Concrete Beam

The elements of beam design is a topic of great interest for structural engineers and contractors. Beam design is integral in the design and construction of a structure. Most structural beams are comprised of wood, steel or concrete. Each of these construction materials reacts differently under the stress of a load. Each also has its own unique advantages.

Elements and Examples of Beam Design: Concrete Beams

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 area also desirable for their ability to absorb sound and vibration.

Elements and Examples of Beam Design: Steel Beams

One very common type of steel beam is the I-beam. These I shaped beam are strong and moderately affordable. Steel beams are capable of supporting heavy loads without experiencing great amounts of deflection by distributing the load of the structure over the flange of the beam. Steel beams may be treated to prohibit corrosion and oxidation, especially when used near or under water, such as in bridge construction.

Elements and Examples of Beam Design: Wood Beams

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.

Elements and Examples of Beam Design: Flitch Beams

Flitch beams are specially constructed beams that join a steel plate with adjacent wood panels to form one composite structural beam. These flitch beams are strong, yet less expensive and lighter than solid steel beams.  The construction of a flitch beam results in a reduction of the overall size of the beam, and the wooden exterior also allows the builder to nail the beam to other existing wooden structures in the home.

Elements and examples of beam designs are plentiful. Beam design and selection are an important part of the construction process and the wide variety of beams to choose from allow a builder to meet the needs of each project more easily.

Structural Analysis of a Beam

Posted by Adam Wilson in Engineering Resources on November 20th, 2007

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:

• Determine the loads
• Calculate the stresses
• Check the allowable stresses against the actual stresses.

Determine the Loads
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 – are loads 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.

Calculating the Stresses
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 it will be first necessary to 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, and the process used to determine their value will be defined in a separate article.  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 the following equations can be used to determine the nominal maximum bending stress and nominal maximum shear stress:

Compare Actual Stresses against Allowable Stresses
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.  So, a beam is adequate if the following is true:

Other Considerations
One major consideration not discussed in this article is that of deflection, or sag in the beam.  A beam might be strong enough structurally, but might deflect so much that it effects the actual performance of the beam.  Deflection is a calculation that is very important and will be addressed in a separate article.

Another consideration when doing any kind of beam design is that of using structural design software.  There are several different engineering design software packages available for beams, columns, or foundation design.  StruCalc, Enercalc, Risa, and BeamChek are a few examples of those structural design software packages.

Josh Parker, E.I.T.
Cascade Design Group