visual inspection of the soil
strata and direct collection of undisturbed samples



Practical limit on depth is about ten feet


soil below that cannot
be directly examined.



Soil Types and Bearing Capacities



Classified according to the Unified Soil Classification System (USC)



System divides soils into major divisions and subdivisons based on grain
size and laboratory tests of physical characteristics



System provides standardized names and symbols.



Bearing capacities are generally specified by code.



Water in Soil



Water in soil can cause several problems for foundations


34



Water can reduce the load carrying capacity of soil



Differential settlement

may occur causing cracking and weakening of
structural and non
-
structural componenets



Hydrostatic pressure:



Puts additional loads on the structural elements



Makes waterproofing more difficult



Ways to minimize the problems caused by excess soil moi
sture



Slope the ground away from the building



Drain storm water away from the building



Foundation drains



Open
-
web matting against the foundation wall breaks water pressure



Layers of large gravel



Soil Treatment



Drainage: can increase the strength of soils and prevent hydrostatic
pressure



Fill: used to replace unacceptable soils


needs to be compacted



Proctor test measures moisture to density for optimum strength/bearing
capacity



Compaction: simple compacti
on of some soil types provides the desired
bearing capacity



Densification: vibration, dropping of heavy weights, pounding piles into
the ground and filling the voids with sand



Surcharging: pre
-
loading the soil with fill to compact the soil or “train”
it



Other Considerations:



Frost



Expansive Soils (bentonite clay)



Repose


typically 45 degrees



Foundation Systems



Two broad divisions of foundations:



Spread footings



Pile or caisson foundations



Spread footings spread the load from the stru
cture and the foundation walls
over a large area so that the load bearing capacity of the soil is not exceeded

35

and settlement is minimized.



One of the most common is the wall footing which is placed under a
continuous foundation wall which in turn suppor
ts a bearing wall.



The joint between the footing and foundation wall is strengthened with a
keyed joint.



Types of spread footings:



Wall footing



Independent column footing



Combined footing



Strap footing



Mat or raft foundation



Independent column footing is similar in concept but supports only one
column.



Required size of both wall and independent column footings is found by
dividing the total load on the footing by the load carrying capacity of the
soil. A safety factor is of
ten used as well. For wall footings, design is
based on a linear foot basis.



Combined footings support two or more columns in situations where the
columns are spaced too close together for separate ones, or where one
column is so close to the property l
ine that a symmetrically loaded footing
could not be poured.



A strap footing or cantilever footing which uses a concrete strap beam to
distribute the column loads to each footing to equalize the soil pressure on
each footing.



Strap footings are also us
ed where the exterior column is next to the
property line but the footing cannot extend beyond the property line.



A mat or raft foundation is used when soil bearing is low or where loads
are heavy in relation to soil pressures. One large footing is desi
gned as a
two
-
way slab and supports the columns above it.



Pile and caisson foundations distribute the load from the building to the ends
of the piles which often bear on bedrock, or to the surrounding soil in contact
with the pile through skin friction o
r a combination of both.



When soil near grade level is unsuitable for spread footings, pile
foundations are used.



Piles are either driven or drilled



Driven piles may be of timber or steel and are placed with pile
-
driving
hammers powered with drop ham
mers, compressed air, or diesel

36

engines.



Drilled piles or caissons are usually called piers.



Drilled piers are formed by drilling out a hole to the required depth and
then filling it with concrete.



A metal lining is used to keep the soil from caving
in during drilling. It
is removed as the concrete is poured or may be left in.



If the soil pressure is not sufficient for a drilled pie of normal
dimensions, the bottom is “belled” out to increase the surface area for
bearing.



Piles are usually placed

in groups or in a line under a bearing wall with the
loads from the building transferred to them with pile caps.



The piles are embedded from 4 to 6 inches into the pile cap which is
designed and reinforced to safely transmit the loads and resist shear a
nd
moment stresses developed.



When two or more piles are used to support one column, the centroid of
the pile group is designed to coincide with the center of gravity of the
column load.



Grade beams are designed and reinforced to transfer the loads from the
building wall to the piles.



Grade beams are often used where expansive soils or clay, such as
bentonite, are encountered near the surface.



The use of carton forms to support concret
e during pouring are used
and disintegrate and form a void shortly after the concrete is cured.



Designing Footings



Three factors when designing footings:



Unit loading


the allowable bearing pressure of the soil is not exceeded
and differential settl
ement is eliminated as much as possible.



Shear





Punching (two
-
way shear) when the column or wall load punches
through the footing.



Fail in flexural shear or diagonal tension the same as regular beams



Bending


when the lower surface cracks under
flexural loading.



Simple spread footings act much like inverted beams with the upward soil
pressure being a continuous load that is resisted by the downward column
load.



This tends to cause bending in the upward direction which induces

37

compression near

the top of the footing and tension near the bottom of
the footing.



The area of a spread footing is determined by dividing the total wall or
column load on it plus its own weight plus any soil on top of the footing by
the allowable soil bearing pressure.



The footing itself is designed for shear, moment, and other loads with
factored loads as required by the American Concrete Institute.



Wall footing design considerations:



Two critical sections



Face of the wall where bending moment is the greatest



At distance, d, from the face of the wall in the footings where
flexural shear is of most concern



The critical two
-
way shear section for column footings is a
distance d/2 from the face of the wall.



(d) is the distance from the top of the footing to the cntroid of the
reinforcing steel, called the effective depth of the footing since the
concrete below the steel does not contribute any structural properties.



(d) for masonry is different than (d) for

concrete walls. For concrete
walls it is measured to the face of the wall.



Maximum allowable flexural shear governs the design depth of wall
footings.



Individual column footings are subject to two
-
way action much like flat
slabs near columns, as well

as one way shear.



Both types of shear must be calculated and the depth of the footing
designed to resist these shear forces.



When both are calculated, the greater shear value is used for design.



One
-
way shear at distance (d) from the face of column,

the factored
soil design pressure is calculated over the rectangular area indicated
by the shear line and the outside face of the footing.



Two
-
way shear is calculated a distance (d)/2 from the face of
concrete in a rectangular area around the pier.



Bo
ttom reinforcing in both directions is required to resist the
moment forces at the face of the column.



Concrete weighs approximately 150 pounds per cubic foot (a one foot
section weighs 1050 pounds)



Soil weighs approximately 100 pounds per square foot
per foot of

38

thickness.



The ACI code requires the design soil pressure be calculated based on
factored loads according to the following formula: U=1.4D + 1.7L.



Types of Retaining wall



Three types of retaining walls



Gravity wall



Gravity wall resist
s the forces on it by its own weight and by soil
pressure and soil friction against its surface opposite to the earth
forces.



Commonly used for low retaining walls up to about 10 feet where
forces on it are not too great.



Cantilever wall



Most common type



Constructed of reinforced concrete



Resists forces by the weight of the structure as well as by the weight
of the soil on the heel at the base slab.



Often constructed with a key projecting from the bottom of the slab
to increase the w
all’s resistance to sliding.



The toe is omitted if the wall is next to a property line or some other
obstruction.



The arm, heel, and toe act as cantilevered slabs, the thickness and
reinforcement increase with increased length because of the larger
mom
ents developed.



Cantilevered walls are economically limited to about 20


25 feet in
height.



Counterfort wall



Walls higher than 20
-
25 feet.



Similar to cantilevered walls but with counterforts placed at
distances equal to or a little larger than one
-
half the height.



Counterforts are simply reinforced concrete webs that act as
diagonal bracing for the wall.



Forces on Retaining Walls



Force on a retaining wall results from the pressure of the earth retained
acting in a horizontal direction to the
wall.



Earth pressure increases proportionally with the depth from the surface.


39



The coefficient of earth pressure depends on the soil type and the
method of backfilling and compacting it.



Coefficient of earth pressure may range from 0.4 for uncompacte
d
soils like sands and gravels to 1.0 for cohesive, compacted soils.



Pressure acts in a triangular form.



Total pressure against the wall can be assumed to be acting through the
centroid of the triangle


or one
-
third the distance from the base of the
triangle.



Assumed equivalent weight of soil is 30 PCF



Additional loads called sur
-
charges may result from driveways or other
forces being imposed on the soil next to the wall.



If the ground behind the wall becomes wet, there is additional pressure
resulting from the water which must be added to the soil pressure



Design Considerations



A retaining wall may fail in two ways



As a whole by overturning or sliding



Individual co
mponents may fail such as when the arm or stem
breaks due to excessive moment.



In order to prevent failure due to overturning or sliding, the resisting
moment or forces that resist sliding are generally considered sufficient
if there is a safety factor o
f 1.5.



To prevent sliding, the friction between the footing and surrounding
soil and the earth pressure in front of the toe (and key, if any) must be
1.5 times the pressure tending to cause the wall to slide.



The thickness, width, and reinforcing of th
e retaining wall must be
designed to resist the moment and shear forces induced by soil
pressures, surcharges, and any hydrostatic pressures,



Design retaining walls to eliminate or reduce the build
-
up of water
behind them.



Connections



The majority of

structural failures occur in the connection of members.



Either the incorrect types of connectors are used, they are undersized, too few
in number, or improperly installed.



Wood Connections



Variable that effect the design of wood connections


40



Load c
arrying capacity of the connector



Species of wood



Type of load



Condition of the wood



Service conditions



Whether or not the wood is fire
-
retardent treated



Angle of load to the grain



Critical net section



Type of shear the joint is subjected to



Spacing of the connectors



End and edge distance to connectors



Species of wood



Species and density of wood affects holding power of connectors



Species are classified into four groups



There is one grouping

for timber connectors



Split ring connectors and shear plates



Four groups for timber connectors are designated A, B, C, and D



Another grouping



Lag screws, nails, spikes, wood screws, metal plate connectors



Four groups for these connectors are des
ignated I, II, III, and IV



Type of Load



Wood can carry greater maximum loads for short durations than for
long durations



Tables of allowable connector loads are for normal duration of ten
years.



Other conditions


multiply by the following factors



0.90 for permanent loading over 10 years.



1.15 for 2 months’ duration (snow loads for example)



1.25 for 7 days’ duration



1.33 for wind and earthquake loads



2.00 for impact loads


41



Condition of Wood



Tabulated values are based on fastening to woo
d seasoned to a moisture
content of 19% or less.



Partially seasoned or wet wood reduces the holding power of the
connector.



Service Conditions



The environment in which the wood joint will be used.



Are they? Dry, wet, exposed to the weather, subject

to wetting and
drying.



Any service conditions other than dry or continuously wet reduce the
holding power of the connector.



Fire
-
retardant treatment



Wood that has been fire
-
retardant treated does not hold connectors as
well as wood that has not been

treated. The adjustment factor for
fastener design loads is 0.90.



Angle of Load



One of the most important variable affecting allowable loads carried by
connectors is the angle of the load to the grain, which is defined as the
angle between the direct
ion of load acting on the member and the
longitudinal axis of the member.



Wood connectors can carry more load parallel to the grain than
perpendicular to it.



If the load is acting other than parallel or perpendicular to the grain, it
must be calculated

using the Hankinson formula or by using one of the
graphs that gives the same results.



Critical Net Section



When a wood member is drilled for one of the many types of
connectors, there is a decrease in area of wood to carry the imposed
load.



The section where the most wood has been removed is called the
critical net section.



It may be necessary to increase the size of the member just to
compensate for this decrease in area.



Type of Shear



Connectors such as bolts and lag screws can be in
single shear, double
shear, or multiple shear.


42



The type of shear condition and the relative thickness of each piece to
the others are especially important when designing bolted connections.



Spacing Connectors



Connector spacing is the distance between

centers of connectors
measured along a line joining their centers.



End and Edge Distances to connectors



End distance is the distance measured parallel to the grain from the
center of the connector to the square
-
cut end of the member.



Edge distance is the distance from the edge of the member to the center
of the connector closest to the edge of the member measured
perpendicular to the edge.



Nails



Nails are the weakest of wood connectors



Nails are the most common for light frame co
nstruction.



For the same penny weight:



Box nails have the smallest diameter



Common wire nails have the next largest diameter



Wire spikes have the greatest diameter



The preferable orientation is to have the fastener loaded laterally in side
grain
where the holding power is the greatest.



If one of the pieces is metal rather than wood, allowable values may be
increased by 25 percent.



Fasteners loaded in withdrawal from end grain are not allowed by
building codes.



Screws



Wood screws used for structural purposes are available in sizes from #6
to #24 in lengths up to five inches.



Also like nails, screws are best used laterally loaded in side grain rather
than in withdrawal from side grain. Withdrawal from end is not
permi
tted.



Design values given in tables are for a penetration into the main
member of approximately 7 diameters.



In no case should the penetration be less than 4 diameters.



Like nails, design values can be increased by 25% if a metal side plate
is used.


43



Lag Screws



Lag screws are also called lag bolts.



Sizes range from 1/4 inch to 1
-
1/4 inch in diameter and from 1 inch to
16 inches in length.



Diameters are measured at the non
-
threaded shank portion of the screw.



Design values for lateral loading
and withdrawal resistance depend on



Species group



Angle of load to grain



Diameter of the lag screw



Thickness of the side member



Length of the screw



If the load is other than at zero degree or 90 degree angle, the design
value must be determined

using the Hankinson formula.



Bolts



Bolts are one of the most common forms of wood connectors for joints
of moderate to heavy loading.



Variables such as the following affect the allowable design values and
the spacing of the bolts:



Thickness of the

main and side members



Ratio of bolt length in main member to bolt diameter



The number of members joined



Two typical conditions are joints in single shear and double shear



Design values given in tables are usually for conditions where the side
members in double shear joints are one
-
half the thickness of the main
member.



When steel plates are used for side members or main members loaded
parallel to the grain, the tabulated
values can be increased 75% for
joints made with bolts 1/2 inch or less, and 25% for joints made with
bolts 1
-
1/2 inch with intermediate diameter values interpolated.



No increase is allowed for loading perpendicular to the grain.



Timber Connectors



There are two types of timber connectors:



Split rings


44



Either 2
-
1/2 inches or 4 inches in diameter and are cut through in
one place in the circumference to form a tongue and slot



Shear plates



2