huangyhg

chord force in a concrete filled diaphragm
i am designing a one story building with tilt-up shear walls and lw composite roof deck for blast resistance. my question is: since i have a concrete filled diaphragm, do i still need to design the perimeter angle for the chord force or is the concrete diaphragm so rigid that the perimeter angle doesn't even have much chord force? any help would be appreciated.
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design perimeter angle for chord force or put extra rebar in diaphragm for chord forces.
just check. treat the diphragm like a simply supported beam and check the stresses. i interned at a pretty big precaster while in college and they always used chord steel (even when not needed by analysis, just for some belt and suspenders). it wasn't a big deal with that because they had to pour a wash around the perimeter anyway.
that is a good question that i wondered before too because it kind of goes against the assumptions of a rigid diaphragm. but, i design for bending in the diaphragm and add chords whether it is assumed rigid or not. there is no such thing as an infinitely rigid diaphragm so there will be some amount of bending. if you go ahead and design for the worst case of a flexible diaphragm then you are covered and don't have to try and analyze what chord force actually exists in a rigid diaphragm.
the chord connection at the tilt-up wall connections does present a problem. the panels must be allowed to move for temperature reasons, and the chord tie must be able to develop a tension. this is a dichotomy.
the solution is to provide a number of grade 4o rebar across the joint that are welded to steel plates embedded into the walls at 2 to 3 feet from the joint. you would want to weld the bars at the during the average temperature for the region. the result would be a compromise, allowing the panels to get longer with increase in temperature above the normal - the bars will just bend to the inside - and the chord tie will be maintained. the chord bars will have to be increased to allow for greater stresses due to the tension in the bars when the temperature drops below normal though.
mike mccann
mccann engineering
i worked at a place that literally did hundreds of big box tilt panel buildings and i think the standard detail was just a continuous l3x3 or l4x4 welded to embeds along the panels to support the roof deck. this was wal-mart and home depot type buildings. not that there ever wasn't a shrinkage restraint issue using that detail, but there was not one over the years i was there that i recall.
well, i just wonder how much thermal expansion really takes place - most tilt-up panels have an exterior skin exposed to the thermal swings and an internal skin that isn't. and over the width of a single panel, how much total movement is there really going to occur?
the embeds are usually what - 6 feet on center or less so the angle doesn't even see the full panel width thermal movement, and being on the inside, may not see much at all.
well, the gap between panels is usually 3/4" with a 1/2" diameter grade 40 rebar inserted in the panel to panel connections, leaving 1/4" for expansion/contraction. for a 20 foot panel seeing 100 degree temperature swings (common in my area), this converts to a little over 1/8" or .15" in lateral movement per panel, under the .25" designed for.
mike mccann
mccann engineering
we never used panel to panel connections other than what was provided by the continuous chord, there was only sealant between the panel gaps. unless additional connections between the panels were required for overturning resistance.
mike,
i like your idea of the rebar across the joint but what about in the case where you have to have the perimeter angle pick up the deck? (i.e. wall running parallel to joists) would you then just provide slotted holes in the angle to allow the panels to expand? you could also add a joist a few inches away from the wall to pick up the deck, but that's not always an option.
i would either stop the angle at the joint, or use slotted holes as you suggest. however, i would not use the angle as the chord

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