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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: Fri, 29 Dec 2000 12:57:48 -0800
I think what most are missing on this problem is a couple of things. 1. The loads are not concentrated at one level but accumulate from each level. While the accumulation of loading is to be applied at each lower level, the distribution of the load is more likely to change from a concentrated load at the upper level and be transferred to the lower level as a uniform load. 2. The blocking (disregarding the rim joist for those who use one where rafters are perpendicular to the plates) creates a composite beam section that considers the bottom plate of the wall above, the sheathing, the blocking and adjacent rafters (discontinuous in shear but tied by the chords in bending) and the double plate below. 3. The highly unlikely event that the joist/truss/rafter maximum live load capacity is reached and consistent over a number of joists, 4. The safety factor included in wood design. 5. The use of the wall "system" with sheathing to act as a beam so as to transfer concentrated loads from upper levels occurring between studs to lower levels as a uniform load by distribution through the wall and sheathing. I think the fault of the rationale used is simply the failure to identify the structural elements as a system and to recognize then you have created a built-up section (double plate, blocking and single top plate with plywood sheathing (floor) used to tie the system from above). As I mentioned above, the concentrated load does not occur at each floor. However, this does not diminish the need to compensate for unusually high loads that may occur from girder trusses or heavy timber trusses or ends of beams that need a post or built-up studs to insure proper transfer of loads from concentrated load to the foundation. Sometimes we lose sight of the fact that conventional wood construction has been around for as long as there have been saw mills. There have been few failures in conventional construction if the historic methods are appropriately followed. I think that we will find from the CUREe studies and, if smart enough to dig deeper, that many of the problems we face are due to quantity demands, improper installation of proprietary hangers, clips and straps and other proprietary products which builders compromise by modifying to fit the needs of the job. Dennis S. Wish, PE From: "Speck, Todd M" <tmspeck(--nospam--at)pbsj.com> To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org> Subject: RE: Design of Top Plates Redistribute the load to where? The load still progress down the studs, through the trusses to the top plates to the studs and so on. I've seen a large number plans where the studs and trusses do NOT align and no additional reinforcing is supplied. I believe, far too often it is ignored. -----Original Message----- From: Oshin Tosounian [mailto:sdgse(--nospam--at)juno.com] Sent: Thursday, December 28, 2000 12:19 PM To: seaint(--nospam--at)seaint.org Subject: RE: Design of Top Plates You are ignoring the rim joist at every level that will redistribute the load from above. So, what you really need to consider is the load from one level only and check your double plate for that load. Oshin Tosounian, S.E. Los Angeles, CA ------Original Message------ From: "Speck, Todd M" <tmspeck(--nospam--at)pbsj.com> To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org> Sent: December 28, 2000 5:33:07 PM GMT Subject: Design of Top Plates John Riley's ledger question brings up a question I've had for some time that no one I've queried has been able to answer. His "Studs are 2x6, D.Fir-Larch(N), @ 16". Load from each floor is about 1100 plf." Assuming the floor trusses are spaced at 24" o.c. My question is this: How does one justify the strength of the top plates? For quick calculations, assuming the roof load is 550 plf you need to transfer 3850 plf (550 plf roof + 1100 4th + 1100 3rd + 1100 2nd) from the second floor trusses to the top plates directly beneath the 2nd floor trusses. with trusses spaced at 24" o.c. that's a 7700 lb point load to the dbl top plate. The studs are spaced at 16" o.c. so there will be a location where the truss bears at midspan of the dbl top plate. I've never had anyone be able to account for the shear and flexure stresses that the dbl top plates are loaded with. Is there something I'm just not seeing or do engineers somehow ignore this?? All help would be much appreciated on this matter, since I've never had anyone give me a decent solution to this.Stan Scholl, P.E.
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