INTRODUCTION TO THE STANDARD DESIGN CODE

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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.