huangyhg

roof angle connection to block wall
i need some clarification regarding roof perimeter connection to masonry block wall.
i have a 8" block shear wall. the roof is metal deck on steel joists(5' c/c spacing) acting as diaphragm . the metal deck is connected to roof angle around perimeter and the angle is welded to joists at every 5' c/c. the joists are normal to the block wall.
my question is "do we need to attach roof angle to the block wall with hilti/anchors or not?". according to me it should be as that's how the shear will be transfered to block wall. but most of the details (done by others) i have seen, does not show this attachement. am i missing something?
also, in case of joists parallel to wall, normally the detail is a cont. plate on top of wall that transfer the shear to wall below.
thanks
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if there is a parapet on the wall, then i attach the angle to the face of the wall with expansion anchors or other anchorage to transfer the shear. else the shear has to go through the joist seats which is not desireable in my opinion.
yes, you want a continuous load path. whatever your resulting shear loads under lateral loads are, they have to be transferred into the masonry bond beams. likewise the tension/compression your diaphragm develops has to be transferred. fwiw, i usually determine the maximum shear along the short sides, and keep the spacing equal around the perimeter.
i agree with haynewp. you don't want to go through the joist seat because they typically aren't attached very well.
also, make sure you call out the metal deck fastening pattern on your plans. this is often omitted (guilty).
you also want to stay away from 24 gauge deck, even if it easily handles the loads. attachment of 24 gauge requires weld washers, increasing the construction cost.
you can go either way. if you transfer load thru the joist seat you have to give the loads to the supplier for the reactions to include in their design and the connection to the structure has to have the capacity to transfer the load. the tension/compression in the diaphragm chords is handled by the continuous angles.
code for my area requires 4'-0" max spacing of supports from cmu wall back to the diaphragm so i would have to use anchors anyway or change the joist spacing to meet this, which would be stupid.
can the lateral load be transferred through the joist seat? if so, then the angle can be welded to the joist and the deck to the angle...
dik
dik,
none of the joist manufacturers i know have any provision for taking lateral shear across the joist seat. most seats are comprised of a built-up section of angles forming what looks like a pair of small channels back to back with a gap between them.
with a perpendicular horizontal shear force, there's really no direct, stiff load path.
for the masonry wall condition described in this post, the previous ideas are the way to go: bolt the angle to the wall.
for conditions where you have braced frames or moment frames the challenge is getting the deck shear down the 2 1/2" (or whatever the seat depth is) to the supporting beam top flange below. we've in the past used the continuous edge angle as a deck collector and then inserted small tube shapes between the deck angle and the beam (like a 2 1/2" tube) at intermittent spacings to achieve this transfer.
jae... i've often used the joist anchorage to transfer shear. for framing, the opposite joist end is typically dropped a cmu and i usually use a bar or l with headed stud anchorages embedded on top of the end walls...
it can be done but i don't like doing it, i have discussed all this with vulcraft before. it is easy to put in an expansion anchor and not have to worry about if the seat was designed correctly by the mfr. to include all gravity and lateral interactions on the seat, and you also have to check the weld from the seat to the anchor plate.
i usually don't have roof joists at a small enough spacing to meet the cmu anchorage to diaphragm requirements anyway. i have also used the hss 2.5x2.5 tube between the joists to transfer the shear when there was no parapet. this is common at the rear of retail tilt-up buildings.

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