huangyhg

bar joists on cd's by eor
i'm reviewing some shop drawings for a set of bar joists on a 6:12 slope and the mfg'r keeps asking for the axial drag force due to the roof slope. i looked at the 2005 sji specification, section 6.1 and could not find anywhere that this information was required to be specified by the eor. the joist mfg'r's catalog states this requirement but i'm not sure i agree that it is my responsibility to provide this axial load if the roof loads are only dl+ll, nothing special. has anyone else come across this issue?
could be just me, but that sure seems like something they could calculate on their own. they've got the loads and the geometry; the resulting drag force should be part of their joist design. i've never had that come up.
i just spoke with the technical director from sji and he agreed with the joist designer...go figure. they basically want every load spelled out and diagrammed. i just wish their specification would spell this out to the eor.
are they asking for the down slope component of the gravity loads?
i don't design steel joists, but i do design glulam trusses. in a case where the trusses is only required to carry gravity loads, i can't imagen asking the eor for the down slope component.
str04, is there a metal deck diaphragm attached to the joists? i have a project going now where i just asked my local joist mfr the same question about whether i should show this component or if he would come up with it and design for it. he stated they don't consider this downward drag if there is a welded diaphragm to the joist top chord, that they consider this component goes into the diaphragm then into the lfrs and bypasses the top chord. i don't know if i completely buy that though.
we make a living by what we get, we make a life by what we give.
sir winston churchill
str04
the reason the joist mfr doesn't want to consider the downward component is that it will probably increase the cost of each joist.
i wonder how many people include the down slope gravity load component in their diaphragm design?
do you think the diaphragm is stiff enough to be relatively close to the axial stiffness of the joist? it would have to be in order for the diaphragm to take load. i would guess that for typical flexible diaphragm this is not the case unless you have small spans.
if the joist is only connected to its end walls, or some beam, there theoretically is zero (or very little) joist stiffness in its axial direction - i.e. the joist has nothing pushing back uphill at its ends. therefore, the diaphragm must take it all.
i agree.
we make a living by what we get, we make a life by what we give.
sir winston churchill
jae - thanks. got it. don't know what i was thinking.
i guess one twist might be if it is a gable building with joists welded on either side to a beam at the ridge, the joist top chord could be acting as a tension drag bringing force into the top joist connection. the force at the ridge may then cancel, assuming the building is symmetrical and loaded the same on each side of the ridge.
but that would be more of a joist connection concern (if you assumed the diaphragm did not take it all in this case since you have a stiff joist reaction point due to opposite forces at the ridge) rather than buckling of the top chord.
we make a living by what we get, we make a life by what we give.
sir winston churchill

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