To Whom it May Concern,
My company is a customer of your software company and I would like to get some technical support regarding one of your products that we have purchased: LEAP Concrete. Specifically, my request for information concerns the design of precast concrete girders composite with a CIP concrete deck slab. The output for Strength (please see attached markup of LEAP output) provides some specific information regarding the cross-section properties at ultimate strength, such as: the total area of prestressing steel, its stress at the ultimate strength condition, its "d" distance from the extreme compression fiber, the distance "c" from the neutral axis and the extreme compression fiber, the value of "a" (0.85c). But no information is provided regarding the compressive stress blocks above the neutral axis used for the strain compatibility analysis. If it is assumed that all concrete above "a" is at 0.85f'c, with stress blocks in the girder web/flange areas using the higher strength of the precast member and the concrete deck using the f'c of the concrete, then the total of all compression forces above the N.A. does not equal the total tensile force below the N.A., which is implied by the LEAP output to be total area of prestressing steel times the value of f_ps given in the table (i.e. all prestressing steel is considered to be acting at the center of gravity of the strands at one value of stress). In other words, when I review the output from a LEAP design, I am unable to confirm that equilibrium is being satisfied. Is it possible that LEAP is using non-linear stress-strain relationships for the concrete above the N.A., similar to discussions in the papers of Seguirant, Brice, and Khaleghi, which are referenced in both AASHTO LRFD and the PCI Design Manual? Or is all the concrete above the N.A. at a rectangular stress distribution at 0.85f'c?
Also, can you please provide me the basis for the calculation of the prestressing steel stress at the ultimate condition. Do you use the simplified equation from Chapter 2 of the PCI Design Manual, or possibly the equation for low-relax strand given in Devalapura and Tadros, 1992, which is also referenced in the PCI Design Manual.
Before I can submit design calculations to a bridge owner, or to a reviewing agency, I must be able to confirm every single number in the LEAP output file. And one of the most important numbers in a precast concrete bridge design is the ultimate flexural capacity of the composite girder. I am unable to confirm the ultimate flexural capacity based on the limited information in the LEAP output file and would greatly appreciate it if you could provide me with some very basic technical assistance regarding your software. I want to explicitly confirm cross-section equilibrium, know exactly the point of actions of all tensile and compressive forces, and confirm, as I must, the flexural capacity by summing the moments of all forces about any horizontal axis.
Thank you for your assistance regarding this matter.
Edward Schauffert, P.E., PhD
C&S Engineers, Inc.
eschauffert@cscos.com
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