As with any materials, steel has both advantages and disadvantages. In the previous post, we discussed advantages and now for the disadvantages:
- Buckling and vibration limit the strength of slender sections
- Slender steel members are prone to distortions due to temperature variations
- Certain design details are vulnerable to fatigue failure
- Relatively high material and fabrication costs
- Without proper provisions, steel is susceptible to corrosion
- Completed steel structure may be sensitive to construction tolerances
Steel is an essential building material used in a variety of structures. Here are some advantages of steel as a building material:
- High strength-to-weight ratio
- It performs well in seismic conditions because steel has high ductility and energy absorption.
- Long spans can be constructed with slender steel members
- In both tension and compression, steel exhibits equal strength and modulus of elasticity
- Steel performs extremely well under shear loading
- Due to its versitility, it can used in the construction of complex and unique structures
- Unlike concrete, steel does not require labor intensive formwork or shoring
- Steel can simultaneously serve a structural and architectural purpose
Designing a steel beam requires choosing a cross-sectional shape that satisfies both strength and serviceability requirements. When considering strength, flexure is often more critical than shear which means that the beam is initially designed for flexure and later checked for shear. We solve real steel design examples of this type of problem in our steel design course.
The first step is to calculate the required moment strength based on load and resistance factor design (LRFD) or allowable strength design (ASD) method. Even though the weight of the beam is part of the dead load, it is unknown at this point. The engineer can either assume a value for the beam weight or ignore it, but the beam weight must be verified after the shape has been selected. Since the weight of the beam is typically a small part of the total load, it may be ignored at the beginning of the design process. In most instances the selected shape will be satisfactory once the beam weight is taken into account and the required moment is recomputed.
The second step is to select a shape that has the strength to resist the required moment. This can be done through 2 ways. The first option is to assume a shape, compute the available strength, and compare the available strength with the required strength. If the available strength is greater than or equal to the required moment strength, the shape is satisfactory, and if the available strength is less than the required moment strength, the shape is unsatisfactory and redesign is necessary. The second option is to choose a shape by utilizing the beam design charts in Part 3 of the AISC Manual
The third step is to check shear strength of the beam per Section G of the AISC Specification.
The last step is to check the deflection
Sources: Steel Design, Fourth Edition, William Segui
Due to its cost and superb mechanical properties, steel is an ideal building material for various types of structures. When designing buildings, carbon steels, high-strength low alloy steels, and quenched and tempered alloy steels are the three main groups of hot-rolled structural steels that are available.
Carbon steels utilize carbon as the primary strengthening ingredient. Based on the percentages of carbon, these steels that can be classified as low-carbon (less than 0.15%), mild-carbon (0.15-0.29%), medium-carbon (0.30-0.59%), or high-carbon (0.60-1.70%). ASTM A36 steel is an example of mild-carbon steel with a carbon content ranging from 0.25-0.29% depending on the thickness of the specimen. The minimum yield stress of carbon steels used in structures is somewhere between 36 and 55 ksi. Raising the carbon content also raises the yield stress but reduces ductility. A reduction in ductility is an undesirable side effect of raising the carbon content because it makes welding more difficult. Besides carbon, manganese, silicon, and copper are key ingredients of carbon steel, and their maximum percentages are 1.65%, 0.60%, and 0.60% respectively.
In addition to carbon and manganese, high strength low-alloy (HSLA) steels contain one or more alloying elements such as columbium, vanadium, chromium, silicon, copper, and nickel. These alloying elements serve to improve strength as well as other mechanical properties of the steel. The total of all alloying elements is typically less than or equal to 5%, hence the term “low alloy”. HSLA steels with yield strengths from 40 ksi to 70 ksi can be found under many ASTM designations. Heat treatment is not used in the manufacture of these steels. In comparison to carbon steels, HSLA steels have greater atmospheric resistance. ASTM A242, A411, A572, A588, and A992 steel are just some examples of high strength low allow steel.
Quenched and tempered alloy steels have relatively high yield stresses, ranging from 70 ksi to 100 ksi. This enhanced performance is the result of heat-treating low-alloy steels which consists of quenching (rapid cooling) and tempering (reheating). ASTM A514, A852, and A709 belong to this group of steels.
Sources Used: Civil Engineering Reference Manual for the PE Exam, 15th Edition, Michael Lindeburg
This example covers the analysis of a structural timber beam that is subjected to dead and snow loads. It is the first example in our timber design course covering beams. We are given the following parameters for the adequately braced deck beam:
Wood Type: No 1 Douglas fir-larch
Wood Size: 2 x 8, Dressed
Dead Load: 15 lbs/ft^2
Snow Load: 20 lbs/ft^2
Moisture Content: > 19%
Span Length: 15 ft
Deck Beam Spacing: 1.5 ft
The first step is to identify all the adjustment factors that will be needed to calculate the allowable stresses per the National Design Specification for Wood Construction with Commentary. The adjustment factors needed to analyze and design a timber or wood beam include the repetitive member factor, wet service factor, size factor, flat use factor, load duration factor, temperature factor, beam stability factor, and incising factor. The repetitive member factor, wet service factor, size factor, and flat use factor are found in the Supplement National Design Specification 2012. The load duration factor, temperature factor, beam stability factor, and incising factor can be found in NDS 2012.
The second step is to find the reference stress values for the particular wood species specified in the problem statement. The reference stress values for bending, tension, shear, and modulus of elasticity for sawn lumber can be found in the NDS Supplement 2012 Table 4A.
The third step is to calculate the section modulus and the cross sectional area for the given beam. The section modulus will be used to calculate the actual bending stress, and the cross sectional area will be used to calculate the maximum actual shear stress.
The fourth step is to calculate the allowable stresses per NDS 2012 Table 4.3.1.
The fifth step is to calculate the design loads and stresses. The design load per ASCE 7 ASD load combinations is equal to the sum of the dead load and snow load. The maximum bending moment and shear are found using the design load. The bending stress is equal to the maximum moment divided by the section modulus, and maximum shear stress is equal to 1.5 times the maximum shear force divided by the cross sectional area.
The last step is to compare the actual bending and shear stresses with the allowable bending and shear stresses for the beam. If the actual stresses are less than the allowable stresses, then the beam is adequate. If the actual stresses are greater than the allowable stresses, then the beam is not adequate.