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RE: Design of Top Plates[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Design of Top Plates
- From: "Structuralist" <dennis.wish(--nospam--at)gte.net>
- Date: Sat, 30 Dec 2000 11:18:39 -0800
Mark, When the "secondary effects" are too difficult to identify simplified analysis is not necessarily the solution. Not trying to be argumentative, the issue often becomes one of defending engineering judgment or compromising at the expense of the clients pocketbook or architects aesthetics. I have found that a letter of explanation, although more costly in time for me to write, will often save the client much more in labor and materials. When stud spacing does not allow for alignment of the joist/trusses the double 2x plate is more accurately treated as a continuous beam over multiple supports. This still does not define the actual condition. Jeff Coronado, S.E. indicated that he did not see the benefit to the rim joist. If you consider the double top plate to be continuous, the rim joist positively connected to the double top plate, then you end up with a joist/truss bearing on the lower flange of a built-up member. Bending is reduced by the depth of the rim-joist. However, in my opinion, the same condition is attained by consideration of continuous blocking as the web separating the single plate above and the double top plate below. The plates become the chord and the depth of the beam is the depth of the joist/blocking. Again, I'm not trying to argue the point. When I built my home, I designed some high windows in place and did not have the room for more than a 3" deep header. The wall was bearing and the roof was constructed of plated trusses spaced at 23" on center. The truss span was, at points, up to 24-feet long. The roof flat gave a reaction on the trusses of approximately 500-lbs. The windows were 36" wide and 18-inches tall (operable casement windows). this placed, at the worst location, (2) trusses within the 36-inch unsupported width. I took each of the arguments that we discussed into consideration. The trusses bear on the lower chord - placing the full height of the truss above the double top plate. On top of this was the roof sheathing (15/32" plywood) and a 30" tall parapet wall (a Sante Fe style home). I rationalized the bottom plate of the parapet wall, the blocking and double top plate of the wall as a built-up beam spanning the 3'-0" and the calculated deflection was negligible. The home has been up for 8-years and although the roof has not been fully loaded with the live load conditions, there is no appreciable deflection in the plate and no difficulty in opening or closing the windows (other than the inconvenience of accessing windows in a 10-foot ceiling). One of the problems that I think we face is trying to much to either simplify the design methodology or to over design the code or structural elements to avoid creative solutions. Sorry to preach but I do wish you all the happiest of New Years and won't debate that issue:o) Dennis S Wish, PE -------------------------------------------------------------------------- From: Mark Pemberton <Markp(--nospam--at)lbdg.com> To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org> Subject: RE: Design of Top Plates Well said! However, most of these secondary effects are difficult to put numbers to. It is therefore usually necessary to take the simplified analysis approach when plan checkers require top plate bending calculations. Mark Pemberton, P.E.
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