Beam design is integral in construction. Architects and structural engineers must balance costs, building codes, and client requests in their designs. Beam design therefore warrants an in depth look. In this post, we’ll review beam materials and examples of beam design, as well as how to perform a beam analysis in three steps.
Most structural beams are made of wood, glulams, pre-stressed concrete, poured concrete, iron, or composite materials. Each of these construction materials reacts differently under the stress of a load, and each has its own unique advantages.
Wood beams are common in residential structures. Wood beams may be notched or jointed together for added strength. Wood beams are inexpensive and easy to alter to a builder’s specifications. However, they are also susceptible to rot and insect infestation. Specially treated wood beams are now available that resist decomposition, moisture and insects, making them an attractive choice in beam materials for most homeowners.
Flitch beams are specially constructed beams that join a steel plate with adjacent wood panels to form one composite structural beam. Flitch beams are strong, yet less expensive and lighter than solid steel beams. The addition of wood elements allows the beams to be nailed to existing wooden structures. The construction of a flitch beam results in a reduction of the overall size of the beam. They are used to support heavy vertical loads while maintaining a strict construction budget. Flitch beams are also very useful when adding additional load carrying capacity to an existing beam.
One very common type of steel beam is the I-beam. The I beam is shaped like a capital I, though the design is also sometimes referred to as a W-shape. The I beam design is the most efficient use of structural steel since it moves the bulk of the steel into the portions of the beam actually resisting the loads. I beams are also strong and moderately affordable. Steel beams may be treated to prohibit corrosion and oxidation, especially when used near or under water. As a result, steel I beams are a very popular choice in construction, but they can be used in residential design.
Concrete beams are most often seen in commercial construction, such as in the erection of multi-level parking decks, hospitals, and large hotels. Concrete beams are also commonly used as bridge and highway supports. Some concrete beams are used in conjunction with steel beams to provide added strength. Newer concrete beams may also contain a hybrid material of traditional concrete mixed with Glass Fiber Reinforced Polymer (GFRP) or Carbon FRP.
Concrete is a strong building material, but it is susceptible to water damage and cracking. Iron bars are often included in the beams to add strength and stability over areas prone to greater stress. Concrete beams are also desirable for their ability to absorb sound and vibration.
Cantilever beam designs create a suspended effect. These beams allow the creation of a bay window, balconies, and some bridges. In cantilever beam designs, the weight load is distributed back into the main beams of the structure, allowing a portion of the structure to extend beyond the supported perimeters of the structure’s foundation.
Hip beam designs are popular in roofing designs. A hip beam provides support for other load bearing beams branching off at symmetrical angles. This design is often used in residential construction.
The process used for determining the adequacy of a wood, steel, or even a concrete beam is essentially the same. Once a beam has been selected the method is as follows:
The first step in the structural analysis of a beam is determining the amount of load, or weight the beam is going to support. There are two major categories of loads:
Live Loads – A live load is a type of load that is temporarily placed on a structure (i.e. loads from snow, wind, vehicles, etc.). The magnitude of live loads will be defined or referenced in a local building code.
Dead Loads – Dead loads are permanently attached to a structure (i.e. loads from building materials, furniture, etc.). Sometimes, the weights of materials are exactly known and can be added together to determine the total dead load. More often, the dead load is assumed and given an approximate weight.
There are two types of stresses that are typically calculated when performing a beam design: bending stress and shear stress. A more complete definition of both bending stress and shear stress can be found here.
In order to calculate the bending and shear stresses, you must first calculate the maximum bending moment and maximum shear that occurs in the beam. The maximum moment and shear will most likely occur at different locations. A top-quality structural beam design software can calculate possibilities in the given beam design by comparing bending and shear stress values to known structural engineering values to ensure structural integrity.
The other two pieces of information needed to determine the stresses will be the section modulus and cross sectional area of the beam being used. The section modulus and cross sectional area can be calculated, or in most cases, can be looked up in tables (like in the National Design Specification (NDS) for wood beams, or the AISC Steel Manual for steel beams). Once all the information has been tabulated, determine the nominal maximum bending stress and nominal maximum shear stress.
In most cases the allowable stresses are tabulated in a design manual of some sorts (like in the NDS for wood, or the AISC Steel Manual for steel). Once the allowable stresses have been located determining the adequacy of a beam is simply a matter of comparing the actual stresses to the allowable stresses.
One major consideration not discussed in this article is that of deflection, or sag in the beam. A beam might be strong enough structurally, yet still deflect so much that it effects the actual performance of the beam. Deflection is a very important calculation and will be addressed in a separate article.
Beam design can be a complicated process. An experienced builder may know which type of beam is used to achieve a desired visual style of a structure, but is that beam able to support the load of the structure adequately? Does it leave open the possibility of expansion of the structure later on? Is there a cheaper beam that would be adequate for the design of the structure?
Structural engineering software such as Vitruvius can help take the guesswork out of the design process. Structural beam design software considers the stiffness, strength, and size of the desired beam. It then calculates the potential weight-bearing load of the designed beam. Calculations based on the desired qualities reveal all viable beam design possibilities. Calculations can also be made that show the cost effectiveness of each beam design option.
The structural beam design software also provides a list of possible beam materials to help provide a stable structure without exceeding the given construction budget. Do you need a solid beam, or is a hollow beam a viable option? Do you need an I-beam design or a rectangular beam? Structural beam design software can help you sort out all the options.
Structural beam design software is a wise investment for any structural engineer, building contractor, or individual building a new home construction. It eliminates potential mistakes in the design of the structure, as well as ensures that structural integrity is maintained during last minute changes in building plans.
Not all structural beam design software is top-quality. Be sure to research the features offered by a structural beam design software program including the support available with the program before selecting a structural beam design software package. If you’d like to give Vitruvius a try, we offer a free 30-day trial.