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Design of Steel Structures

Department of Civil Engineering

University of Engineering & Technology, Taxila

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Chapter1

Fundamentals of Steel Design

Design of Steel Structures

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Basic Design Equation

In design, the applied forces and moments

due to external loads are equated to the

maximum resistive forces and moments with

a FOS which is always greater than or equal

to one.

The concept may be summarized by the

following design equation:

Load Effects X Factor of

safety (F.O.S)Max. Internal Resistance

offered by Material of

the Structure

=

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Load effects are defined as the forces,

stresses and deformations produced in a

structural component by the applied loads.

A simply supported beam of span L

subjected to a point load P can be analyzed

to get the maximum bending moment ofPL/4.

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However, this bending moment will only be

produced if the material of the beam is strong

enough to develop the required strength.

This means that the answer of analysis may

be true for bigger steel girder but may not be

true for small wooden batten.

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Factor of Safety

3. To cover uncertainties in material strength.

4. To cover, in part, poor workmanship.

5. To cover unexpected behavior in case the

theory is not fully developed.

6. To cover natural disasters.

7. The stresses produced during fabrication and

erection.

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Factor of Safety

8. Presence of residual stresses and stress

concentrations.

In case of allowable stress design, the factor

of safety is applied in the form of safety factor

(), while in case of LRFD, it is applied in theform of overload factors and the resistance

factor ().

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Nominal Strength

Nominal strength (Rn) is defined as the

strength of a structure or component to resistload effects determined by using formulas

given in the specifications.

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Types of Design

Load and Resistance Factor Design (LRFD),

Strength Design or Limit State Design

Allowable Stress Design (ASD)

Plastic Design

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1. Load & Resistance Factor Design

(LRFD)

Major part of FOS is applied on load actions

called overload factor.

Minor part of FOS is taken on RS of designequation called resistance factor or capacity

reduction factor ().

Resistance factor () is lesser than or equal

to 1.0 and is applied on material strength.

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(LRFD)

The design equation is checked for each

strength and serviceability limit states one-

by-one.

Limit state is defined as the limiting stage in

the loading after which the structure

cannot fulfill its intended function due tostrength or serviceability considerations.

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(LRFD)

Analysis of structures for loads is performed

considering the structure to be within elastic

range.

However, inelastic behavior, ultimate failure

modes and redistribution of forces after

elastic range are considered in this method.

This is more realistic design as compared

with the old Allowable Stress Design.13

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(LRFD)

Nominal strength (Rn) is defined as the

strength of the structure or its component

determined by using formulas given in

specifications.

Any particular load effect increased by theload factors is called the Required Strength

(Ru).

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(LRFD)

The nominal strength reduced by the

resistance factor (Rn) is called the Design

Strength.

The design equation in case of LRFD

becomes:

Ru ()Rn

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Advantages of Using LRFD

LRFD is another tool for steel design, which

provides a flexibility of options to the designer

in selecting the design methodology.

Economical in case dead loads are larger,

compared with live loads.

Every type of load may be given a different

FOS depending upon its probability of

overload, number of severe occurrences and

changes in point of application.16

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Advantages of Using LRFD

Behavior at collapse including ductility,

warning before failure and strain hardening

etc.

This is not directly possible in ASDbecause

here the structure is considered at service

stage and not approaching close to collapse.

More safe structures result due to better

awareness of behavior near collapse.

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Disadvantages of Using LRFD

Elastic behavior considered for load analysis

and ultimate plastic behavior taken for

material strengths are not compatible,

however, percentage difference is less.

Engineers experienced in ASD have to

become familiar with this technique.

Old books and design aids become

ineffective.

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Disadvantages of Using LRFD

Validity of previous designs is still to be

checked according to ASD.

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2. Allowable Stress Design (ASD)

F.O.S is taken on right side of the basicdesign equation. This is denoted by .

Allowable strength (Rn/) is defined as thenominal strength divided by the safety factor.

Loads Effects Material Resistive ForcesFOS

=

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Required ASD Strength (Ra) is the load

effect obtained from the service loads without

any additional factor.

The design equation for ASD becomes:

This method is now gradually replaced by

LRFD for the structures, where behavior near

collapse is fully understood.

Ra Rn/

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It is still preferred by some engineers for

important structures like atomic reactors and

pre-stressed concrete.

It is included in the specifications as an

alternate method of design.

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Disadvantage of ASD

Latest research and literature is very much

limited.

Same factor of safety is used for different

loads.

The failure mode is not directly predicted.

With some overloading, the material stressesincreases but do not go to collapse. (The

failure mode cannot be observed).

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Disadvantage of ASD

The ductility and warning before failure

cannot be studied precisely.

Results cannot be compared with

experimental tests up to collapse.

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3. Plastic Design

It is somewhat similar to the LRFD but here

the analysis for loads is performed

considering the collapse mechanism of the

structure.

Full reserve strength due to indeterminacy of

the structure and inner elastic portion of the

structure is utilized.

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3. Plastic Design

Inelastic material behavior is considered in

the analysis and design.

Deflections and other serviceability

conditions become more important along with

the strength requirements.

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DESIGN STRENGTH

In LRFD, design strength of all elements is

obtained as resistance factor multiplied with

maximum stress that can be developed

multiplied with sectional area or section

modulus.

The design strength is also called the loadcapacity, or sometimes only capacity, of a

member.

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DESIGN STRENGTH

An example to explain the difference

between the member capacity and the

applied load is that of a bottle.

This bottle may have a fixed liquid retaining

capacity of suppose 1 litre.

However, it may be empty at times meaning

that the amount of liquid retained in it is zero

litres but the capacity of the bottle still

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DESIGN STRENGTH

Any amount of liquid may be poured in this

bottle that is not exceeding 1 litre.

Similarly, load capacity of a member existswith a fixed value.

The applied load may have a different value

with only one condition that the applied load

must be lesser than or equal to the member

capacity for stability.

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CAPACITY ANALYSIS OF

STRUCTURES

Knowing the material properties and

dimensions of the member, finding the

maximum loads that can be applied on the

member using the design equation is called

Capaci ty Analys is or Analys is of

Structures.

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DESIGN OF STRUCTURES

Knowing the expected loads and span

lengths of the members in the basic design

equation, finding the required material

properties and cross-sectional dimensions is

called Design of Structu res.

In steel structures, the design mainly consists

of a selection out of already available

sections in the market.

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DESIGN OF STRUCTURES

Structural Design may be defined as a

mixture of art and science, combining the

experience and intuitive feeling for the

behavior of the structure with a sound

knowledge of the principles of statics,

dynamics, mechanics of materials, and

structural analysis, to produce a safe

economical structure which will serve its

intended purpose.

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Objectives of Structural Designer

Design is a process by which an optimum

solution is obtained satisfying certain criteria.

Minimum cost

Minimum weight

Minimum construction time

Minimum labour

Maximum efficiency of operation

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The structural designer must learn to arrange

and proportion the parts of his structures so

that they can be practically erected and will

have sufficient strength and reasonable

economy.

These important items, called safety, cost

and practicabilityare discussed next:

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1. The structure must safely support the loads

to which it is subjected.

The deflections and vibrations should not be

so excessive as to frighten the occupants.

2. The designer must keep the construction,

operation and maintenance costs at the

lowest levelswithout sacrificing the strength.

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3. Designers need to understand fabrication

methods and should try to fit their work to the

available fabrication facilities, available

materials and the general construction

practices.

Some designers lack in this very important

aspect and their designs cause problems

during fabrication and erection.

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Designer should learn everything possible

about the detailing, the fabricationand the

field erection of steel besides the loads,

mechanics, and the expected material

strengths.

The designer must have information

concerning the transportation of the

materials to site, labor conditions,

equipment for erection 39

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problems at site, field tolerances and the

required clearancesat the site.

This knowledge helps to produce reasonable,practical and economical designs.

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Procedure of the Structural Design

The structural framework design is the

selection of the arrangement and sizes of

structural elements so that service loads

may be safely carried.

Structural designer has to complete thefollowing steps to get a successful design:

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The above design procedure for a whole

structure requires iterations and the main

steps are listed below:

1. The functions to be performed by the

structure and the criteria for optimum solution

of the resulting design must be established.

This is referred to as the planning stage.

2. The general layout of the structure is

decided.

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3. Different arrangements of various elements

to serve the functions in step 1 are

considered.

The possible structural forms that can be

used are studied and an arrangement

appearing to be best is selected for the first

trial, called preliminary structural

configuration. Only in very rare cases, it has

to be revised later on.

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4. Loading conditions are considered and the

loads to be carried by the structure are

estimated.

5. Based on the decisions of the earlier steps,

trial selection of member sizes is carried

out depending on thumb rules or assumed

calculations to satisfy an objective criterion,

such as least weight and cost.

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6. Structural analysis involving modeling the

loads and the structural framework to obtain

internal forces, stresses and deflections is

carried out.

7. All strength and serviceability requirements

along with the predetermined criteria for

optimum are checked. If any check is notsatisfied, the member sizes are revised. This

stage is called evaluationof the trial member

sizes.46

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8. Repetition of any part of the above sequence

found necessary or desirable as a result of

evaluation is performed in this stage called

redesign.

9. The rivets, bolts and welds along with other

joining plates and elements are designed.

The process is termed as the design of

assembly and connections.

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10.It is determined whether or not an optimum

design has been achieved, and the final

decisionis made.

11.Drawings are prepared to show all design

details. An estimate for the required

quantities is also made. This stage of design

is called preparation of design documents.

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Procedure of the Structural Design

The important sub-steps in the design of

parts (step 7 above) are shown in the form of

a flow chart in Fig 1.1

Objectives of the design must always be kept

in mind while using this flow chart.

The selection of trial section in step 2depends on the main objectives, availability

of material, construction requirements and

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Collect and list all the known data

Select trial section

based on assumed stresses/effectiveness of cross-sectional

alternatively, selection tables may be used

Apply all stability checks

Perform strength checks

Perform serviceability checks

Accept section if all checks aresatisfied, other-wise revise

Write Final Selection

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Limit State

Limit state is defined as the stage in the

loading after which the structure cannot

fulfills its intended functiondue to strength

or serviceability considerations.

The term limit state is preferred compared

with failure because in most cases of limitstates, the actual failure or collapse does not

occur.

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Limit State

Limit states are generally divided into two

categories, strength and serviceability.

Strength or safety limit states meansconditions of loading corresponding to

maximum ductile flexural strength (i.e.,

plastic strength), ultimate ductile shearstrength, buckling, fatigue, fracture,

overturning and sliding, etc.

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Limit State

Serviceability limit states are those

concerned with occupancy of the building,

such as the deflection, vibration, permanent

deformation and cracking.

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Limit State

The structure should not cross any strength

or serviceability limit for a perfect design. All

the applicable limits are to be checked by

using the available procedures.

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End of File

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