Faculty of Engineering
Civil Engineering Department
Steel Structure
Chapter 2: Design of steel structure
Fall 2020/2021
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Objectives of structural design
The primary goal of structural design is to size members and components of a system to adequately
and safely sustain loads.
However, how do we reliably and accurate assure this safety?
Answer: address uncertainty
Examples of uncertainty:
Loads Resistances
Load estimation Material quality
Possible overloading Fabrication tolerances
Different probabilities of Construction
different load cases Etc.
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Objectives of structural design
3 things we should take into consideration during design:
1- Safety:
Resistance ≥ Effect of applied loads
Both sides of the inequation should be evaluated for the same conditions and units.
For example, compressive stress on soil should be compared to bearing capacity of soil.
Resistance are determined by: Materials used
X-sections (dimensions)
Goal in design: Prevent the limit state to be reached.
In fact, limit state is a condition beyond which a structural system or a structural component cesses
to fulfill the function for which it is designed. It could be :
• Strength limit states: flexure, torsion, shear, fatigue, settlement, Bearing… (load carrying capacity)
• Serviceability: Buckling, cracking, stability, deflection (performance)
• Special: Fires, earthquakes, explosions.
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Objectives of structural design
3 things we should take into consideration during design:
2- Cost:
Lower cost without strength sacrifice
• Choose members with standard-size present in the market.
• Choose members which need less maintenance.
3- Practicality:
Study the transportation, labor conditions and equipment available for erection to provide
problems during fabrication and erection.
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Failures in engineering structures
➢ It is rarely due to faults in the materials
➢ It is usually caused by the insufficient attention to details of connections, deflections,
erection problems, foundation settlement.
Failure in connection Failure of beams Failure in compression
"One bolt is no bolt..!!"
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Failures in engineering structures
➢ It is rarely due to faults in the materials
➢ It is usually caused by the insufficient attention to details of connections, deflections,
erection problems, foundation settlement.
Failure in tension Local failure
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Specifications
➢ Like other structures, steel structures design are controlled by codes:
AISC: American Institute of Steel Construction
AASHTO: American Association of State Highway and Transportation Officials
ASTM: American Society for Testing Materials
Those codes protect public safety, the engineer, the owner and constitute a guide to facilitate
engineers work. They should determine an acceptable level of safety against exceeding each limit
states.
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Loads
➢ Dead loads
Magnitude that remains in one position.
Dead loads consist of the permanent construction material loads comprising the roof,
floor, wall, and foundation systems, including claddings, finishes, and fixed equipment.
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Loads
➢ Live loads
Live loads are produced by the use and occupancy of a building. Loads include those
from human occupants, furnishings, nonfixed equipment, storage, and construction
and maintenance activities.
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Loads
➢ Environmental loads
a) Snow: Depends on many factors , including geographic location, roof pitch, the
shape of the roof, etc…
b) Wind: Act as pressure on vertical surfaces. They also depend on factors as
geographic location and height.
c) Earthquake loads: Depending on seismic zones, it is necessary to consider seismic
forces in design for all types of structures, especially the horizontal component that
can be severe.
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Design of steel members
▪ Use members with standard size
▪ Use members of the same sizes although some of them may slightly over designed
▪ Use sections that are easy to erect and without difficult conditions to maintain;
▪ Select steel members that will fit with the requirements made by pipe, ducts,
conduits… present in the building;
▪ Have all the informationn related to transportation problems: greater lengths and
depths that can be shipped by truck or rail, clearance available under bridges, loads
or bridges, power lines leading to the project;
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Philosophies for design steel members
The AISC provides 2 methods for designing structural steel members and their connections:
➢ LRFD: Load and Resistance Factor Design
➢ ASD: Allowable Strength Design or WSM: Working Stress Method
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Philosophies for design steel members
ASD method (Based on safety factors)
The main assumption in this method is that the behavior of structural material is
restricted with in the linear-elastic region and the safety of it is ensured by restrecting
the stresses coming on the members by working loads.
Thus, the allowable stresses will come in the linear portion of the stress- strain curve,
and a factor of safety Ω is introduced.
𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 (𝑅𝑛 )
Ω 𝑠𝑎𝑓𝑒𝑡𝑦 𝑓𝑎𝑐𝑡𝑜𝑟 = >1
𝐸𝑓𝑓𝑒𝑐𝑡 𝑜𝑓 𝑙𝑜𝑎𝑑 (𝑄)
𝑅𝑛 : Nominal strength
Theoretical strength with no safety factors or resistance factors
𝐸𝑓𝑓𝑒𝑐𝑡 𝑜𝑓 𝑙𝑜𝑎𝑑 should produce stresses that is a fraction of the yield
stress fy (for example 0.5)
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Philosophies for design steel members
ASD method (Based on safety factors)
Apply service loads to structure (no load factors)
The service loads are not multiplied by a factor, they are summed up for various
feasible combination and the largest values 𝑅𝑎 so obtained are used to compute the
forces in the members
𝑅𝑛
≥ 𝑅𝑎
Ω
Combined loads for ASD:
• D
• D+L
• D+ (Lr or S or R)
• D+0.75 L + 0.75 (Lr or S or R)
• D+ (0.6 W or 0.7 E)
• 0.6 D + 0.6 W
• 0.6 D + 0.7 E
• D + 0.75 L + 0.75 (0.6 W)
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Philosophies for design steel members
LRFD method
This method consider the variability not only in resistance but also in the effects of loads.
Design such that : Factored forces ≤ Factored capacities
So, a resistance factor (φ< 1) is multiplied by the nominal strength of a member.
And on the other hand, possible service load groups are formed, and each service load is multiplied by a
load factor, normally larger than 1. The magnitude of the load factor reflects the uncertainty of the
particular load. The largest values determined in the manner are used to compute moments, shears… (Ru)
∅𝑅𝑛 ≥ 𝑅𝑢
DL LL Load variability
Low High
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Philosophies for design steel members
LRFD method
This method consider the variability not only in resistance but also in the effects of loads.
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Philosophies for design steel members
𝑅𝑛 /Ω : ASD capacity
φ𝑅𝑛 : LRFD capacity
𝑅𝑛 : Nominal capacity
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