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RE: Re: FW: seismic loads on post tensioned conc parking stru[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Re: FW: seismic loads on post tensioned conc parking stru
- From: hsprague(--nospam--at)aspen.klaalov.com (Harold Sprague)
- Date: Fri, 31 Jul 1998 08:49:10 -0600
I guess I am still not clear on the question. Are you looking for text references? Or just general tips? I'll give you both. Post tensioned parking garages have some quirks, but are not that much different from other concrete structures used for parking. Shear walls used for lateral resistance are very good, but the placement is a bit tricky. You have to allow for the creep and shrinkage, and you also have to allow for thermal cycling. Often times designers use the stair towers in the corners and use delay pours. The delay pours work for creep and shrinkage, but the corner locations provide a lot of restraint and crack the walls excessively due thermal cycling. I generally like to let the stair towers in the corners move independently, and pull the shear walls closer to the center while being mindful of the torsion. The shear walls are not all that stiff in the direction perpendicular to the walls, and can flex to accommodate for thermal cycling, shrinkage, creep, and deformation compatibility. The details at the stair wells then need to allow for differential movement, and deformation compatibility. Another major thing is to provide mild steel to account for stress reversals due to nonlinear movement. It is all in the code, but many people misinterpret this section. The code treats it as a cook book. One of the provisions is to put bottom mild rebar in at the slab to beam interface. Normal linear analysis indicates no tension, but in an earthquake there will be stress reversals. One other important consideration, especially if you use shear walls. Take care at the columns. Often times designers take the bottom bars from the girders straight into the columns and terminate them about 6" into the column (zero moment). Just due to creep, shrinkage, and thermal cycling; the slab and beams will get shorter. This will put tension into the bottom of the girder to column connection. Unless the bottom bars extend deep enough into the column for development (which they seldom do) there will be a nasty looking diagonal crack develop. It will start at the bottom of the girder to column interface and project upward through the column at about a 45 degree angle. Calculate the amount of movement anticipated. Add the displacement to the frame, and provide hooks or terminators, and confining steel to control the cracks. It might not be necessary for your load path, but the cracks create a lot of lower end pucker in owners and is cheaper than epoxy injection. This happens in any seismic zone about 4 months after construction. Another consideration is unbonded vs. bonded strand. I prefer bonded especially in high seismic applications. It behaves more like conventional reinforcement in elements that go nonlinear. You can also reduce your mild rebar requirements, but there are some areas that I like to leave it in due to nonlinear movements. Specific references: ACI 362 Parking Structures, by Chrest, Smith, and Bhuyan. Post Tension Design Manual by the PTI There are some pamphlets by the PTI Texts on post tensioning by Lin or by Nawy (sorry about my laziness and not looking up the specific titles) I hope this helps. Regards, Harold Sprague, PE Krawinkler, Luth & Assoc. P.S. If the guy has the horsepower between the ears and knows what he is doing, I don't care if he is licensed as a dentist. -----Original Message----- From: GRileyPE(--nospam--at)aol.com [SMTP:GRileyPE(--nospam--at)aol.com] Sent: Thursday, July 30, 1998 11:00 PM To: seaint(--nospam--at)seaint.org Subject: Re: Re: FW: seismic loads on post tensioned conc parking stru So, back to the original topic of discussion, is there anybody out there with information on the seismic design of post tensioned concrete parking structures? And for all of you snobs out there who are still convinced that Ross Ellena can't possibly be a "real" engineer because he is a licensed architect, try to "out-calc" him in post tensioned concrete in seismic zone 4! Bonne Chance Greg Riley, P.E.
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