It is about steel design

1. Steel Structures Design Sessional
Course Code: CE 3202
Lecture No: 02
Presented by-
Nadira Islam Nila
Lecturer, Department of Civil Engineering
Northern University of Business & Technology Khulna

2. The specifications of most interest to the structural steel designer are those published by
the following organizations.
• American Institute of Steel Construction (AISC)
• American Association of State Highway and Transportation Officials (AASHTO)
• American Railway Engineering and Maintenance-of-Way Association (AREMA)
• American Iron and Steel Institute (AISI)
 Steel Design Specifications
 Design Methodology
The fundamental requirement of structural design is that the required strength not
exceed the available strength. That is,
Required strength ≤ available strength

3. Design for Strength
LRFD - Load and
Resistance Factor
Design
ASD - Allowable
Strength Design

4. In this method a member is selected that has cross‐sectional properties such as area and
moment of inertia that are large enough to prevent the maximum applied axial force,
shear, or bending moment from exceeding an allowable, or permissible, value. This
allowable value is obtained by dividing the nominal or theoretical, strength by a factor of
safety.
This can be expressed as,
Allowable Strength =
𝑁𝑜𝑚𝑖𝑛𝑎𝑙 𝑆𝑡𝑟𝑒𝑛𝑔𝑡ℎ
𝑆𝑎𝑓𝑒𝑡𝑦 𝐹𝑎𝑐𝑡𝑜𝑟
 Allowable Strength Design (ASD):

5.  Load and Resistance Factor Design (LRFD)
In this method load factors are applied to the service loads, and a member is selected that
will have enough strength to resist the factored loads. In addition, the theoretical strength
of the member is reduced by the application of a resistance factor. The criterion that must
be satisfied in the selection of a member is
𝐹𝑎𝑐𝑡𝑜𝑟𝑒𝑑 𝑙𝑜𝑎𝑑 ≤ 𝑓𝑎𝑐𝑡𝑜𝑟𝑒𝑑 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ
In this expression, the factored load is actually the sum of all service loads to be resisted
by the member, each multiplied by its own load factor. For example, dead loads will have
load factors that are different from those for live loads. The factored strength is the
theoretical strength multiplied by a resistance factor.
𝛴(𝑙𝑜𝑎𝑑𝑠 ×𝑙𝑜𝑎𝑑 𝑓𝑎𝑐𝑡𝑜𝑟𝑠 ) ≤ resistance × resistance factor

6.  Classification of Structural Steel:
• Compact: Section reaches its full strength (yield) before local buckling occurs.
Strength of section is governed by material strength.
There is no possibility of local flange or web bucking to prevent attainment of the
full sections yield strength. In common terms, the beam will not have a local failure
(i.e. your web buckles) before the beam has global failure.
• Non-compact: Only a portion of the cross-section reaches its full strength (yield)
before local buckling occurs.
• Slender: Cross-section does not yield before local buckling occurs. Strength is
governed by buckling .
Important Note: It is almost always avoided.

7. If:
λ ≤ λp (the section is compact)
λp < λ ≤ λr (the section is non-compact)
λr < λ (the section is slender)
where:
λ = Width to thickness ratio (dependent on section type and element)
λp = Limiting ratio to see if a section is compact/ upper limit for compact
category
λr = Limiting ratio to see if a section is non-compact/ upper limit for non-
compact category
 Classification of Structural Steel:

8.  Classification of Structural Steel:
• Hot rolled steel
• Cold rolled steel
• Built-up section
• Hot Rolled Steel:
Hot rolling is a mill process which involves rolling the steel at a high temperature
(typically at a temperature over 1700° F), which is above the steel’s recrystallization
temperature. When steel is above the recrystallization temperature, it can be shaped
and formed easily, and the steel can be made in much larger sizes.
The final shape of the material after cooling is unpredictable, making it less suitable
for an aesthetically focused design.

9. Hot rolled steel is typically cheaper than cold rolled steel due to the fact that it is
often manufactured without any delays in the process, and therefore the reheating
of the steel is not required (as it is with cold rolled). When the steel cools off it will
shrink slightly thus giving less control on the size and shape of the finished
product when compared to cold rolled.
• Hot Rolled Steel:
 Uses of Hot Rolled Steel:
 Hot rolled products like hot rolled steel bars are used in the welding and
construction trades to make railroad tracks and I- beams.
 Hot rolled steel is used in situations where precise shapes tolerance are not
required.

10. • It requires less processing making the material cheaper to purchase.
• The steel comes with a textured surface, slightly rounded edges and unique
shapes.
 Benefits of Hot Rolled Steel:

11. Fig: Hot rolled structure sections

12. • Cold Rolled Steel:
Cold rolled steel is essentially hot rolled steel that has had further processing. Cold
rolled steel is manufactured at temperatures below its recrystallization temperature,
typically at around room temperature. Because the steel is manufactured at a much
lower temperature, there’s no need to worry about the steel shrinking or changing
form or appearance. The steel is processed further in cold reduction mills, where the
material is cooled (at room temperature) followed by annealing and/or tempers
rolling.
This process will produce steel with closer dimensional tolerances and a wider range
of surface finishes. The term Cold Rolled is mistakenly used on all products, when
actually the product name refers to the rolling of flat rolled sheet and coil products.
Cold rolled steel is often used for more technically precise applications, or where
aesthetics are important.

13. Cold rolled processes include:
Drawing – increases the yield and tensile strengths, often eliminating further costly
thermal treatments
Turning – gets rid of surface imperfections
Grinding – narrows the original size tolerance range
Polishing – improves surface finish.
These processes will produce steel with closer dimensional tolerances making it a
stronger, more durable material than hot rolled.
Cold rolled steel delivers with an oily finish, a very smooth surface and very sharp
edges.
• Cold Rolled Steel:

14.  Uses of Cold Rolled Steel:
Any project where tolerances, surface condition, concentricity and straightness are
the major factors.

15. Fig: Angle Section Fig: Channel Section
Fig: Stiffened Zee Section Fig: Hat Section
Fig: Zee Section
Fig: Stiffened Hat Section

16. • Built-up Members:
Built -up members are obtained by connecting two or more plates or shapes which
then act as a single member. Such members may be made necessary by requirement
of the area, which can’t be provided by a single rolled shape, or by the requirement
of rigidity because for the same area, much greater moment of inertia can be
obtained with built-up sections compared to single rolled shapes, or by the
requirement of suitable connection, where the width or depth of member necessary
for proper connection can’t be obtained in a standard rolled section.
 When Built up members are required? (must include the answer of this
question in your lab report)
 Difference between stiffened & unstiffened compression elements? (must
include the answer of this question in your lab report)