huangyhg

line up multi-story wood studs and trusses?
i designed a 4-story wood dorm with 2x6 studs and wood trusses both at 16" o.c. on the drawings i had a note that the studs and trusses should line up to avoid loading the double top plates in bending and shear. the contractor has submitted shop drawings for prefabricated wall assemblies. my question is am i being unrealistic in asking things to line up? the building is fairly typical dorm construction in that the long sides of the building are load-bearing, along with the two corridor walls. it will obviously require a great deal of coordination on the wall fabricators part to insure that studs along 4 different walls line up.
it doesn't sound unrealistic to me...but what kind of tolerance(s) do you expect? did you specify any tolerances on your drawings? what guidance did you give to the contractor via the plans & specs?
in the absence of specific (and realistic) guidance in the plans and specs, you will probably have to live with the tolerances (if any) in the local building code as well as locally accepted practice -
if the prefabricated wall assemblies are engineered by the manufacturer, they should specify their requirements. are the floor trusses and roof trusses part of this package? if so, then the manufacturer usually provides a layout drawing with all components and location of supporting members (studs) and floor/roof framing
i would be concerned about how the studs and trusses align wherever i have hold-downs. with a 4 story wood structure, i am assuming that wind controlled for the lateral load. i am also assuming that the shearwalls have some uplift at the ends, unless thae walls are long enough.
with that said, in most wood construction, the studs and floor system do not always align, and with a 16" spacing on the studs and trusses, i would not expect there to be any problems if they don't align now.
in the type of construction you describe, i generally see a ribbon board running perpendicular to the floor trusses. generally, a 2 x 4 is used, but i have required a 2 x 6 on occasion. i assume the stud lands halfway between two floor trusses and check the 2 x 4. in your case, a four story building, i am guessing that even a 2 x 6 ribbon board will be inadequate at the lower floors. therefore, asking for one stud below each truss is the right thing to do.
daveatkins
mrengineer,
on timber projects i have designed in the past, i have not included a statement similar to yours for the following reason. think of it like this. you have 2 - 2x6's spanning 16". unless you have large point loads (such as mechanical equipment), i would not expect a failure of the double top plates. if there are such loads, i would also check the axial capacity of a stud (it may fail before the double top plate).
also, if you decide to accept the alternate design submitted by the contractor, i would require that the drawings be signed and sealed by whoever designed it. i would not want to accept the liability for the design of another engineer.
good luck.
the double top plates should be checked for the load coming down on them! this will control sometimes when there is just a roof load coming down on them. it doesn't matter if the 2x6 are only spanning 16" they will fail in shear or sometimes bending. most contractors use the cheapest pieces of woods for the plates so they do not have the capacity unless you spec something stronger.
with the structure being 4 stories i would be concerned at the first supported floor especially. you will be bringing down three stories on the top plate. (the loads will accumulate as they go down.) on the same note even if the studs do line up you should check for crushing of the top plate perpendicular to the grain. a lot of times this will control and you will have to call out a better species of lumber for your plates.
i may not have been clear in my response. i did not mean to imply that the top plates should not be checked, but rather i would expect the studs to fail in compression prior to the top plates failing in bending or crushing.
the other thing to keep in mind is that although the loads due accumulate, it is a highly indeterminate structure and there will be a loading sharing effect of adjacent
one solution is to specify studs at 12" o.c. with roof and floor trusses being spaced at 24" o.c. you should be ok.
i don't know any way to show that a double top plate will check for studs at 16" o.c. for a three or four story building when all the roof and floor loads are being carried by the exterior walls.
that said i have never heard of a double top plate failing. however when i have been the engineer of record on this type of projects i have done several things, none which made the contractor happy. i've specified triple plates which is an expensive option because it does allow for the use of standard length precut studs.
another thing i have done is detail the bearing with 1"plywood 24" deep nailed to the back of the floor trusses. the plywood then distributes the load from the roof or higher floors to the studs.
what you do depends alot on your level of comfort and i know from talking to other engineers that a lot of people are unsure how to handle the same problem
no doubt a great deal of coordination will be required between the contractor-wall fabricator-truss fabricator. you have noted the requirements on your drawings. the contractor and suppliers have not indicated any exceptions to what is indicated. the contractor has submitted shop drawings for pre-fabricated walls. i believe this indicates that the contractor is accepting the responsibility for coordinating this issue. with pre-fab walls, i would have other questions. will the wall diaphragm be nailed up correctly to assume a continuous diaphragm? will hold downs be installed in the correct locations? will hold downs be able to be installed correctly? if a truss happens to not line up over a stud (and it is out of allowable tolerance), a stud can be installed in the field below this stud.
there is some load sharing between the top plates and the sheathing as well.

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