Method to Model Post Tensioned Bridges and Defining Draped Strands in LARS Bridge

 Many bridges utilize a combination of prestressed strands and draped post tensioned tendons.  When the load rating models are created, the post tensioned tendons should be modeled as “equivalent” prestressed strands.  This document is intended to serve as a guide for modeling these types of bridges in LARS Bridge.

 The post tensioned tendons should be modeled as an area of steel based on the center of gravity (c.g.) since LARS doesn’t support input of strand information by force c.g.  The method of modeling the prestressed strands and post tensioned strands into LARS Bridge is shown in the example below.

 Example – Bridge 1230158:

 The example below uses information from the rating report for bridge 1230158 and illustrates entry of prestressed and post tensioned strands into LARS Bridge.  It is broken into two parts: 1) Strand Information and 2) Strand geometry.

 1)      Strand Information – the three data items shown below are very similar to those in existing calculations

2)      Strand Geometry – Since LARS Bridge currently does not have a means to enter draped strands directly, the strand geometry needs to be entered as chords that simulate the parabolic draping curve.  A spreadsheet has been developed (LARSDrapedStrands.xlsx) that takes as input the member length (L), height at centerline (HC) and height at the left end of the member (HL).  The assumption is that the curve is divided into 10 chords and is symmetric (so only the left 5 segments need to be entered).  The spreadsheet performs these calculations.  Given the input described above the three values (L, HC and HL are entered and a table of segment end heights (measured from the bottom of the member) and lengths are calculated.  This information is then entered directly into LARS Bridge as described in the procedure shown below.  A view of the spreadsheet for one member in bridge 1230158 is shown below.  The member from LARS Bridge and the segment locations are shown below.

 

Procedure for defining draped strands using equivalent chords

1)      Calculate the area of steel needed for normal prestressed and "equivalent" post tensioned strands.  See segment 1 in Table 1 above.

2)      Calculate the centers of gravity of beam strands at the left (HL) and center (HC) of the member.  See segments 2 and 3 in Table 1 above.

3)      Determine the draped strand geometry to be entered into LARS by following these steps (note that we are assuming 10 segments will represent the draped strands over the entire length of the member):

1. a.      Using the spreadsheet above, enter the calculated values for HL and HC, along with the member length L in the yellow shaded cells
2. b.      The segment dy Left and dy Right values will appear in the shaded table shown above along with the length of the individual segments

4)      Enter the information for each segment representing the draped strand center of gravity into LARS as follows:

1. a.      Select the Description à Member Section Description à Steel Command
2. b.      Change Type to Prestress-Area
3. c.       Enter the Area that was calculated in Table 1, number 1.
4. d.      Make sure the Symmetry Option is checked
5. e.      Change the Variation setting to Straight Line
6. f.       Make sure the Locate By option is set to Contiguous to Last L->R
7. g.      Enter dy Left with the from option set to B (bottom)
8. h.     Enter dy Right with the from option set to B (bottom)
9. i.      Enter the segment length L.
10. j.      Press the Add button.  The segment will appear coming from the left and right sides of the member
11. k.     Repeat steps a. to h. for each segment in the table (5 total taking symmetry into account)

Look for the the following files in the "Files" tab:

     1.    Excel spreadsheet to compute the ranges in the files:  LARSDrapedStrands.xlsx

     2.  Sample LARS Bridge model:  1230-158-G2only.bmd