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RE: WOOD MOMENT FRAMES, Rw = 3[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaoc(--nospam--at)seaoc.org>
- Subject: RE: WOOD MOMENT FRAMES, Rw = 3
- From: "Dennis S. Wish" <wish(--nospam--at)cwia.com>
- Date: Tue, 14 Apr 1998 10:28:50 -0700
This has been an interesting discussion. My vote is to leave the Rw=6 for these reasons. A box system designed with plywood shearwalls would normally require an Rw of 8. However, most of us already work with a degree of safety which we attribute to construction deficiencies - and therefor use an Rw of 6 to boost the lateral shear to 18.5% of the structures weight. In most residential structures of modest means, the Rw is a moot issue in seismic zone 3 or 4 since the majority of lateral shear originates from wind rather than seismic forces. The allowable windo load theoretically lowers the Rw factor closer to 3 or whatever you have it based upon the basic wind speed and other exposure. Assume for the moment that the structure is governed by seismic and with an Rw of 8. We increase the lateral load by at least 50% when using an Rw of 6 - essentially increasing the stiffness requirment of the shear elements. If the wind load is stronger, then the "demand" to the structure begins to approach an Rw of 5,4,3 - depending again upon the location and exposure. My point is that many of us have already comprimised with an Rw of 6 when the code allows an Rw of 8. When and where wind governs - which is often (at least in one direction or down one or more lines of shear) - we design for this worst case. Therefore, I am not in favor of creating a pole with three or four times the stiffness of the wood lateral system unless wind dictates. One other point - personally, I don't determine the lateral load for the entire structure and distribute it proportionately into each line of shear. Instead, I analyize and compare wind to seismic in each line of shear and design that line for the worst condition. The front of a garage - jutting out of a building, will exhibit very low lateral loads based upon seismic weight, but will most often govern by wind loads - this makes the issue of Rw moot. One last point. If those of you who feel that the Rw should be reduced to 3, how do you compensate for a structure controlled by wind with an embedded post in one line of shear and plywood shearwalls in the others. Surely, natural frequencies are reached whether the motion is wind or seismic related. Therefore, I don't understand why there is a need to raise the lateral load to a structure at more than three times what code allows? With an Rw=8 in seismic zone 4 the base shear would be approx 0.137Wd, Rw=6 would be 0.183Wd and with Rw=3 would be 0.366Wd. Rather than discussing Rw values, I understand this in terms of stiffness of the resisting member - the same as using an allowable story drift of 0.0025h for masonry and any other element used to resist shear in line with the masonry. For conditions of wood and steel, allowable deflections increase to 0.005h. Therefore, I can understand increasing stiffness in the SAME LINE of shear where one element is stiffer than the other. Sorry to ramble, I hope you see my point. Dennis S. Wish PE La Quinta, California wish(--nospam--at)cwia.com ICQ# 6110557 http://wwp.mirabilis.com/6110557 "The death of democracy is not likely to be an assassination from ambush. It will be a slow extinction from apathy, indifference, and undernourishment." Robert Hutchins -----Original Message----- From: Parkerres [mailto:Parkerres(--nospam--at)aol.com] Sent: Tuesday, April 14, 1998 8:05 AM To: seaoc(--nospam--at)seaoc.org Subject: Re: WOOD MOMENT FRAMES, Rw = 3 Bill - Thanks for the repsonse. I, too, feel that the Rw=3 for the rest of the building, or at least that direction, is excessive. I will adopt the City's shrug method and design accordingly. Personally, I think it would be clearer if the Code left Rw=6 for cantilvered columns, and then added a note in the text saying that cantlivered column systems should be designed for twice the calculated load. (This is similar to masonry shear walls, which are designed for 1.5 times the calculated load). This would eliminate the Rw=3 problem for the building, yet still give the desired design results for the cantilevered columns. The Code could then explicitly deal with the question of whether or not the doubled load also applies to the footing design and to the drift design. Bruce Resnick, SE Parker Resnick Str. Eng.
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