Lecture 1.
INTRODUCTION TO THE STANDARD DESIGN CODE
Content of lecture:
T
ypes of steel buildings; steel as a structural material
Rolled and built

up steel sections and their properties
Principles of Limit States Design;
British Standard Code: design conc
epts and
requirements
Types of steel buildings
S
teel buildings are composed of:
(1) Beams and girders;
(2)
Ties;
(3)
Struts, columns or stanchions;
(4)
Trusses and lattice girders;
(5)
Purlins;
(6)
Sheeting rails;
(7)
Bracing.
The problem in structura
l design consists of:
(1)
Estimation of loading;
(2)
Analysis of main frames, trusses or lattice girders, floor systems, bracing and
connections;
(3)
Design of the elements and connections using design data from step
(2);
(4)
Production of arrangement and
detail drawings from the designer's sketches.
Figure 1. Common types of steel buildings
Design methods
Steel design may be based on three design theories;
(1)
Elastic design;
(2)
Plastic design; and
(3)
Limit state design.
British standard codes of practice
The design is based on the actual behavior of materials and structures in use
and is in accordance with:
BS
5950:
The 'Structural Use of Steelwork in Building;
Part
1

Code of
Practice for Design in Simple and Continu
ous Construction: Hot Rolled
Sections.
BS
4360:
Weldable Structural Steels.
This gives the mechanical properties for
the various grades of structural steels.
BS
6399:
Part
1,
Code of Practice for Dead and Imposed Loads, CP
3:
Chapter
V,
Part
2,
Wind
Loads.
Figure 2. Multistory office building
Figure 3. Factory and multi

story building
STEEL AS A STRUCTURAL MATERIAL
Steel is popular material because of the several factors:
Great strength,
Good ductility because of yielding,
High stif
fness,
Easy fabrication and
Relatively low cost.
Table 1.
Advantages of steel structures

Item
Comments


Ease of erection
No formwork
Minimum carnage
Speed of erection
Much of the structure can be
prefabricated away from the site
Largely self

supporting during
erection
Modifications
Extensions/strengthening
at a later da
te
relatively straightforward
Low self

weight
Permits large clear spans
Good dimensional
Prefabrication in the shop ensures
control
accurate work
Figure 5.Typical tensile test specimens
Figure 6. Typical stress
–
strain diagrams for structural steel
PROPERTIES OF STEEL
1.
Stresses and deformations
Young’s modulus,
E
limit of proportion
ality,
p
upper yield point
yu
and lower yield stress
yL
(or yield stress
y
);
yu
/
yL
= 1.05

1.10
Three strength gr
ades

43,
50
and
55(value of
ult
in kgf/mm
2
)
.
strength, ductility, impact resistance
and
weldabilit
y. The
mechanical properties for steels are set out in
BS
4360.
2.
Residual stresses

the result of uneven heating and cooling of structural
members will
normally contain.
3.
Fatigue

occurs in structures subjected to fluctuating or cyclic loads (crane
girders, bridges and offshore structures, etc.) through progressive growth of
a crack. The failure load may be well below its static value. To help avoid
fat
igue failure, detail should be such that stress concentrations and abrupt
changes of section are avoided in regions of tensile stress.
4.
Brittle fracture
a.
Fire protection

provided by encasing the member in a fire

resistant
material such as concrete.
b.
Co
rrosion protection. The main types of protective coatings are:
Metallic coatings (sprayed

on coating of aluminum or zinc)
Painting
Figure 4. Model of profiled rolls used in final rolling of H

section
Figure 7. Rolled and formed sections
Figure 8. Compound sections
Figure 9. Built

up sections
Figure 10. Cold

rolled sections
Section properties
(1)
The exact section dimensions;
(2)
The location of the centroid if the section is asymmetrical about one or
both axes;
(3)
Area
of cross section;
(4)
Moments of inertia about various axes;
(5)
Radii of gyration about various axes;
(6)
Moduli of section for various axes, both elastic and plastic.
Steelwork Design,
Guide to BS
5950:
Part 1:
1985,
Volume 1, Section Prop
erties,
Membe
r Capacities, Constrado, 1985
For the symmetrical I section the section properties are as follows:
(1)
Elastic properties:
Area
A = 2BT + dt
Moment of inertia XX axis
I
x
=
BD
3
/12

(B

t) d
3
/ 12
Moment of inertia YY axis
I
y
=
2
TB
3
/12
+
dt
3
/12
Radius of gyration XX axis
r
x
=
(I
x
/A)
0..5
Radius of gyration YY axis
r
y
= (I
y
/A)
0..5
Modulus of section XX axis
Z
x
=
2
I
x
/D
Modulus of section YY axis
Z
y
=
2
I
y
/B
(2)
Plastic moduli of section = algebraic sum of the first moments of
area
about the equal area axis. For I section:
S
x
=
2
B T (D

T)/2 + td
2
/4
S
y
=2
TB
2
/4 + dt
2
/4
For asymmetrical sections the n. a. must be located first. In elastic analysis the
n. a. is the centroidal axis while in plastic analysis it is the e
qual area axis (see
procedures from Strength of Materials).
Other properties of universal beams, columns, joists and channels:
1.
Buckling parameter (
u
);
2.
Torsional index (
x
);
3.
Warping constant (
H
)
4.
Torsional constant (
J
)
Figure 11. Beam sections
LIMIT STATE DESIGN
British standard codes of practice
The Limit State Design is based on the actual behavior of materials and structures in
use and is in accordance with
BS
5950:
The Structural Use of Steelwork in
Building, Part
1.
Code of Practice fo
r Design in Simple and Continuous
Construction: Hot Rolled Sections.
British Standards give the design methods, factors of safety, design loads, design
strengths, deflection limits and safe construction practices. Also reference must be
made to other rele
vant standards, including:
(1)
BS
4360:
Wieldable Structural Steels.
(2)
BS
6399
:
Part
1,
Code of Practice for Dead and Imposed Loads
.
(3)
CP
3
:
Chapter
V,
Part
2,
Wind Loads.
The central concepts of Limit State Design
(1)
The separate limit states.
(2)
The design is based on the actual behavior of materials and performance of
structures.
(3)
Design should be based on statistical methods with a small proba
bility of the
structure reaching a limit state.
Ultimate Limit States (ULS)
(I)
Strength (includ
ing general yielding, rupture, buckling and transform
ation
into a mechanism);
(2)
Stability against overturning and sway;
(3)
Fracture due to fatigue;
(4)
Brittle fracture.
When the ultimate limit states are reached, the whole structure or part of it col
lapses.
Working loads
(S
pecified, characteristic or nominal
loads
) are the actual loads the
structure is designed to carry (
95%
probability of not being exceeded).
Dead loads
. These are due to the weights of floor slabs, roofs, walls, ceilings,
partition
s, finishes, services and self

weight of steel structure.
Imposed loads
. Loads caused by people, fur
niture, equipment, stock.
Wind loads
. These loads depend on the location and building size (CP 3:
Chapter
V:
Part
2).
Dynamic loads
. These are caused ma
inly by cranes and earthquake.
Factored loads
are used in design calculations for strength and stability (
Section
2.4.1 of
BS
5950:
Part
1).
Factored load
=
working load
牥汥癡湴 潶o牡r氠汯慤l晡f瑯t
Table 2. Overall load factors (
Table
2 of
BS
5950:
Part
1)

Loading
Factors
f

Dead load
14
Dead load restraining uplift or overturning
1.0
Dead load, w
ind load and imposed load
1.2
Imposed load
1.6
Wind load
1.4
Crane loads
Vertical load
1.6
Vertical and horizontal load
I .4
Horizontal load
1.6
Crane loads and wind load
1.2


Load combinations:
(1)
The main load for design of most structures is dead plus imposed load.
(2)
The load combination of dead plus wind load is used with a load factor of
1.0 for dead and 1.4 for wind load.
(3)
It is improbable that wind and imposed loa
ds will simultaneously reach
their maximum values and load factors are reduced accordingly.
Structural stability
To ensure stability structures must be checked using factored loads
for the following two conditions (Clause 2.4.2 of BS 5950
)
:
(1)
Overturn
ing
.
The structure must not overturn or lift off its seat.
(2)
Sway
.
To ensure adequate resistance two design checks are required:
(a)
Design to resist the applied horizontal loads.
(b)
A separate design for notional horizontal loads.
Structural in
tegrity
Ensure that the structure complies with the Building Regulations and
has the ability to resist progressive collapse following accidental
damage (Section
2.4.5
of BS
5950).
Serviceability limit states (SLS)
(1)
Deflection;
(2)
Vibration (for exam
ple, wind

induced oscillations
);
(3)
Repairable damage due to fatigue;
(4)
Corrosion and
durability.
Deflection under serviceability loads of a building should not impair the
strength or efficiency of the structure or its components or cause damage to t
he
finishings (
BS
5950:
Part
I,
Clause
2.5.1)
The servicea
bility loads used are the unfactored imposed loads except in the
following cases:
(
1)
Dead + imposed + wind. Apply 80 % of the imposed and wind load.
(
2)
Crane surge + wind. The greater effect
of either only is considered.
Design methods for buildings
The design of buildings must be carried out in accordance with one of the
methods (
Clause
2.1.2 of
BS
5950)
Simple design
.
The structure is assumed to be pin jointed for analy
sis. Bracing or
shear walls are
necessary to provide resistance to horizontal loading.
Rigid design
.
The connections are assumed to be capable of developing the strength and/or
stiffness required by an analysis assuming full continuity. The analysis may be
made using ei
ther elastic or plastic methods.
Semi

rigid design
.
Practical joints are capable of transmitting some moment and the method takes
this partial fixity into account.
Experimental verification
.
Where the design
of
a structure or element by calculation in
accordance with any
of the above methods is not practicable, the strength and stiffness may be
confirmed by loading tests.
In practice, structures are designed to either the simple or rigid methods of design.
Semi

rigid design has never found general favo
ur with designers.
References:
1.
Morris, L.J., Plum, D.R. “Structural Steelwork Design to BS 5950” .
Longman Scientific & Technical. 1988.
2.
MacGinley, T.J., Ang, T.C. “Structural steelwork: Design to limit State
Theory”. Buterworth. 1987.
3.
Nethercot, D. “
Limit States Design of Structural Steelwork”. Van Nostrand
Reinhold. 1987.
4.
The Steel Construction Institute. “ Steelwork Design Guide to BS 5950: P.I:
1985, Vol. I, Section Properties and Member Capacities”. 1987.
5.
BS 5950: Structural Use of Steelwork in Bu
ildings, P. I. 1985.
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