Structural Steel- refers to a number of steel
that, because of their economy and desirable
mechanical properties, are suitable for loadcarrying members in structures.
The customary way to specify a structural steel
is to use an ASTM (American Society for Testing
and Materials) designation.
For ferrous metals, the designation has the
prefix letter “A” followed by two or three
numerical digits (e.g. ASTM A36, ASTM A514).
There are three groups of hot-rolled structural
steels for use in buildings.
Carbon Steels- use carbon as the chief
strengthening element with minimum yield
stresses ranging from 220 MPa to 290 MPa. An
increase in carbon content raises the yield
stress but reduces ductility, making welding
more difficult.
Ductility- Ability of the material to undergo
large inelastic deformations without fracture.
Toughness- Ability of the material to absorb
energy and is characterized by the area under a
stress-strain curve.
Weldability- Ability of steel to be welded
without changing its basic mechanical
properties.
Poisson’s Ratio- Ratio of the transverse strain to
longitudinal strain. Poisson’s ratio is essentially
the same for all structural steels and has a value
of 0.3 in the elastic range.
Shear Modulus- Ratio of the shearing stress to
shearing strain during the initial elastic
behavior.
High-strength low-alloy steels (HSLAS)- have
yield stresses of 480 MPa to 840 MPa. In
addition to carbon and manganese, these steels
contain one or more alloying elements such as
columbium, vanadium, chromium, silicon,
copper, and nickel.
Quenched and Tempered alloy steels- have
yield stresses of 480 MPa to 690 MPa. These
steels of higher strength are obtained by heattreating low-alloy steels.
The heat treatment consists of quenching (rapid
cooling) and tempering (reheating).
ASTM Designation
For hot-rolled structural shapes, plates, and
bars, such tests shall be made in accordance
with ASTM A36/A36M;
For sheets, such tests shall be made in
accordance with ASTM A568/A568M;
Yield Strength (Fy)- Unit tensile stress at which
the stress-strain curve exhibits a well-defined
increase in strain (deformation) without an
increase in stress.
Tensile Strength (ULTIMATE) (Fu)- Largest unit
stress that the material achieves in a tension
test.
Modulus of Elasticity (E)- Slope of the initial
straight-line portion of the stress-strain
diagram. It is usually taken as 200,000 MPa for
design calculation for all structural steel.
For wide flange, bearing pile, S-shapes,
channels, and tees: the letter indicates the
shape, the first number indicates the nominal
height, and the second number indicates the
weight per 1 foot of length
For instance, the W12x36 is a wide flange shape
that has a nominal height of 12 in and weighs
36 lb/ft.
For steel tubes, pipes, plates, and angles: the 3
numbers indicate the height, width, and
thickness of the steel.
Wide Flange (W12x36)- Flange surfaces are
parallel; flange thickness is not necessarily equal
to the web thickness.
Bearing Pile (HP14x73)- Flange surfaces are
parallel; flange and web have equal thicknesses.
American Standard Beam (S15x50)- The inner
flange surface is sloped.
Channel (C12x30)- Standard AISC flanges have
sloped inner flange surfaces.
Tee- WT12x38, ST12x38, MT12x38
Lecture 2
WT shapes are cut from a wide flange.
The centroid of an area is analogous to the
center of gravity of a homogeneous body. The
centroid is often described as the point at which
a thin homogeneous plate would balance.
ST shapes are cut from American Standard
Beams.
MT shapes are cut from non-standard I-shapes.
Hollow Steel Section or Steel TubeHSS12x6x0.5, TS12x6x0.5
Either nomenclature is acceptable; however,
HSS is more common.
Angle- L2x2x0.5, L6x3x0.5
Angles come in an equal leg or unequal leg
sizes. The diagram at right shows an unequal
leg.
Pipe- Pipe26STD
Plate (0.5x2x300- Very small plates can also be
called bars.
Combined Sections- These are sections made
up of combining structural shapes. Also,
referred to as Built-up sections.
There are three basic types of construction and
associated design assumptions permitted, and
each will govern in a specific manner the size of
members and the types and strength of their
connections:
Type 1, commonly designated as rigid-frame
(continuous frame), assumes that beam-column
connections have sufficient rigidity to hold
virtually unchanged the original angles between
intersecting members.
Type 2, commonly designated as simple
framing (unrestrained, free-ended), assumes
that in so far as gravity loading is concerned,
ends of beams and girders are connected for
shear only and are free to rotate under gravity
load.
Type 3, commonly designated as semi-rigid
framing (partially restrained), assumes that the
connections of beams and girders possess a
dependable and known moment capacity
intermediate in degree between rigidity of Type
1 and the flexibility of Type 2.
Moment of Inertia- It is also known as the
Second Moment of Area.
The moment of inertia of a beam’s cross section
measures the ability of the beam to resist
bending.
Parallel Axis Theorem- also known as the
transfer axis theorem
If the moment of inertia with respect to a
centroidal axis is known, then the moment of
inertia with respect to another axis parallel to
the first can be calculated using the parallel axis
theorem
Radius of Gyration- is an imaginary distance
from the centroidal axis at which the entire area
can be assumed to exist without affecting the
moment of inertia.
Polar Moment of Inertia- measures the ability
of the area to resist torsion or twisting.
The moment of inertia of such cases can be
determined by the use of the formula and more
conveniently by graphical solution using Mohr’s
circle especially if principal moments of inertia
are known.
On a set of rectangular coordinate axes choose
one axis on which to plot values of 𝐼𝑥 and 𝐼𝑦 and
the other on which to plot 𝐼𝑥𝑦 . These axes are
called principal axes.
Lecture 3
The fundamental requirement of a structural
design is that the required strength do not
exceed the available strength; that is
𝒓𝒆𝒒𝒖𝒊𝒓𝒆𝒅 𝒔𝒕𝒓𝒆𝒏𝒈𝒕𝒉 ≤ 𝒂𝒗𝒂𝒊𝒍𝒂𝒃𝒍𝒆 𝒔𝒕𝒓𝒆𝒏𝒈𝒕𝒉
•
Allowable Strength Design
•
Plastic Design
•
Load and Resistance Factor Design
This approach is called the allowable stress
design. The allowable stress will be in the elastic
range of the material. This approach is also
called elastic design or working stress design.
Working stresses are those resulting from the
working loads, which are the applied loads.
Working loads are also know as service loads.
LRFD is similar to plastic design in that strength,
or the failure condition, is considered. Load
factors are applied to the service loads, and a
member is selected that will have enough
strength to resist the factored loads.
The factored load is a failure load greater than
the total actual service load, so the load factors
are usually greater than unity.
The factored resistance 𝜙𝑅𝑛 is called the design
strength