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Re: Tension Stress Graphs
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- Subject: Re: Tension Stress Graphs
- From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
- Date: Thu, 7 Dec 2000 13:05:02 -0500
Tension stress graphs ---> Confusing terminology 1. If *you* are being asked to do the prestress design using your system(s), then I could assume that these graphs are the theoretical *tension stresses* in the concrete due to loading, DL and LL and possibly other loads, that the prestressing has to resist, and does not include the prestressing force. Does it appear that the graph represents an envelope? 2. Tendon tension stress, for all intents and purposes (except for friction in P/T), is uniform. If the "Tension stress" graph is not uniform, or nearly so, then it would not be the tendon tension stress. 3. As Jason said, ask whomever sent you the graph what it means. That way any misinterpretation is eliminated. In P/S design, I prefer to specify the final prestress force and its c.g., and let the prestress fabricator select the strand pattern that will satisfy that profile and temporary stress conditions. If the prestresser cannot satisfy the c.g. and final prestress force with his/her system, then he/she can submit their design. A. Roger Turk, P.E.(Structural) Tucson, Arizona Jason Emoto wrote: -=-=--=-=-=--=-===-===-===-===-===-===-==== <<<frp 2000 wrote: I received "flexural" demands for some prestressed beams. What I received were : Tension Stress Graphs (length on x-axis, psi on y-axis) Are these equivalent to the moment curve divided by the Section Modulus ?? i.e. stress = M/S = Mc/I , where c = distance to furthest tension fiber>>> Any response you get from the list will be an educated guess. You should ask the person who sent you the submittal for clarification. That said, here's my guess. ACI code limits tensile and compressive concrete stresses in prestressed members at service loads (at transfer of prestress and after losses). The intent of the graphs may be to show compliance with these stress limits. The concrete stresses are computed by: f = P/A + M/S where P = axial compression due to prestress force M = bending moment due to profile of prestressing, self-weight and applied loads The prestressing steel stresses are also limited by code, but should not vary drastically along the length of most members. Some variation will occur due to friction, but it should be obvious by looking at the graphs whether they show tendon stresses. Jason Emoto -===-====-===-===-===-===
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