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How to Design a Steel Beamby Tom Kujawa

Introduction

Designing a steel beam is not as complicated as you may think. There are essentially 6 Steps to designmost steel beams:

1. Material - Choose the appropriate grade of steel for the beam you will be designing.2. Shape - Select the shape of steel beam you would like to design.3. Span - Enter the distance you are trying to span.4. Bracing - Not to be overlooked! Bracing is critical in determining the capacity of a beam.5. Load - Enter loads base on their type and load case.6. Design - In the United States, there are two common methods of beam design (ASD and LRFD).

Select the method you would like to use and specify deflection limits.

In the WebStructural Beam Design Module we've given each of these steps an icon to allow you toefficiently work through the design process:

1. Material

There are many different grades of steel, but usually one common grade for each shape type. Themost commonly used type of Steel I-Beam is the W (wide flange) Shape. W Shapes are usually madefrom Grade A992 steel. For our example, we will be designing a W Shape steel I Beam, so we selectA992 from the Material List.

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2. Shape

There are many types of steel shapes, even numerous types of I beams. For our example, select aW8X15. This beam is roughly 8" deep (or tall), that is the first number following the x. This beamweighs 15 lbs. That is the second number in the shape name. Typically, the lighter the beam, the less itwill cost, so to design the most cost effective beam, you will want to choose one that weighs the leastbut meets you design criteria. After we enter all the design criteria, we can change the shape if it is notthe right size for our design.

3. Span

Span is the distance between points of support for a beam. A beam is often just a single span andsupported at both end, but a single beam can also be supported at more than just both ends. It canbe supported along it's length or it can cantilever beyond it's end support. To add or edit span length inWebStructural, 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 supports can also be easily changed in WebStructural. Simply by clicking on a support. You cantoggle through Pinned, Fixed, and Free Supports. For our example, change the right most support toFree to create a cantilever.

4. Bracing

Bracing is an incredibly important, yet often overlooked aspect of beam design. When a member isbent, tension and compression forces are introduced. For a simple span beam (one spanning betweentwo pinned supports), the top of the beam will be in compression. It is these compression forces thatcan cause a beam to buckled out-of-plane. To understand buckling, think of compressing a short rulerbetween your hands. Now think of compressing 3' long ruler. Which one will flex and buckle? Clearlythe longer, more slender one. It is this slenderness that is directly related to buckling. If we are able tobrace 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. Spansupports are automatically assumed to be bracing locations. Remember, continuous bracing assumesthat the compression side of the beam is braced. If you have a continuous beam, look at your momentdiagram in the report to verify that this is truly the case. You can always conservatively assume the

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5. Loads Load Cases

A beam can carry loads from many different sources. Below is a list of some of the more commontypes 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 youare in right now. American Society of Civil Engineers publishes a book (ASCE 7) with guidance for theamount 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 toconstruction or maintenance activities.

Snow Loads (S) are exactly want they sound like, loads cause by snow. Local building codes oftendictate the appropriate ground or design snow loads to use. These are typically basic loads. Drift andunbalanced 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.

Load Types

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 unitsfor Uniform Loads is kips per foot (1 kip = 1000 lbs.). Uniform loads are often used to simplify repetitiveand closely spaced point loads such as floor joists or roof rafters. To calculate the appropriate uniformload to apply to a beam, simply multiply the beams tributary area by the appropriate area load. Arealoads and other structural loads are established by the American Society of Civil Engineers ASCE7document and are given as pounds per square foot (psf).

Linear Loads: Linear Loads are very similar to uniform loads, but rather than having a constantmagnitude, vary along their length. Linear Loads also have units of force per length. Linear loads can beused to represent triangular snow drifts or beams with joists framing in at a skewed angle, or manyother 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 anotherbeam, 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 timeslength. The default in WebStructural is kip-feet. Moments are more complex to those less familiar withthem, but consider a column welded to the top of a steel beam. If a force is applied to top of thecolumn, it will cause the beam it is attached to bend as well, just like a lever. This bending type reactionis a moment. If the force is in the direction of the beam axis (or span direction), it can be input as amoment in WebStructural. If the force applied is perpendicular to the beam axis, then a torsionalmoment 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 ofsupport from our beam is a wall 10' away on one side, and another beam 20' away on the other. Wewill enter a uniform dead load D = .625 k/ft = [25 psf x (10'/2 + 20'/2)]/1000lb/k and also enter liveload L = 1.5 k/ft [100psf x (20'/2 + 30'/2)]/1000lb/k. Also make sure to include self weight.

Load Factors

Depending on which design method you choose, (see below) the loads you enter will be factoredappropriately. Simply enter the service (unfactored) loads.

6. Design Design Method (ASD or LRFD)

Structural steel can either be designed by Load and Resistance Factor Design (LRFD) and AllowableStress Design (ASD). Both methods yield similar results. Engineers have their opinions about the prosand cons of each method, but both are currently allowed in the United States. Simply choose themethod you would like to use.

Design Equations

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

Deflection is an important measure of beam performance. Beams that have excessive deflection can be

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strong enough to carry their design loads, but perform poorly in service. Excessive deflections can leadto user complaint including bouncy floors, cracked building finishes, instability for mechanicalequipment, etc. The International Building Code (IBC) dictates the minimum deflection for variousmembers and load types. Deflections are typically described as a ratio or L (span) over some value toallow for comparison and standardization. Example: The deflection ratio for 0.5" delection in a 12' beamequals L/288 = 12ftx12in/ft (span) 0.5" (deflection). Now consider a beam that deflects 0.75" and is18' long. It has an equivalent deflection ratio or L/288. In theory, these beams have the samedeflection performance even though the longer span beam has a greater deflection. That is becausethe deflection is less noticeable over the greater distance. L/100 is often considered to be near the limitof deflection that is detectable to the human eye. L/360 is usually considered a minimum accepatlbedeflection due to live loads on floors, but that is just a minimum.

Reporting

Once you have input all of your criteria, all you have to do it click "Design This Beam". WebStructuralwill perform a finite element analysis for your specific beam span, support and loading conditions. It willdetermine the design forces and calculate the design capacities of the beam using the AmericanInstitute of Steel Construction (AISC) standards. If you originally selected an appropriate beam size, youwill see a lot of green and bending, shear, and deflection capacity ratios will be less than 1.0. Thesevalues are a percentage of capacity. So if your report reads "Bending 0.79" the beam configurationyou 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 thedesign criteria. This means you need a bigger beam! Simply choose a different shape and click "DesignThis Beam".

At WebStructural we strive to give you high quality design calculations with transparent reports thatyou can rely on. If you found this how-to useful please share it with your colleagues.

Finally, we encourage you to keep checking back for updates, and let us know if you have anysuggestions or requests.

Happy Engineering,

The WebStructural Team

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