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Section 2.1
Steel Materials
Last Revised: 11/04/2014
It is assumed that you have an understanding of basic steel stress-strain behavior and the nature and purpose of American Standards for Testing Materials (ASTM) standards from the pre-requisite courses.
There are a variety of structural steels available. As we saw during the overview in BGSCM 1.3, there are also a variety of available shapes. As it turns out, not all shapes are readily available in all types of steel. In BGSCM 1.3 you were introduced to SCM Table 2-4. You should turn to that table now.
SCM Table 2-4 relates the available ASTM steel designations to the available shapes. The black shaded boxes indicate the preferred specification/shape combination. The grey shaded boxes indicate that you can probably get the indicated specification/shape combination. If the box is white, then the shape is not available in that material.
The selection of ASTM material specification may be driven by special project requirements. For example, the preferred alternatives are probably the best choice for steel that is to be protected inside a building envelope. In a case where steel will be exposed to the elements or in some other more corrosive environment, you may chose to use another steel such ASTM A588. You might also chose to use a different type of steel where high strength will reduce the steel demand on the project.
It should also be noted that the list of available steel changes from time to time. For example, in the early to mid 1900s the only steel specification was ASTM A6 steel (which is no longer on the list!). Later other steels were developed including ASTM A36 and ASTM A572 which were dominate in the building structure arena for many years. Around the year 2000, ASTM A992 steel was introduced which economically meets the requirements of both ASTM A36 and ASTM A572. ASTM A992 is now the dominant steel specification for several shape categories. Knowing something of the history will allow you to evaluate buildings that were constructed before the current specifications were made available.
A Note about Symbols
Before getting into the actual material properties it is useful to note at the SCM has traditionally used the symbol "F" or "f" to denote stresses. Most mechanics courses use the symbol sigma, s, to denote stress.
Capital "F" is used for stress related material properties derived from a stress-strain curve. The typical values are the specified minimum yield stress (Fy) or the specified minimum tensile strength (Fu). Whenever you see the capital "F" you know that you are looking at a material constant or permitted value.
In the case of the ASTM requirements, the word "specified" is used to denote the required stress level. Note that the actual yield stress and tensile strength for a given piece of steel may exceed the minimum requirements. For example it was common for A36 steel (with Fy = 36 ksi) to have actual yield stresses of 40 ksi or more. For design purposes, however, minimum specified values are always used since the actual values for the particular piece of steel to be used for a particular member is rarely known at the time of design and it is impractical to change the design based on the actual yield strength for each individual member.
For denoting computed stresses, the lower case "f" is used. For example, fb is the computed bending stress (M/S), fa is computed axial stress (P/A), and fv is the computed shear stress (VQ/Ib).
Primary Steel Properties
For the purposes of the Specification for Structural Steel Buildings, four quantities are particularly important for a given steel type: the minimum yield stress (Fy), the specified minimum tensile strength (Fu), the Modulus of Elasticity (E) and the Shear Modulus (G). Some of these values are shown below on a typical stress-strain curve from a tension test in Figure 2.1.1. These quantities appear in the limit state equations found in the SCM. Other properties such as the Poisson's ratio, coefficient of thermal expansion and ductility are also important considerations in analysis and design.
Figure 2.1.1
Typical Stress-Strain Curve
Modulus of Elasticity, E
The Modulus of Elasticity (E) is a measure of a material's axial stiffness. Stiffness and strength are not the same thing. As it turns out, the value for the modulus of elasticity (E) is considered to be the same for all steel types. There is no difference in stiffness with type of steel. The value used by the SCM is 29,000 ksi. (See SCM pg 16.1-xxvii, the symbols section.)
Shear Modulus, G
The shear modulus, G, is a measure of the shear stiffness of the material. It is a constant for all steels and has the value of 11,200 ksi (see SCM pg 16.1-xxx).
The fact that all steels have the same E and G means that for members whose design is controlled by a stiffness limit states (i.e. deflection or vibration), the type of steel used is unimportant. In these cases, there is no advantage to using a higher strength steel over one with lower strength. You will particularly notice this in long span beams where deflection becomes the controlling limit state.
Minimum Specified Yield Stress, Fy
The minimum specified yield stress, Fy, is just that: the MINIMUM specified yield stress. It can be, and often is, higher than specified as previously stated. The ASTM specifications only require that the yield stress be at least the specified value. Even though it is likely to be higher than specified, unless you have tests verifying the actual yield stress for your steel, you can only use the minimum ASTM specified value in the calculations.
The values for Fy (and for Fu) are listed in SCM Table 2-4. You will note that some times there is a range of values listed. In this case, you should use the lower value of the range in your computations unless you have a good reason to do otherwise and can be assured that the steel used in the project meets the higher value.
Minimum Specified Tensile Strength, Fu
The minimum specified tensile strength, Fu, is often referred to as the ultimate strength of the material. It is the largest value of stress that the material will support. This value is commonly used to determine the maximum or nominal strength of a member.
As noted above, Fu can also be found in SCM Table 2-4.
Note that all steels have the same stiffness, E, but not the same strength, Fy and/or Fu. Using high strength steel is only an advantage if the limiting criteria are strength related instead of stiffness related.
General Steel Properties
In addition to the four basic properties of steel there a number of other properties that you will need to know about.
Poisson's Ratio, n
Poisson's ratio is often use in structural analysis but does not find its way into the strength and serviceability equations of the SCM. The value of n typically used for steel is 0.3.
Coefficient of Thermal Expansion, e
The coefficient of thermal expansion, e, is necessary in computing deformations and forces in structures as a result of changes in temperature. It does not show up in the strength and serviceability limit state equations found in the SCM. While most building frameworks are kept at nearly constant temperature inside a building envelope, there are still many cases where steel structures are subject to widely ranging temperature values and, as a result, see significant deformation which, in turn, can result in large forces within the members.
Table 17-11 (see SCM page 17-23) lists the coefficients of thermal expansion for a number of materials, including steel. For mild steel the value of e is .0000065 in/in/deg F.
Other Things to Know When Using Steel
Part 2 of the SCM covers a number of topics that you need to be knowledgeable about when you use steel as a structural material. We will point out a few here. You need to read the relevant sections of Part 2 to gain a basic understanding. Follow up with many of the references given in Part 2 will give you detailed information about each item.
Tolerances
Tolerance is the allowed departure from a specified dimension. How much deviation can be "tolerated". It is important to realize that as-built dimensions may vary a little from those dimensions placed on the design drawings. Exact dimensions are hard to come by in the milling, fabrication, and erection processes. This is recognized by the profession. Standards for allowable deviations are stated in a number of ASTM specifications and in the AISC specification. In most cases, tolerances are not a major issue in the building design process, but can be in special situations.
One special case is the interface of the supporting structure with architectural features such as curtain walls. Building facades are frequently very susceptible to tolerance issues. Take the time to ready the SCM section (pg 2-33) on this topic. Building deflections and thermal expansion may cause distress to supported features. Details that allow for differential movement must be devised when this is the case. In this introductory text such details will not be considered, however it is an excellent topic for a more advanced course in structural steel design.
Camber, Sweep and Straightening
The SCM section (pg 2-37) on this topic is also very good. It is very common to intentionally bend steel members to compensate for dead load deflections or to fit curved architectural features. Knowing what you can and cannot do with pre-bending or straightening steel members is important when laying out framing systems for buildings and other structures.
Fire Protection and Engineering
One of the major weaknesses of steel as a structural material is its susceptibility to fire induced loss of strength as well as a change in material properties after heating and cooling. The SCM refers you to the AISC Design Guide 19, Fire Resistance of Structural Steel Framing. Fire behavior is an extensive topic that needs to be considered when designing steel buildings. In this first steel design text however, we will not be spending any time on the topic as we will have our hands full learning the strength and serviceability requirements of the AISC specification. Just be aware that a steel structure essentially loses all its strength in a normal building fire situation. Fire safety techniques focus on slowing down the heating process so that occupants can safely exit the building during a fire event. A dramatic example of the effect of fire on steel was the collapse of the twin towers in New York after the terrorist attacks on that city. The collapse was largely the result of the core supporting structure losing its strength during the fire that followed the plane crashes.
Cold Weather Effects
The SCM is fairly quiet on the subject of cold weather effects on steel. There is a item on pg 2-41 that introduces the subject (item 7 on the list). In most building frames cold weather is not an issue since the frames are encased in a heated building envelope. However, an understanding of the effect of cold on steel is important anywhere the steel will be exposed to cold temperatures. This may occur in a refrigerated facility or in exterior structures in cold regions.
The general effect of cold on steel is to cause a reduction in ductility. A slight increase in the strength parameters occurs and stiffness is unaffected. These changes start to occur as the steel temperature drops below +40 degrees F and are very pronounced when temperatures approach +10 degrees F.
The ductility issue is extremely important. In buildings and other civil engineering structures, local stress concentrations are not normally considered since ductility will allow local yielding to redistribute the stresses. For situations where local stress concentrations are common, the AISC specification tends to use a low reduction factor, f, in LRFD and a larger factor of safety, W, for ASD. These are not sufficient when ductility is substantially reduced.
Failure of cold steel tends to be brittle, which means that it generally happens suddenly and without notice. Fractures tend to originate at locations of high stress concentration and propagate rapidly through the member. This behavior is very undesirable.
Approaches for dealing with this problem involve using steel with a formulation that is less susceptible to cold embrittlement or by severely reducing the computed capacity of the members. The AISC specification does not account for cold weather behavior. Large organizations that regularly have exposed steel structures in cold environments often develop their own criteria for determining member capacity when exposed to cold.
For example one company with a large inventory of exposed structures in the arctic has developed an allowable stress design specification that essentially reduces all allowable stress to 25% of what they would be in warm conditions. This is on the assumption that the stress concentration factors for structures designed to their specification (which includes detailing requirements) will rarely exceed 4.0. The result of the specification is that steel is treated as a non-ductile material. The resulting structures tend to be much heavier than would be found in more temperate climates.
Corrosion Protection
Another problem with steel as a structural material is it susceptibility to corrosion. The SCM (pg 2-38) discussion on this topic is a good introduction to the topic. You should read this section.
Fatigue and Fracture Control
As a crystalline material, steel is very susceptible to brittle fracture due to fatigue. There are other mechanisms that lead to brittle fracture as well. The SCM discussion on pg 2-40 is a must read section for engineers designing steel structures which see significant cyclic loading.
Wind and Seismic Design
Both wind and seismic events have very high strain rates which can lead to brittle behavior. In addition, current practices in seismic design typically do not design structures to resist forces elastically. There is an inherent dependence on material ductility in the design philosophies for wind and seismic conditions. Special detailing is required to ensure ductile behavior of steel structures in these events. There have been significant advancements in this area in recent years. These detailing requirements, again, are topics for a more advanced course, however, you should take the time to read the SCM discussion on pg 2-42.