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RE: Two-way flat plate moment frame question
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- Subject: RE: Two-way flat plate moment frame question
- From: "Craig Leech" <cleech(--nospam--at)nyc.rr.com>
- Date: Wed, 22 Oct 2003 15:24:03 -0400
I am sure that if you ask any two engineers they will disagree as to an appropriate "effective width" of slab beams to be used for lateral analysis. Your best bet is to go out and look in academic databases. There you will find a variety of options to choose from. We use a set of recommendations similar to a paper by Jacob Grossman ACI v94, no 2 (march-april) 1997. There is no "correct" method. Most do not have allowances for reinforcing etc and are just geometry based. The two things to worry about are strength and serviceability. For serviceability you just need a method that is proven to be close. A variation of Grossman's formula have been used to design several hundred highrise buildings in NYC (+30 stories). Whatever your choice make sure it is justified by experiment. Computer modelling, especially elastic finite element, does not well describe the stiffness of the system. For strength, things are a little easier as your guess is, to some extent, self fulfilling. You end up putting in reinforcing based on your choice of width and your "real" effective width depends on the amount of reinforcing (stiffness) you provide. Punching shear is the worry. Be careful and be conservative. The worst thing you can do is ignore that there is a lateral slab beam. Presuming you have reinforcing, the beam is there, it will attract moment and it will increase punching shear stresses. If you don't use it for your lateral system, or the code won't let you, that is your choice. But you should assume something for a punching shear check. Craig -----Original Message----- From: Clifford Schwinger [mailto:clifford234(--nospam--at)yahoo.com] Sent: Tuesday, October 21, 2003 10:58 PM To: seaint(--nospam--at)seaint.org Subject: Two-way flat plate moment frame question I have a question about "slab-beams" in non post-tensioned two-way flat plate moment frames. Section 13.5.1.2 in ACI 318-02 has this somewhat vague statement: "For lateral loads, analysis of frames shall take into account effects of cracking and reinforcement on stiffness of frame members." The commentary doesn't really clarify anything except to say "For nonprestressed slabs, it is normally appropriate to reduce slab bending stiffness to between one-half and one-quarter of the uncracked stiffness." One-half to one-quarter the stiffness of what? The full tributary width of the slab framing into the columns on the moment frame? The commentary statement seems to imply that when you have building with square column bays (let's say 30' x 30') then the moment of inertia of the equivalent "beam" in my moment frame is equal to 0.25 x "I"gross where "I"gross is the gross moment of inertia of entire tributary width of the floor slab framing into the column (i.e., a 30' wide "slab-beam"). This seems to agree with section 10.11.1 where simplified approximate moments of inertia for various moment frame components are listed. For flat plates and flat slabs "I"effective = 0.25 x "I"gross. What isn't stated is a clear definition as to the width of the moment frame "slab-beam" member for which "I"gross is computed. Is the width of the "slab-beam" equal to the full tributary width of the slab that frames into the columns in the direction for which the moment frame is being analyzed - or is it something less? If the full tributary width of slab is considered as the "I"gross beam width then how is the column stiffness (or beam stiffness?) modified to account for the torsional flexibility of the slab-to-column connection? Is the torsional flexibility accounted for in the "0.25" factor that's applied to "I"gross? I'm figuring that maybe the "0.25" factor is comprised of the product of two numbers - 0.5 x.0.5 = 0.25. The first "0.5" being an adjustment that modifies the effective width of the "slab-beam" to one-half the actual slab width (this modification reduces the beam width to account for the torsional flexibility of the slab-to-column connection). The second "0.5" factor may be an adjustment for converting "I"gross to "I"effective. I don't want to overestimate the stiffness of the moment frame beams, but then again I don't want to underestimate the stiffnesses of the slab-beams either. If you underestimate stiffness of flat-plate moment frame slab-beams, the column k-factors will quickly go through the roof (due to the wimpy beams)! Does any of this make sense? TIA, Cliff Schwinger __________________________________ Do you Yahoo!? The New Yahoo! 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