Need a book? Engineering books recommendations...
Return to index: [Subject] [Thread] [Date] [Author]
RE: Rigid plywood diaphragms
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Rigid plywood diaphragms
- From: "Sasha Itsekson" <itsekson(--nospam--at)jps.net>
- Date: Sun, 22 Nov 1998 02:16:23 -0800
Thank you Charles Greenlaw, Michael Cochran, Ernie Natividad and Dave Puskas for your detailed and insightful responses. To tell you the truth, I am not that much worried about the possible litigation. But I do understand the merit behind the rigid or rather semirigid behavior of wood structures with closely spaced shear walls. As somebody said the building does what it wants under the loading. Our job is to use our engineering judgement, available research and codes to safely design them. I will in fact use tributary area method as the base for my horizontal shear distribution (it does not take that much time and it seems to be still working). It should not be that difficult to put together the spreadsheet using UBC Standards formulas for shear wall deflection either. I am wondering though if it would give me the realistic shear distribution ( I don't think so) and would it be any closer to the actual performance of the system than the customary method above. Let's not forget that we are talking about the RELATIVE RIGIDITY of shear walls along different lines of resistance. For the sake of the argument let's look at the direction when framing is parallel to shear walls. So the overturning moment and boundary member forces are the function of the unit shear times the height of the wall. I can drop out several components of the total deflection from my relative rigidity calcs that would cancel out. These elements may include deflection due to oversized holes in holdowns (which in fact are 2 to 3 times more than the holdown deformations itself), plywood shear deformations, slip in sill bolts. I liked Charles's suggestion to use the surplus capacity rather than the tabulated capacity of the shear wall. If we only could design all of our walls stressed to the same percentage of the allowable... (:<). But that's not going to happen. Well, it's late, and I don't have any answers yet, only thoughts. Anybody else? Regards, Sasha Itsekson -----Original Message----- From: Mlcse(--nospam--at)aol.com [mailto:Mlcse(--nospam--at)aol.com] Sent: Friday, November 20, 1998 11:08 PM To: seaint(--nospam--at)seaint.org Subject: Re: Rigid plywood diaphragms In a message dated 11/20/98 3:03:10 AM EST, itsekson(--nospam--at)jps.net writes: < ............. DO YOU THINK THAT IT IS APPROPRIATE TO ESTIMATE THE RELATIVE THE RIGIDITY OF THE PLYWOOD SHEARWALL (OR CLOSE APPROXIMATION OF IT) BY USING THE PRODUCT OF WALL'S RATED CAPACITY TIMES THE LENGTH?............ Regards, Sasha Itsekson >> I am not sure it is completely appropriate, but it might get you rather close depending on how you handle the holdown assembly. The code provides allowable shear design values for wood sheathed walls with nails at a certain spacing (6", 4", 3", etc.) For a given wall length, using a certain nail spacing, and assuming a certain load in the wall in terms of pounds per square foot (so you can calculate nail slip), you can then calculate the shear wall deflection. If you know the deflection, you can calculate the rigidity. If you omit the holdown portion of the shear wall deflection you might be able to directly use the walls rated capacity (pounds/ft) times the wall length to represent the wall stiffness. The problem as I see it is that for the short shear walls, less than 1:1 aspect ratio, the holdown assembly (deflection of holdown body, oversized holes in post, crushing of sill, shrinkage of lumber) is a major contributing factor to the total shear wall deflection. For multiple shear walls in the same line, you will not obtain the same equal distribution of shear to all walls (say all walls have 300 pounds per foot) until all the slack in the holdown assembly, as described above, has been removed by the wall uplifting and reaching the capacity of the holdown. The even distribution also assumes all walls along the same line have the same nail spacing. By the time you reach the holdown capacity, you probably have exceeded the allowable code drift limits and now possible damage control issues. I think you would have to do the tributary analysis first, including holdowns and calculating shear wall deflections. Since you now have the shear wall deflections, you can assign stiffness to each wall and then do a rigid analysis. This is a lot of work to obtain an envelope of design forces. I don't think you can do the rigid analysis first because of the holdown assembly deflection problem. If you can eliminate the holdown assembly deflection issues, then you could probably do the rigid analysis first. Would you rerun your analysis if the forces in the shear wall exceed it's rated capactiy that you assumed. Using the shear wall unit capacity method for stiffness will probably be closer to a complete rigid analysis when all the shear wall aspect ratios exceed 1:2. As the aspect ratios get larger (1:1, 1.5:1, 2:1, etc) the holdown becomes a very siginificant factor and the rigid analysis will become most likely more inaccurate. The other problem I see is that I would not include the nonshear wall elements in the analysis such as the gypboard walls at this time. I don't know if this would keep you out of litigation, but I think it would address the fact that you at least considered the possibly of a rigid diaphragm, especially if it governs the design of some of the walls when compared to a tributary analysis. Michael Cochran
- References:
- Re: Rigid plywood diaphragms
- From: Mlcse
- Re: Rigid plywood diaphragms
- Prev by Subject: Re: Rigid plywood diaphragms
- Next by Subject: Re: Rigid plywood diaphragms
- Previous by thread: Re: Rigid plywood diaphragms
- Next by thread: RE: Rigid plywood diaphragms
- About this archive
- Messages sorted by: [Subject][Thread][Author][Date]