huangyhg

snow drift loading at parapet with a kicker
just thoguht i'd see what people have to say about this subject. i have a storefront with a facade constructed out of light gage framing. the light gage framing forms the parapet and due to the height of the parapet is braced back to the roof structure (near the mid-pt) with a lt gage kicker at a 45 degree angle. the kickers are then covered with the roof structure effectively leaving a void beneath them.
so now the question. how would one place a snow load with with a drift at the parapet. btw i'm looking to size roof members (joists in particular most of the time). i've seen it handled two ways:
1. completely ignoring the 45 degree roof structure and calcualting the normal flat roof snow laod with the windward drift. this seems very conservative because you are effectively loading the area at the void.
2. placing the flat roof snow load with the windward drift agaisnt the parapet wall and basically cutting out the section of the drift in the void below the kicker? then taking the area of the snow over teh kicker and applying it to the roof.
thoughts,
chris


the cosmic "correct" answer is somewhere between your option 1 and 2.
steer towards the conservative side as assurance/confidence is lacking.

i can envision the drift possibly beginning to pile up at the base of the 45 degree connection to the roof and not making its way all the way up the 45 degree angle. i agree with being pretty conservative in this case, i always felt snow drift was pretty high on the 'possibility of causing you a lawsuit scale'.
for joists perpendicular to the parapet wall, your #1 choice is worst case for shear. i might also look at a drift beginning at the base of the kickers for moment check on the joists. i don't know that there would be any less total snow drift just because of the 45 degree angle being present, i would think the drift would just pile up differently. else i would have to see some research showing me otherwise. thus i don't agree with your #2 choice.
i lean toward #2, although i can certainly see a case for being a little more conservative. i lean toward # 2 because in the past i have heard other engineers propose adding a shed roof on top of an existing roof, when a taller addition is added--to prevent snow drifting on the existing lower roof. this is basically the same situation.
daveatkins
thank you all for the input. based on these answers i'm gonna run both cases and see what kind of results i get and then use some engineering judgement. for sure i will not use the large conservative loading on a joist thats tributary area carries no snow (@ the kicker void). other than that i will probably use the conservative approach.
dave: your answer is a very valid point. i have heard the same theory about adding a shed roof on top of and existing lower roof to help minimize drifting when a new taller building is constructed adjacent.
thanks for your thoguhts.
chris

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