AASHTOWare BrDr 6.8

Multicell Box Tutorial MCB1-Post-Tensioned Multicell Box Example

MCB1 - Post-Tensioned Multicell Box Example

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Topics Covered

Post-tensioned concrete Multicell Box Data Entry

Integral with substructure

LRFR analysis and results

This example describes entering a post-tensioned multicell box superstructure into AASHTOWare BrDR. The

superstructure is modeled as integral with the pier.

Analysis Methods

Post-tensioned concrete multicell box (MCB) superstructures can be analyzed in the following manners:

LRFR

Full box section including each individual webline

Open bridge BID 27 “MultiCell Box Examples” in the sample database.

Create a new MCB (multicell box) superstructure definition.

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Enter the following data for the superstructure definition. Select ‘Integral with substructure’ and mark Support 2 as

being integral. Also be sure to select the ‘Post-tensioned’ checkbox. This will display the PT windows in the UI.

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Select the following LRFR options on the Control Options window.

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Open the Load Case Description window and use the ‘Add Default…’ button to create the following load cases.

Create a stress limit for the beam concrete.

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Create the following Lump Sum loss.

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Create the following structure cross section.

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Now that all of the dimensions are entered, click the ‘Compute Properties’ button.

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Make a copy of this cross section by using the right‐click menu

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Rename the new cross section and revise the depth to 8’.

The schematic can be used to view the cross sectoin.

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Create the following tendon profile.

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Open the Cross Section Range Properties window and assign the cross sections as follows.

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Effective supports allow you to shift the specification check point at the centerline of the support to the located

entered below. Shear will be checked at a distance dv from the location entered below.

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Select ‘Cross Section Range Properties’ in the BWS tree and open the following Schematic from the toolbar.

Open the Structure Typical Section window and locate the superstructure definition reference line as follows.

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Enter the following data for the structural overlay. The overlay is applied in the self load DC load case.

Enter the barriers.

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Use the Compute button to enter the following lane positions.

The Structure Typical Section will appear as follows. The webs are not visible in the schematic because the cross

section at the start of the structure was marked as ‘Solid’.

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Enter the following diaphragm locations on the Framing Plan Details window. The diaphragms only contribute to

the dead load on the structure. They do not provide a structural role in the box analysis. Enter the diaphragm

thickness and the AASHTO engine will compute diaphragm load based on the box cross section properties and

diaphragm thickness.

The Framing Plan schematic appears as follows:

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Open the Supports window to view the following. Support 2 is marked as Integral since we specified that on the

Superstructure Definition window. There is no data to change here.

We will leave the Live Load Distribution factors blank so they will be computed by the AASHTO engine at runtime.

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Enter the following reinforcement in the top and bottom slabs of the box.

Create the following stirrup definition.

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Open the Web 1 Shear Reinforcement Ranges window. Select the input reference type as ‘Centerline of Bearings’.

Click the Stirrup Wizard button and enter the following data.

Select ‘Span 2’ in the Wizard and enter similar data for Span 2.

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Click the ‘Apply All’ button to create the stirrup ranges for each span.

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Span 1 will show the following data.

Open the Web 2 Shear Reinforcement Ranges window. Select ‘Web 1” in the “Link With” field. The data from

Web 1 will appear in this window as read only. If data is changed in the Web 1 Shear Reinforcement Ranges

window in the future, those changes will be reflected in this window. Do the same for Web 3, linking it to Web 1.

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Now that the superstructure definition has been defined we can create the pier that is integral with the superstructure.

Open the Substructures tab on the Bridge Alternative window.

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Enter the following substructures locations.

Open the Substructures tab on the Superstructure window and select the following supports. This is necessary

because a Bridge can contain multiple Superstructures and Substructures. This tab identifies which substructure

units support which superstructures.

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Now open the Superstructure Alternative window and select the Superstructure Definition that we just created.

The following reminder will appear when you click OK to close the Superstructure Alternative window. Click OK

to close this reminder.

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Now open the Pier window and click OK.

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Now create a solid shaft pier alternative.

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No data needs to be changed on the resulting Pier Alternative window so click OK to close it.

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In this training, we are only concerned with modeling the pier geometry so its stiffness can be included in the

superstructure analysis. Therefore we will not enter the reinforcement details for the pier. Enter the geometry of the

pier.

Open the Cap window and verify the correct cap concrete material is selected.

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Enter the following for the pier cap geometry.

Open the Column Components window and select the following concrete material.

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Open the Column Geometry window and enter the following data.

The pier is now sufficiently defined to be considered in the superstructure analysis. The column will be considered

fixed at the base of the column. This percent fixity can be adjusted on the Pier Model Settings window if desired.

The FE model created during the superstructure analysis will include an element modeling the column length and

stiffness.

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We can now sit on our superstructure definition in the BWS tree and launch an LRFR Design Load Rating. The full

multicell box width is analyzed for flexure and shear and then each webline is analyzed for shear. The Analysis

Results window shows the critical rating factors considering the full box and each webline.

Spec check details are available for the full box and each webline.