Hybrid roof diaphragm
Hybrid roof diaphragm
(OP)
Please see the attached two sketches. Half the building framing is plywood over light gage trusses and the rest of the half is metal deck on bar joists. I have enough lengths of cmu walls on both ends to get masonry shear walls to work for lateral loads. But the problem is the connection between the plywood and metal deck. I really don't want to use a steel moment frame in the middle and would like to keep the steel frame along the middle as gravity only.
So the question is, can the two types of roof diaphragms be connected in a way that they act as continuous piece from shear wall to shear wall?
So the question is, can the two types of roof diaphragms be connected in a way that they act as continuous piece from shear wall to shear wall?






RE: Hybrid roof diaphragm
Based on your sketch, I would say it is easier said than done. You might need a couple of different elements connected together to make it work, but it seems doable.
RE: Hybrid roof diaphragm
Fore the connection of the wood to the steel beam,, add a 3X nail plate under the wood truss to the top of the steel beam, connected with Nelson sill plate anchors, and add blocking between the top of the steel g=beam and the plywood diaphragm over the trusses. Make appropriate connections of the blocking to both.
Mike McCann
MMC Engineering
RE: Hybrid roof diaphragm
BA
RE: Hybrid roof diaphragm
RE: Hybrid roof diaphragm
RE: Hybrid roof diaphragm
BA
RE: Hybrid roof diaphragm
EIT
www.HowToEngineer.com
RE: Hybrid roof diaphragm
1. to act as a diaphragm one needs a top and bottom chord(top & bott fla)....ok as far as the metal deck is concerned, but when we get to the plywood portion there is no web connection to the chord(fla)...in fact the web is 45 degrees to centerline of girder...so I may not be able to treat this as a continous
diaphragm(girder)....at first glance, it is like having a girder with a continous web for ,say, half the span and then a prctically open web area for the other half....granted, this web zig-zags out and back in this area and I have never tried to analyze something like this before.......so to get an upper bound of the problem, I assumed the metal deck cantilevered of one wall and carrying it's own horizontal shear load and half the horizontal shear load of the plywood area, ofcourse, one needs bracing/shear wall for a portion of the metal deck in the other orthoganal dir....
RE: Hybrid roof diaphragm
The pitched roof portion of the structure must be made into a semi-rigid box capable of carrying the required wind shear from the end of the steel deck to the west shearwall.
If the plywood deck is a competent diaphragm at 45o to the horizontal plane, it provides an indirect path which can carry shear from the west wall to the steel beam. If it is not adequate or is too flexible, horizontal diagonal bracing can be placed between the bottom chords of the pitched light gauge trusses to complement diaphragm action. Alternatively, a ceiling diaphragm could be introduced.
It is still necessary to provide a pair of continuous chord members between shearwalls. The bottom chord of the edge truss and the top chord of the edge open web steel joist could form a chord, but they are at different elevations, so a detail must be devised to connect them in such a way that they are capable of transmitting the calculated force taking into account the eccentricity.
One way to do this would be by means of a column at each edge of the roof designed to resist a moment of F*e at the top where F is the force and e is the eccentricity.
Another way could be by introducing a parapet at the flat roof portion with the chord member raised up to eliminate the eccentricity.
BA
RE: Hybrid roof diaphragm
RE: Hybrid roof diaphragm