Understanding Structural Steel Design
If you’re planning a new structure, or if you’re renovating an existing one, you need to understand the basic structural steel fabricator design techniques. For example, you need to know the difference between Allowable Stress Design and Allowable Strength Design. You also need to be aware of LRFD (Load and Resistance Factor Design) and splicing between steel columns.
Allowable Stress Design vs Load and Resistance Factor Design
Load and Resistance Factor Design (LRFD) is a structural design methodology. The LRFD is a tool used in structural design to determine the correct strength and stiffness of a steel structure. LRFD provides an easier and more accurate solution based on statistical data.
Another method, called Allowable Stress Design, is also used to design a structure. It compares actual stresses with a theoretical stress that is considered acceptable.
This is a very useful technique to measure the strength of a structure. However, it is not mandatory to use this strategy. Depending on the materials and conditions of a particular structure, engineers can choose a strategy that will keep material stresses in the elastic range.
Allowable Stress Design and Load and Resistance Factor Design are essentially the same, with some minor differences. Unlike ASD, LRFD is not mandatory to use. But because it is more accurate, it is becoming more widely accepted.
In addition, LRFD is not limited to steel structures. Moreover, this design methodology is also a good way to incorporate new information regarding loads. For instance, it is possible to use LRFD to design a wooden structure.
Allowable Stress Design vs Allowable Strength Design
Allowable Stress Design and Allowable Strength Design are two names given to the same ol’ concept. They are similar in that they are both used to evaluate the strength of a structural member. However, they differ in several ways.
The allowable stress design is the simplest approach to calculating the maximum stress a structural member can safely handle. It is calculated by adding dead and live loads. This procedure works as well for timber designs as it does for steel ones.
Load and Resistance Factor Design (LRFD) is an alternative design strategy. Unlike ASD, it makes use of load factors. In addition, it consists of proportioning the elements of a structure to obtain a more accurate factor of safety. LRFD is not for the faint of heart, and can be a challenge for a structural engineer.
Another is the AISC Allowable Strength Design. While not a replacement of the old allowable stress design, it uses a quasi-safety factor approach to make it more applicable to modern design techniques.
Splices between steel columns
Splices are used to connect steel columns. They are commonly used for a number of reasons. They can be bolted or welded. The choice of splices depends on the construction sequence, type of load, type of material, and stiffness. Typical uses include constructing tall buildings, as well as reducing construction costs.
When designing splices, it is important to consider the effect of the splice on the column. For instance, the splice may be a weak point that creates a problem. In addition, it is necessary to consider the effect of the splices on the column’s load carrying capacity.
Axial compression and bending forces are transferred through the column through the contact of the splice plates. For large loads, a division plate or cover plate is needed to restore the continuity of flexure about the principal axes. Typically, the thickness of a division plate is governed by the bending strength.
If the moment splice is being designed, the moment capacity should be provided by the end plate. This can be sized for a flexure similar to a baseplate with moment.
LRFD (Load and Resistance Factor Design)
Load and Resistance Factor Design is a design method used to ensure structural reliability. It uses a probabilistic approach to determining the factors that affect the reliability of a structure. Compared to the Allowable Stress Design (ASD), LRFD produces a more consistent factor of safety.
The LRFD method is designed to keep the stress within the elastic range of the material. It also combines the effects of loading and resistance into one factor. This means that the resistance of a member can be safely compared to its ultimate strength.
LRFD takes into account the uncertainties associated with materials and construction methods. It enables the engineer to select a probability of failure for the loads that the structure is designed to handle.
ASD is similar to LRFD, except that it uses two factors instead of just one. In ASD, the strength of each load is combined into a single factor. For this reason, ASD is more conservative than LRFD, and it can lead to less reliable designs.