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Re: URM[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaoc(--nospam--at)seaoc.org>
- Subject: Re: URM
- From: "wish" <wish(--nospam--at)cwia.com>
- Date: Tue, 6 Jan 1998 16:43:16 -0800
Mark, I haven't done a multi-story URM for a while but I would like to respond. I am doing so without my UCBC in hand so bare with me here. Your answer depends upon the capacity you assign to your crosswalls. The amount of crosswalls is also a function of the DCR chart which places you in regions based upon the aspect ratio of the diaphragm less any openings. The DCR analysis will determine how much crosswall you need to provide to bring your model in compliance with the region that it exists. The actual capacity of the crosswall will therefore, depend upon how much available wall you have and the maximum capacity of twice the diaphragm in the line of the wall. To be exact, I believe it was the original ABK methodology back in the very early eighties that determined the need for a crosswall to be less stiff than a "shearwall" but only flexible enough to deflect a maximum of 1" per story. One inch was the magic number that we designed crosswall frames for. To stiff and the element became a shear wall. Remember, Special Proceedure is not a uniform distribution, but uses crosswalls as dampers. To be more exact, you would analyze your crosswall for deflection (I used the '91 UBC Standard). Intermediate floor crosswalls were historically attached to the floor framing above and were considered adequate crosswalls when considering plaster finishes on both sides. It was not necessary to further connect or drag the diaphragm into a plaster crosswall (or insure that the framing was even blocked). Where framing was running parallel to the wall, the assumption was that the plaster wall was adequately connected to the diaphragm above unless you could prove otherwise. I never considered it a positive connection when I needed more diaphragm capacity into a plywood crosswall and assured extra connections of the wall to blocking and a positive connection to the diaphragm above. UCBC (if I recalled) only allowed a maximum capacity for 3/8" sheathed plywood per side of walls. If I further remember, the diaphragm capacity at floor level (500 plf for straigth and 600 for diagonal sheathing) rarely was less than the sheathed capacity (somehow 320 plf sticks in my mind for 3/8" on each side of the wall). Since the crosswall picks up the capacity of the diaphragm from both sides of the wall the diaphragm at 2*Vu would exceed the crosswall capacity Vc. Therefore, additional drags or collectors would not be required required. Crosswalls at roof level were a different story since the twice the diaphragm capacity of straight sheathing is less than a crosswall sheathed one side with plywood. The second problem is that upper-story walls need to be extended to the roof by nature of the use of carpenter trusses, separate ceiling and roof joists or the use of trusses with attic spaces. Now to make it more complicated. Assume you design a crosswall frame which you intend to give the entire diaphragm capacity at both sides (approximatly 2*600plf*D) Depending upon the length of the frame, you would design appropriate collectors and drags to develop the diaphragm. Now, I would assume that you could deviate from the code and design a plywood crosswall to take the entire diaphragm capacity into one wall so long as you can justify the deflection at 1". This is tricky, but I think it is in keeping with the intent of the code. In this case you might use collectors to develope the capacity you need. Finally, in multi-story apartment buildings, it was never necessary to make a positive connection of a plaster crosswall to the floor above. We just added the lengths of qualifying walls at each level and considered that our actual crosswall capacity (total) to compare in the DCR charts. Hope I havn't confused anyone since I have been away from multi-story URMs for over five years (I did a few but must be loosing my memory in my old age). Dennis Wish PE PS. I realize that some of this may no longer be acceptable since Northridge. You need to check current code to see if this is still an acceptable judgment. -----Original Message----- From: Mark D. Baker <shake4bake(--nospam--at)earthlink.net> To: seaoc <seaoc(--nospam--at)seaoc.org> Date: Tuesday, January 06, 1998 3:12 PM Subject: URM >To all the URM people out there: > >UCBC, retrofit of unreinforced masonry buildings, special procedure: > >Since a new crosswall is not considered a shear wall and its intent is >to decrease the displacement of the diaphragm at the center of the >diaphragm relative to the end shear walls, is it necessary to provide a >collector element dragging load into the crosswall? > >Regards, > >Mark D. Baker >Baker Engineering