Steel strucure lec # (3)

2/22/2013 1
Objectives Of Structural Designer
Design is a process by which an optimum
solution is obtained satisfying certain criteria.
Some typical criteria are:-
a. minimum cost
b. minimum weight
c. minimum construction time
d. minimum labour
e. maximum efficiency of operation to
owner, etc.
Prof. Dr. Zahid A. Siddiqi

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If a specific objective criterion can be
expressed mathematically in the form of an
objective function, then optimisation
techniques may be employed to achieve the
goal.
The criterion of minimum weight is almost
always satisfied in all steel structures.

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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 practicality, are briefly discussed in
the following slides.

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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 or cause unsightly cracks.
2. The designer must keep the
construction, operation, and
maintenance costs at the lowest level
without 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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Designers should learn everything possible about
the detailing, the fabrication, and 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, labour
conditions, equipment for erection, problems at
site, field tolerances and the required clearances
at 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.
The important steps in the design of separate
members are shown in the form of a flow
chart in Figure 1.1.

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Write final selection
Accept section if all checks are
satisfied, other-wise revise
Perform serviceability checks
Perform strength checks
Apply all stability checks
Select trial section
based on assumed stresses/
effectiveness of cross-section.
Alternatively, selection tables may be used
Collect and list all the known data

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The complete 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. The Loading conditions are considered
and the loads to be carried by the structure
are estimated.
5. Based on the decisions of 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 or cost.

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6. The Structural analysis involving modelling
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 not satisfied, the member
sizes are revised.
This stage is called evaluation of the trial
member sizes.

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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 decision is 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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LOAD FACTORS AND
LOAD COMBINATIONS
It is almost impossible that all loads like live load,
snow load, wind load and earthquake all occur
together with their maximum intensity.
A load combination combines different types of loads
depending on the probability of occurrence of these
loads acting simultaneously, considering their
expected intensity in the combination compared with
the maximum load intensity.

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The factors of safety are also included in the
LRFD load combinations and hence the
output of the expressions is a design load.
The alphabets used in the combinations
mean different types of nominal service loads
and the numerical values with them are the
load factors.
When intermediate floors have full live load,
any type of roof load may be considered
equal to half of its normal service load
intensity.

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Similarly, in case of maximum intensity wind storm,
live load may be half.
The last combination, given afterwards, is very
important for uplift of structure or reversal of forces.
The wind load on roof is upwards in majority of the
cases and if the downward gravity load is less, the
structure may be blown up or sagging bending may
change into hogging bending.
A list of most commonly used combinations are as
under:

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LRFD Load Combinations
When the loads S, R, H, F, E and T are taken
equal to zero and wind loads are taken from the
previous codes, the load combinations are
reduced to the following form:
1. 1.4 D
2. 1.2 D + 1.6 L + 0.5Lr
3. 1.2 D + 1.6Lr + (L or 0.8 W)
4. 1.2 D + 1.3 W + L + 0.5 Lr
5. 0.9 D + 1.3 W

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ASD Load Combinations
The simplified ASD load combinations are as
follows:
1. D
2. D + L
3. D + Lr
4. D + 0.75L + 0.75Lr
5. D + 0.8W
6. D + 0.6W + 0.75L + 0.75Lr
7. 0.6 D + 0.8W

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TYPES OF STRUCTURAL STEEL
Steels are divided into four categories
depending on the carbon percentages (C) as
follows:
1- Low carbon steel C < 0.15%
2- Mild carbon steel C = 0.15 − 0.29%
3- Medium carbon steel C = 0.30 − 0.59%
4- High carbon steel C = 0.60 −1.70%

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E-value for steel = 185 GPa to 230 GPa
(Average 200 GPa)
Unit weight = 7850 kg/m3
= 77 kN/m3
For comparison, the unit weight of concrete is
23.6 kN/m3

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Most of the structural steel falls into the mild carbon
steel or simply mild steel (MS) category.
Hot rolled structural shapes may be made to
conform to A36M, A529M, A572M, A588M, A709M,
A913M and A992M.
Sheets are manufactured according to the standards
ASTM A606, A1011MSS, HSLAS and HSLAS-F.
Bolts are made according to ASTM standards A307,
A325M, A449, A40M and F1852.

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Most commonly used structural steel is A36M
having the following properties:
Fy = 250 MPa
Fu = 400 MPa
E = 200 GPa
Weld electrodes are classified as E60, E70, E80,
E100 and E110. The letter E denotes electrode.
The two digits indicate the ultimate tensile strength
in ksi. The corresponding SI equivalents are E425,
E495, E550, E690, E690 and E760.

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HOT ROLLED STRUCTURAL SHAPES
These are the steel cross-sectional shapes
that are hot rolled in the mills. Some of these
shapes are shown in Figure 1.2, whereas, the
steel bars, plates and hollow sections are
reproduced in Figure 1.3.
HSS are hollow structural sections that are
prismatic square, rectangular or round
products of a pipe or tubing.

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Tee Section
Thicker than
flange
HP-Section
Slope » 0o
W Section
16.7% Slope
S-Section Angle-Section
16.7% Slope
Channel Section
Figure 1.2. Common Steel Structural Shapes.

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Pipe Section Structural
Tubing
Bars
Plates
Figure 1.3. Hollow Steel Sections, Bars and Plates.

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1. W Shapes
The letter ‘W’ stands for an I-shape with wide
flange. The cross-section is doubly symmetric
in the form of the letter “I” (Figure 1.4). The
width / depth ratio varies from about 0.3 to
1.0.
The US Customary designation W16 x 40
means that the nominal depth of the section is
16 in. and the weight per unit length of the
section is 40 lbs/ft.

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Flange
Web
Less or no slope
Figure 1.4. Typical W-section.
The equivalent SI designation W410 x 60 means that
the W-section has a nominal depth of 410 mm and a
weight of 60 kgf/m.
This kilogram-force weight per unit length may be
converted in kN/m by multiplying it with the factor
9.81/1000.

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Nominal height is the rounded off height to
be used for common use. Actual depth of the
section may be in decimals and somewhat
different from this depth.
2. S Shapes
16.7° Figure 1.5. Typical S-section.

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* Doubly symmetric I-shapes.
* Previously called standard I-beams or
American Standard Beams.
* The inner edge of the flange has a slope of
approximately 16.7°.
* An S510 x 112 section means that the section
is S-shape having nominal depth of 510 mm
and weight of 112 kgf/m.
* The width / depth ratio varies from about 0.25
to 0.85.

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3. M Shapes
* Miscellaneous I-shapes.
* Doubly symmetric I-shapes not classified as
W or S shapes.
* Relatively lightweight used for smaller spans
and lesser loads.
* An M310 x 17.6 means that it is M-shape
section having nominal depth of 310 mm and
weight of 17.6 kgf/m.

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4. C Shapes
The C-shapes have the following
distinguishing features (Figure 1.6):
16.7°
Figure 1.5. Typical C-section.

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* Channel shapes with standard
proportions.
* Inner flange slope is the same as that
for the S shapes (16.7°).
* Previously called Standard or
American Standard Channels.
* A C150 x 19.3 is a standard channel
shape with a nominal depth of 150mm
and a weight of 19.3 kgf/m.

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5. MC Shapes
These sections have the following properties:
* Channels not classified as C-shapes.
* Previously called Shipbuilding or
Miscellaneous Channels.
6. L Shapes or Angle Sections
The various types of angle sections are shown
in Figure 1.6 and their salient features are
given below:

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a
b Figure 1.6. Typical Angle-Sections.
* The single angle sections are in the form
of letter ‘L’.
* If a = b, these are called equal angle
sections.
* If a ¹ b, these are called unequal angle
sections.

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Sides of the angle are called ‘legs’ or ‘arms’.
L89 x 76 x 12.7 is an unequal leg angle with
longer leg dimension of 89mm and shorter
leg dimension of 76mm with a leg thickness
of 12.7mm.
Double angle sections are combination of
two angles with longer or shorter sides close
to each other.
Double angle sections are denoted by 2Ls.

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2L89 x 76 x 12.7 means two angles 2L89 x
76 x 12.7 placed side by side in one of the
ways shown in the figure.
7. T Shapes
Figure 1.7. Typical Tee-Section.

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* These are called structural tees.
* These are obtained by splitting W, S or
M shapes and are called WT, ST or MT
shapes, respectively.
* A WT205 x 30 is a structural tee with a
nominal depth of 205mm and a weight
of 30 kgf/m and is obtained by splitting
the W410 x 60 section.

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COLD - FORMED SHAPES
These sections are formed from thin high
strength steel alloy plates under normal
temperature.
Some of the common shapes of these
sections are drawn in Figure 1.8.

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Channels Zees I-Shaped Double
Channels
Angle
Hat Sections
Figure 1.8. Commonly Used Cold Formed Shapes.

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BUILT-UP SECTIONS
Sections made by combining two or more standard
hot rolled sections, joined together at intervals with the
help of direct welding, stay plates or lacing, are called
built-up sections.
Examples are four angles section, double angle
section and double channel section shown in Fig. 1.9.
However, double angle section is sometimes excluded
from built-up section category and is considered as a
regular hot rolled member because of difference of its
behaviour from other built-up sections.

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4-Angle Box
Section
Figure 1.9. Some Examples of Built-Up Sections.

2/22/2013 43
CLADDING
The exterior covering of the structural
components of a building that are made up of
steel sections is called cladding.
This covering may be made up of reinforced
concrete, wood, aluminium or any other
architectural and lightweight material.

Steel strucure lec # (3)

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