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VisualAnalysis Tutorials: Deflection Checks

Introduction:

The purpose of this tutorial is to familiarize you with the deflection checking capabilities of the design software. The task will be to design the girders and columns of the structure shown below according to AISC-ASD provisions. The girders cannot have a relative deflection greater than L/360 for the Live load case, L/240 for the D+L case, and L/240 for the 0.75(D+L+W) case. The columns cannot have a drift greater than 0.75 in.

Hint: VisualAnalysis does not automatically check "story drift" requirements in the design checks performed on members. You may create story drift "reports" and manually make changes to your model to control the drifts.

Getting Started

Set up the plane frame model shown below. The story height is 10 feet. The bay width is 16 feet. Split the girders into four members each to model beams spanning perpendicular to the plane and framing into the members for each span. Now combine the four members into one beam for each bay resulting in a total of two beam type members. Combining the members will make checking deflections simpler as you will see below.

Modeling Discussion:

It should be noted that you could have achieved basically the same model using one single member for each of the spans and using concentrated member loads at offsets of 4, 8, and 12 ft along each member's length. We are just modeling the structure this way to show you that if for some reason you did need to split the members up, you could.

The columns should be W14x68 A992 Grade 50 shapes (preliminary size). Release the moment at the tops of the exterior columns and pin support all column bases. The girders are W 10x26 A992 Grade 50 shapes (preliminary size).

 

 

Loading

Dead loads: Apply -2 kip Y Direction concentrated loads to the exterior nodes and -4 kip Y Direction concentrated loads to the interior nodes of the girder.

Live loads: Apply -4 kip Y Direction concentrated loads to the exterior nodes and -8 kip Y Direction concentrated loads to the interior nodes of the girder.

Wind +X loads: Apply a 1.25 k/ft uniform load in the X-direction to Column1, the left most column.

Recall that both the girders and columns need to be designed according to AISC-ASD provisions. With this in mind, go to the "Load Case Manager" and select the "ASCE 7-05 ASD" and "Deflection Checks" from the Automatic Building Code Combinations area at the top right of the Load Combinations tab. Close the "Load Case Manager" when you're done.

 

 

 

 

Note: The Deflection Checks building code is just an example that we include with VisualAnalysis--you may wish to define your own deflection load combinations manually or with a custom building code!

Creating Design Groups

Switch to a design view. For this tutorial we will not use the automatic design groups created by the software so first select all members and choose Design | Remove Member(s) from Group.

Create three steel design groups:

Exterior Columns

Interior Columns

Girders

Setting Up Design Parameters

Exterior & Interior Columns:

Choose one of the exterior columns in the design view to make the Exterior Columns design group the active design group for editing in the Modify tab of the Project Manager. Using the Modify tab set up the following design parameters for the group.

Steel: Make sure the shape category is set to AISC Steel/W and the Specification to AISC ASD (2005). Set the Member Type to "Other."

Bracing: We will assume that these columns are unbraced in all three directions, that the columns are a part of a moment frame in the y direction (the X-Y plane), and part of a braced system in the z-direction (the Z-Y plane). With this in mind set the Sidesway y option to Yes and the Sidesway z option to No. Since we are working with a plane frame and therefore can't model the members framing into our frame in the Z-Y plane let's override the Ky value setting it to 1.0 (the max value for a braced frame). Don't override the Kz value, so it will be calculated automatically.

Constraints: None were specified, so leave these unchecked. As mentioned above, the Specification is AISC ASD.

Overrides: Leave the Overrides parameters unchanged.

Deflection Discussion: The only parameter left to set up is the allowable Beam Deflection. Beam deflection limits offer a series of options. Depending on how you set up the deflection limits, the deflections include not only overall member displacement but also deflection along the member's length. For this model, set all Strong Deflections to a limit of 0.75 in. for Total Deflection. For more information on Beam Deflection please refer to the Deflection Criteria topic located under Essentials\VisualDesign Concepts in the User's Guide (Help | Contents). For this tutorial, the columns will not have a deflection limitation.

Girders:

Choose one of the girders in the design view to make the Girder design group the active design group for editing in the Modify tab of the Project Manager. Using the Modify tab set up the following design parameters for the group.

Steel: Make sure the shape category is set to AISC Steel/W and set the Specification to AISC ASD (2005). Also verify that the member type is set to "Beam."

Bracing: We will assume that these members are braced on the top and bottom flange at there quarter points. With this in mind, you can specify Quarter Points for the top and bottom bracing patterns.

Constraints: None were specified, so leave these unchecked. As mentioned above, the Specification is AISC ASD.

Overrides: Leave the Overrides parameters unchanged.

Beam Deflections: Since the beams are modeled as combined members, you can specify the span ratio limits directly. Pick Member Span Ratio for the Strong Type of deflection and specify limits of L/360 for the "L only" case, L/240 for the D+L case, and L/240 for the "other' case. Set the limit for the "W or S' case to 1 to be sure it does not control. The "other' limit will be used for the 0.75(D+L+W) case.

Beam Deflection Discussion:

Modeling the girders as combined members made specifying the deflection limits easy. It would be fine to model them as individual members as well, but specifying the deflection limits would be a little more difficult.

Using the "individual members" method, the "member span ratio" deflection limit wouldn't be an adequate method. In this method, the actual deflection is calculated as the deflection of each member relative to its end points (dmember shown below).

This actual deflection is then compared to the fraction L/blank limit you set in the Project Manager, where L is the individual member's length not the overall span length (L=4' instead of 16' in our example). This isn't the type of deflection we are looking to check because it doesn't take into consideration the fact that the endpoints of the members have deflected from their original position.

The "member deflection" limit also would be inadequate. This method checks the same local deflection of the member. The only difference is that you enter a limiting value instead of a limiting span ratio.

The "total span ratio" limit could be used, but it would require some adjustments to correct for the shorter individual member spans compared to the overall span.

The total deflection method would be the best option. Using this option, you could calculate the value of L/360 for the overall beam (all four members) and enter this as the value for the total allowable deflection for the members of the group. Note that this method takes into account deflection of the endpoints of the members as well as the deflection of the members with respect to their endpoints. This means that axial deflection of the columns will be included in the deflection of the girders. The axial deflection should be negligible, so it shouldn't cause a significant error.

Designing

After you have set up the model and design parameters as described above, analyze the model and design the members. If you change a members shape, remember to synchronize the changes.