-
8/2/2019 Section1-3S03-2008
1/12
1-1
1 Structural Steel Properties and Steel ProductsIntroduction
A typical steel frame is shown below. Steel structures can be rapidly erected by
fastening together prefabricated components using simple connection devices (see
connection figure below). It is the responsibility of the designer to determine the
orientation and size of the members so that adequate performance is achieved at
minimum cost.
-
8/2/2019 Section1-3S03-2008
2/12
1-2
Steel Metallurgy
Steel is the most widely used metal in Civil Engineering. Steel is a versatile
structural material used to construct buildings, bridges, towers, pipelines etc.
Composition is:1) > 95% IRON
2) carbon: C leads to strength and hardness but
ductility and toughness
3) manganese: affects properties in a manner similar to C, except
Mn toughness
4) others: silicon, phosphorus, sulphur, copper, nickel, chromium,
cadmium
-
8/2/2019 Section1-3S03-2008
3/12
1-3
Production of Steel and Steel Products
Steel products are produced in a four step process:(1)Iron ore is placed in a furnace with limestone and a form of carbon called
coke, and heated so that iron and carbon are mixed.
(2)Excess carbon, silicon, sulphur and phosphorus are removed.(3)Hot steel is cast into slabs. At this point oxygen bubbles may be removed
(a process called killing) by adding aluminum, manganese, and silicon.
If the steel is not killed, it CANNOT be used for structural applications.
(4)Steel slabs are then rolled to produce typical steel structural productslike wide-flange sections, angles, bars, plates, etc. This called hot
rolling.
-
8/2/2019 Section1-3S03-2008
4/12
1-4
The quality of steel depends in its final chemical content and its uniformity. An
important trade-off that generally holds:
Common Steel Sections
As a designer, you must be familiar with common steel products that areavailable.
A standard designation is used to identify steel sections in Canada:W 250 x 73
W = wide flange S = standard WWF = welded wide flange
L = angle C = channelWT = structural T,
cut from W shape
HP - H-pile
WRF - welded, reduced flange
MC - miscellaneous channels
SLB - super light beams
-
8/2/2019 Section1-3S03-2008
5/12
1-5
The properties and dimensions of each shape are tabulated in Chapter 6 of the
CISC Steel Handbook.
Example: W530 x 92 A =
Ix =
Zx =
d x (b x t) x w =
-
8/2/2019 Section1-3S03-2008
6/12
1-6
HSS (hollow structural sections)
produced as eitherClass C:
Class H:
HSS sections come in rectangular, square, or round cross-section.
NOTE:
Round HSS Pipe. Round HSS is usually 350W, pipe can have Fy = 172 MPa, 207
MPa, or 241 MPa.
Mechanical Properties of Steel
Strength and ductility are measured using a standard testing procedure
Typical tension test specimen:
-
8/2/2019 Section1-3S03-2008
7/12
1-7
Ductility is perhaps the most important mechanical property of steel. It is ameasure of the ability to undergo large deformations without breaking.
Can be measured by the ratios:
-
8/2/2019 Section1-3S03-2008
8/12
1-8
Yield strength is critical for design:
Toughness is the capacity of a material to absorb large amounts of energy. Itis important for structures subject to impact, earthquakes, or low
temperatures. Measured (roughly) using the Charpy V-notch test (CVN):
-
8/2/2019 Section1-3S03-2008
9/12
1-9
Fatigue is the damage induced in a structure due to repeated load cycles(e.g. trucks driving over a bridge deck) that are much LESS than the material
yield stress obtained from a tension test. The load cycles initiate cracks that
grow in length with load cycling. When the crack is long enough, brittle
fracture occurs. In steel structures, fatigue cracks will typically initiate at
welds. Fatigue resistance is quantified using the stress-life curve:
-
8/2/2019 Section1-3S03-2008
10/12
1-10
Canadian Steel Grades
A designer ensures that a structure has sufficient strength, stiffness, ductility,toughness, and fatigue resistance by specifying a grade of steel for a
particular application.
The grades of steel have been standardized by the Canadian StandardsAssociation (CSA) through standard CSA G40.21-M, Structural Quality
Steels. The CISC Handbook summarizes the grades readily available to the
Canadian designer (from Table 6.1):
- Weldable steels = semi-killed
- Tough steels = killed
- General steels = not killed and therefore should not be welded
- Atmospheric corrosion resistant steels = also called weathering steels.
Corrodes at the rate of other steel grades. Does not require painting.
American designers and suppliers use ASTM standards, which are NOTidentical to CSA standards.
The ultimate strength, Fu, is often required to calculate the resistance ofconnections. CISC HDBK Table 6-3 lists Fu for selected steel grades:
-
8/2/2019 Section1-3S03-2008
11/12
1-11
Typically the lower bound value of Fu is used for design Note that not all shapes are available in all grades of steel.
-
8/2/2019 Section1-3S03-2008
12/12
1-12
Quality Control of Steel Grades
The steel supplier at the mill must conduct tests on the steel being producedto ensure that it falls within the specified grade. The results are reported on
Mill Certificates. The strength values reported on the Mill Certificate may
be higher than the specified strengths, but should not be used for design. As
pointed out in the Canadian Highway Bridge Design Code:
Actual values of yield and ultimate tensile stresses reported on mill
certificates shall not be used for evaluation. Instead, the strength used shall be the
guaranteed minimum value as specified for the grade of steel shown.
