by Tom Kujawa
Designing a steel beam is not as complicated as you may think. There are essentially 6 Steps to design most steel beams:
In the WebStructural beam design app each of these steps has an associated icon to allow you to efficiently work through the design process:
There are many different grades of steel, but usually one common grade for each shape type. The most commonly used type of Steel I-Beam is the W (wide flange) Shape. W Shapes are usually made from Grade A992 steel. For our example, we will be designing a W Shape steel I Beam, so we select A992 from the Material List.
There are many types of steel shapes, even numerous types of I beams. For our example, select a W8X15. This beam is roughly 8" deep (or tall), that is the first number following the x. This beam weighs 15 lbs. That is the second number in the shape name. Typically, the lighter the beam, the less it will cost, so to design the most cost effective beam, you will want to choose one that weighs the least but meets you design criteria. After we enter all the design criteria, we can change the shape if it is not the right size for our design.
Span is the distance between points of support for a beam. A beam is often just a single span supported at both end. However, that's not always the case. Beams can be supported anywhere along their length or they can be cantilever beyond their end supports. To add or edit span length in WebStructural, simply click the Span button or the span dimension and add a span to the left or right, then adjust the length as appropriate. For our example, add a span to the right and make it 4'-0". Adjust the first span to equal 12'-0".
Span support conditions can also be changed in WebStructural by simply clicking on a support. Cliking the support will toggle through Pinned, Fixed, and Free support conditions. For our example, change the right most support to Free to create a cantilever.
Bracing is an incredibly important, yet often overlooked aspect of beam design. When a member is bent, tension and compression forces are introduced. For a simple span beam (one spanning between two pinned supports), the top of the beam will be in compression. It is these compression forces that can cause a beam to buckled out-of-plane. To understand buckling, think of compressing a short ruler between your hands. Now think of compressing 3' long ruler. Which one will flex and buckle? Clearly the longer, more slender one. It is this slenderness that is directly related to buckling. If we are able to brace a beam against this type of buckling, then we can often achieve greater bending strength. WebStructural allows you to specify bracings in many different common configurations. Span supports are automatically assumed to be bracing locations. Remember, continuous bracing assumes that the compression side of the beam is braced. If you have a continuous beam, look at your moment diagram in the report to verify that this is truly the case. You can always conservatively assume the beam in completely unbraced.
A beam can carry loads from many different sources. Below is a list of some of the more common types of Load Cases:
Dead loads (D) are those which are always present. Think of a concrete slab, or the weight of a wall. Those loads are always present and do not change.
Live Loads (L) are typically occupancy type loads. You are a type of Live Load in the structure you are in right now. American Society of Civil Engineers publishes a book (ASCE 7) with guidance for the amount of live load that should be used for different structures.
Roof Live Loads (Lr) are similar to Live Loads, but are specific to the roof and are typically related to construction or maintenance activities.
Snow Loads (S) are exactly want they sound like, loads cause by snow. Local building codes often dictate the appropriate ground or design snow loads to use. These are typically basic loads. Drift and unbalanced conditions should be accounted for as needed.
Other Loads are less common in beam design but can include Wind (W), Seimic or Earthquake (E), Rain (R), Lateral Earth (H), etc.
Beams can be loaded in many ways, but most loadings that cause flexure can be described as either:
Uniform Loads: These loads have units of force per unit length. With WebStructural, the default units for Uniform Loads is kips per foot (1 kip = 1000 lbs.). Uniform loads are often used to simplify repetitive and closely spaced point loads such as floor joists or roof rafters. To calculate the appropriate uniform load to apply to a beam, simply multiply the beams tributary area by the appropriate area load. Area loads and other structural loads are established by the American Society of Civil Engineers ASCE7 document and are given as pounds per square foot (psf).
Linear Loads: Linear Loads are very similar to uniform loads, but rather than having a constant magnitude, vary along their length. Linear Loads also have units of force per length. Linear loads can be used to represent triangular snow drifts or beams with joists framing in at a skewed angle, or many other triangular and trapezoidal type loading.
Point Loads: Point loads have units of force. The default in WebStructural is Kips (1 kip = 1000 lbs.). Point loads can be as simple as a reaction from another member such as a beam framing into another beam, or a column sitting on a beam.
Moments: Moments are loads which cause rotation in the axis of a beam and have units of force times length. The default in WebStructural is kip-feet. Moments are more complex to those less familiar with them, but consider a column welded to the top of a steel beam. If a force is applied to top of the column, it will cause the beam it is attached to bend as well, just like a lever. This bending type reaction is a moment. If the force is in the direction of the beam axis (or span direction), it can be input as a moment in WebStructural. If the force applied is perpendicular to the beam axis, then a torsional moment will be introduced. WebStructural does not currently allow for torsional loads to be input.
For our example lets use a dead load of 25 psf and a live load of 100 psf. Let's say the next point of support from our beam is a wall 10' away on one side, and another beam 20' away on the other. We will enter a uniform dead load D = .625 k/ft = [25 psf x (10'/2 + 20'/2)]/1000lb/k and also enter live load L = 1.5 k/ft [100psf x (20'/2 + 30'/2)]/1000lb/k. Also make sure to include self weight.
Depending on which design method you choose, (see below) the loads you enter will be factored appropriately. Simply enter the service (unfactored) loads.
Structural steel can either be designed by Load and Resistance Factor Design (LRFD) and Allowable Stress Design (ASD). Both methods yield similar results. Engineers have their opinions about the pros and cons of each method, but both are currently allowed in the United States. Simply choose the method you would like to use.
WebStructural will automatically factor your loads and apply them in the appropriate design equations. You can view these equation and exclude ones you don't want to include in analysis if you wish.
Deflection is an important measure of beam performance. Beams that have excessive deflection can be strong enough to carry their design loads, but perform poorly in service. Excessive deflections can lead to user complaint including bouncy floors, cracked building finishes, instability for mechanical equipment, etc. The International Building Code (IBC) dictates the minimum deflection for various members and load types. Deflections are typically described as a ratio or L (span) over some value to allow for comparison and standardization. Example: The deflection ratio for 0.5" delection in a 12' beam equals L/288 = 12ftx12in/ft (span) 0.5" (deflection). Now consider a beam that deflects 0.75" and is 18' long. It has an equivalent deflection ratio or L/288. In theory, these beams have the same deflection performance even though the longer span beam has a greater deflection. That is because the deflection is less noticeable over the greater distance. L/100 is often considered to be near the limit of deflection that is detectable to the human eye. L/360 is usually considered a minimum accepatlbe deflection due to live loads on floors, but that is just a minimum.
Once you have input all of your criteria, all you have to do it click "Design This Beam". WebStructural will perform a finite element analysis for your specific beam span, support and loading conditions. It will determine the design forces and calculate the design capacities of the beam using the American Institute of Steel Construction (AISC) standards. If you originally selected an appropriate beam size, you will see a lot of green and bending, shear, and deflection capacity ratios will be less than 1.0. These values are a percentage of capacity. So if your report reads "Bending 0.79" the beam configuration you selected is at 79% of flexural capacity (according to AISC).
If your report is red, then your capacity ratios are greater than 1.0 and the beam does not meet the design criteria. This means you need a bigger beam! Simply choose a different shape and click "Design This Beam".
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The WebStructural Team