Live loads specified in codes do account for ordinary impact loads

tobascothwackUrban and Civil

Nov 15, 2013 (3 years and 8 months ago)

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Live loads specified in codes do account for ordinary
impact loads


When structural members are subject to unusual
vibration or impact we have to account for them
outside the code specs

Type of
member

Source of Impact

Percent
increase

Supporting

Elevators and elevator machinery

100

Supporting

Light machines, shaft,

or motor driven

20

Supporting

Reciprocating

machines or power
-
driven
units

50

Hangers

Floors or balconies

33


Structures supporting cranes:


Maximum wheel loads


Allowance for impact


Multiple cranes


Traction and braking forces


Use of crane stops


Cyclic loading / Fatigue


Crane live load is its fully rated
capactity


Max vertical wheel load


Monorail, cab operated, remote operated


increased by 25% for impact


Pendant operated overhead


Increased by 10% for impact


Impact increases do not have to be applied to
supporting columns, only runway



Electic

powered trolleys


≥ 20% (crane rated load + trolley weight + hoist weight)


Assume applied by wheels at top of rails


Acts normal to the rails


Distributed, as appropriate to stiffness of rail support


Bridge or monorail with hand
-
gearing


No need for lateral load increase


Runway must be designed for stop forces


Velocity of crane at impact taken into account


Fatigue and serviceability concerns



AISC Design Guide 7


AISE Standard No. 13


Caused by changes in dimensions/geometry of
structures due to


Behavior of material


Type of framing


Details of construction


e.g.


Foundation settlement


Temperature changes


Shrinkage restrained by adjoining structures


Loads may act simultaneously


Building codes specify various combinations that must
be considered


Depends on whether allowable stress design (ASD) or
Load and Resistance Factor Design (LRFD) is used


SEI/ASCE 7
-
02 provides guidance.


D = dead load


L = live floor load, including impact


Lr

= roof live load


S = roof snow load


R = rain load

(initial rainwater or ice, exclusive of
ponding
)


W = wind load


E = earthquake load


T = restraint load


D


D + L + T


D + (
L
r

or S or R)


0.75 [ L + (
L
r

or S or R) + T ] + D


0.75 (W or 0.7E) + D


0.75 [ L + (W or 0.7E) + (
L
r

or S or R) ] + D


0.6D + W


0.6D + 0.7E


Because E was calculated for LRFD it is reduced by 0.7
for ASD design.


1.4 D


1.2(D+T) + 1.6L + 0.5(L
r

or S or R)


1.2D + 1.6(L
r

or S or R) + (L or 0.8W)


1.2D + 1.6W + L + 0.5(L
r

or S or R)


1.2D + E + (L or 0.2S)


0.9D + 1.6W


0.9D + E


International Building Code


International Code Council, Falls Church, VA


NFPA 5000, Building Construction and Safety Code


National Fire Protection Association, Quincy, MA


National Building Code of Canada


National Research Council of Canada, Ottawa, ON


Or local code


Most fires are accidental or carelessness


Start small and require fuel and ventilation to grow


Noncombustibles

(concrete, steel, brick) are not fuel


Combustibles (paper, wood, plastics) are fuel


Fire loading is the amount of fuel, measured in
equivalent pounds of wood per square foot of floor
area


Fire severity is the duration of the fire, in hours of
equivalent fire exposure


More modern approaches of fire load are expressed in
terms of potential heat energy


Fire loading correlates well with fire severity


Reasonable estimate for conventional wood frame
construction:


7.5


10 lb/ft
2


Reasonable heavy timber estimate


12.5


17.5 lb/ft
2


Consequently building codes limit permitted size
(height and area) of combustible buildings more than
non
-
combustible buildings.


BUT ventilation is an important factor as well.


0
1
2
3
4
5
6
7
8
0
10
20
30
40
50
60
70
Fire Severity (hrs)

Fire Load (lbs/ft2)

Type

of Occupancy

Fire

Load (lb/ft
2
)

Fire Severity (hrs)

Assembly

5
-
10

0.5


1

Business

5
-
10

0.5
-
1

Educational

5
-
10

0.5
-
1

Hazardous

Variable

Variable

Industrial


Low hazard

0
-
10

0
-
1


Moderate

hazard

10
-
25

1
-
2.5

Institutional

5
-
10

0.5
-
1

Mercantile

10
-
20

1
-
2

Residential

5
-
10

0.5
-
1

Storage


Low hazard

0
-
10

0
-
1


Moderate hazard

10
-
30

1
-
3


Fire Resistance
: Relative ability of construction
assemblies to prevent spread of fire to adjacent spaces,
and to avoid structural collapse


Fire resistance requirements are a function of
occupancy and size (height and area)


Fire resistance is determined experimentally


ASTM E 119


Uses “standard” fire exposure


Specified in terms of time of exposure


Time during which an assembly


continues to prevent spread of fire,


does not exceed certain temperature limits, and


Sustains its structural loads without failure


Typically expressed in hours



Fire Resistance Directory, Underwriters Lab


Fire Resistant Ratings, American Insurance Services
Gp
.


Fire Resistant Design Manual, Gypsum Association


No building is fireproof.


Avoid this term


In general, steel can hold 60% of yield strength at
1,000 F


Failures rarely occur because during a fire building is
rarely loaded at design load.


This is not recognized in the code


structures are
assumed to be fully loaded during testing.


Thus, when building codes specify fire resistant
construction, fire protection materials are required to
insulate the structural steel.

0
0.2
0.4
0.6
0.8
1
0
500
1000
1500
2000
2500
% Yield Strength

Temperature (F)


Gypsum


As a plaster, applied over metal lathe or gypsum lathe


As wallboard, installed over cold
-
formed steel framing
or furring



Effectiveness can be increased significantly with
lightweight mineral aggregates (vermiculite,
pearlite
)


Mix must be properly proportioned and applied in required
thickness and the lathe correctly installed


3 kinds:


Regular, Type X, and proprietary


Type X:


Specially formulated cores for fire resistance.


Proprietary


Such as
Typc

C, even greater fire resistance


Type of wall board must be specified clearly.


Type and spacing of fasteners (and furring channels if
applicable) should be in accordance with specs



Most widely used


Lightweight mineral fiber and cementitious material


Sprayed onto beams, girders, columns, floor decks,
roof decks


SFRM: Spray
-
applied Fire Resistive Materials


Generally proprietary formulations


Follow manufacturers recommendations!


Underwriter’s Laboratories specifies these



Cohesion/Adhesion are critical


Must be free of dirt, oil, loose scale


Generally light rust is OK


Paint can cause problems


Wide range of systems available to protect floors,
beams and girders


Fire resistance ratings published by UL


Require careful integration of ceiling tile, grid and
suspension system


Openings for light fixtures, air diffusers, etc. must be
adequately limited and protected.


Sometimes code requires individual structural element
protection, thus suspended ceilings are not permitted.


Concrete used to be common, but not highly efficient
(weight and thermal conductivity)


Concrete floor slabs are common for tops of flexural
members.


Concrete sometimes used to encase columns


for architectural or structural purposes,


or for protection from abrasion or other physical damage


AESS:
easthetic

choice


Steel


Insulation


Steel skin


Gives appearance of steel surface but has protection


Water filled tubular columns


Patented in 1884, but not used until 1960 in the 64 story
US Steel Building in Pittsburgh


Flame shielded spandrel girders


Standard fire test is not representative of the exposure
for exterior structural elements.


Can only be used if code allows engineered solutions


Columns are interconnected with a water storage tank.


In fire, water circulates by convection, keeping the
steel temperature below the critical value of 450
°
C.


This system has economical advantage when applied to
buildings with more than 8
storeys
.


If the water flow is adequate, the resulting fire resistance
time is virtually unlimited.


In order to prevent freezing, potassium carbonate
(K
2
CO
3
) is added to the water.


Potassium nitrate is used as an inhibitor against
corrosion.


Interior

Exterior

Painted girder


Major confusion from concept of Restrained and
Unconstrained ratings


Only in ASTM E119 and US codes


No other country uses this


Part of problem is max test size is 15’
x

18’


not full scale


When testing problems arise:


Floor slabs and roof decks are physically continuous over
beams and girders, but this is too big


Beams join columns and girders in a number of different
ways


can’t test them all


ASTM E119 includes 2 test conditions: Restrained and
Unrestrained


Restraint is against thermal expansion


This allows for thermal stresses from surrounding structure


Most steel framing is tested as Restrained


Unrestrained:


Single span and simply supported end spans of multiple bays


Open web steel joists or beams, supporting precise units or
metal decking


Wood construction


Rate of temperature change depends on mass and
surface area.


The weight to heated perimeter ratio is significant


W/D


W = weight per unit length


D = inside perimeter of fire protection material


W/D = Thermal Size (lbs/ft/in)

0
0.5
1
1.5
2
2.5
3
3.5
4
0.5
1.5
2.5
3.5
Fire Resistance (hrs)

W/D (lb/ft/in)

1/2"
5/8"
1"
1 1/4"
1 1/2"
1 7/8"
2"
2 1/2"