Drift Loading Cause By Tall Structure

[BadgerPE]

Hey all!

Hope everyone is ready for a long weekend (at least those in the U.S.)! I am doing a little preliminary research for a small project that we are going to be starting shortly. It is a corridor built along a series of silos. My question becomes, is there any additional literature available on the topic of drift loading near a tall structure >50'+ above the lower roof. If I apply ASCE's drift loading for the indicated structure on plan, I get a maximum drift load of about 30 psf (pg=30psf). My gut tells me that I should be adding a much larger quantity of snow to the roof, but I am unsure of how to quantify it. I feel as though there should be somewhere in the area of 100 psf max snow load. If anyone has any additional information that would be great. Applicable codes for the region are ASCE 7-05 and IBC 2009.
Thanks Much!

 

[BadgerPE]

Wow must be "Friday". That title left much to be desired. Should read "Drift Loading Caused By Taller Structure"

 

[SteveGregory]

You might want to consider the impact of ice or snow sliding off the roof of the silos and hitting the corridor roof. This force could be much higher than a snow drift of 100 psf. I can't help you with the impact factor because there are too many variables.

I hope this doesn't ruin your weekend.

 

[RFreund]

Where is all the extra snow going to come from? Just because the heights are very different does not mean more snow. You need more area (thus more snow) that is above the lower roof. If the existing building is taller than it looks like there would be more roof area to consider.
However if you wanted to argue for a higher load you would take height of the windward and leeward snow and add them together.

EIT

 

[BadgerPE]

One thing that has me concerned is the offset of the silos in plan view. I feel as though more snow may collect on the corridor roof because as the snow blows between the silos it will be slowed down and may collect on the roof.

SG: Impact loads are something I will have to evaluate.

 

[spats]

You worry too much. There isn't enough length of upper or lower roof to cause any type of a significant windward or leeward drift... and how is snow going to slide off the silo roofs? Even if the tops of the silos are dead flat, they would never accumulate any real snow because of their exposure. Design for flat roof snow load and you can still sleep comfortably at night.

 

[DST148]

Crackerjack47 : I checked snow drift based on the data you provided and your figure of 30 psf is correct using the provisions of ASCE 7-05.

Another approach could be (very conservataive) - Assume all the (balanced) snow from the six silos drifts on the entire length of the corridor in the form of a triangular loading.
Impact should not be of concern for snow drift with a elevation difference of about 50 feet. Ice - may be.
Canadian codes are very exhaustive when it comes to snow loads. They have provisions for calculating drifts from higher roofs with parapets (parapets and screen walls affect snow drifts significantly), canopies adjacent to tall buildings etc.

 

[SteveGregory]

I live in Virginia, not Canada and I saw the disastrous results from several cases of sliding snow dropping 6'-10' from an upper metal roof with snow guards in most cases. The impact of the snow/ice mixture damaged or destroyed a couple of canopies and a roof over an elementary school cafeteria.

 

[DST148]

SteveGregory - I agree with you that it is prudent to account for any dynamic effects associated with sliding snow. But that usually occurs with low roofs abutting sloping and slippery high roofs. Impact usually is not a concern for snow drifts with a elevation difference of 50 feet.
By the way I live and work here in U.S.

 

[RFreund]

I'd be interested in how you would analyze or design for the falling snow case. Steve - When you say there are too many variables, what are they or where do you start. Is there a particular example that you could point us (me) to just to give a general idea.

Sorry - I'm not trying to hijack the thread.
Thanks.

EIT

 

[SteveGregory]

DST, The picture and the examples that I cited were for the condition you described and not a 50' difference.

However, there is another condition where the 50' or 250' number comes into play. For a radio or television transmitter site, there is usually a small building near the tower. Ice buildup on the legs of the tower can become like dangerous missles when a section breaks off. The ice can slice right through a roof if it is not designed correctly or a person.

RF, For starters, How much of the snow is fresh stuff and how much has melted and refrozen as solid ice?