Snow Drift on Existing Roof

[BubbaJ]

I am working on a project where it is proposed that an addition will be approximately 12 feet taller than the existing building. Due to the length of the existing roof, there is potential for a 4.5' drift to form at the junction.

Unfortunately, the existing structure is joists and joist girders, which were not designed for this type of loading. Since reinforcement of the girders is almost impossible I was considering another avenue. I have heard of similar cases where a sloped roof, mono-slope type structure was erected on the existing roof that represents the anticipated shape of the potential drift.

Have any of you heard of this practice?

 

[COEngineeer]

adding more girder(s) is not an option?

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[BubbaJ]

What do you mean by adding more girders?

This is an industrial facility and is currently under production. It is not viable to add columns, etc.

 

[csd72]

I think COEngineer means can you add more joists in the drift area.

I cant think of any other option. Adding a section of sloped roof will just transfer your drift to another part of the roof.

csd

 

[BubbaJ]

Adding more joists doesn't solve the issue of the overloaded joist girder.

 

[COEngineeer]

Ok, what do you mean my joist girder? Dropped beam right? If you have dropped beam, isnt it easy to sister another beam or two next to it? Or perhaps add another one somewhere else. Usually you are ok with the shear and bearing at the support (usually the deflection controls) so you can just sister on another beam on both side and bolt them together without supporting the end of the new members.

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[COEngineeer]

sorry I just assume it was wood, but if it was steel then add channels on both side of the existing beam and maybe at the bottom. Do shear flow analysis if necessary.

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[Lion06]

COEngineer-
The joist girder is most likely a steel bar joist (just heavier), and it is probably at the same elevation.

I don't think you can add channels to this condition, but possibly a king post on the bottom chord will help? This is done for vibrations quite frequently, maybe it will help for your strength concerns.

 

[csd72]

How about running the channels either side between the diagonals of the joists, then put angles between to support the top chord of the joist girder.

csd

 

[BubbaJ]

That is correct, I am sorry I was not clearer. It is a steel structure, the joist girder is an open web steel bar joist.

 

[lkjh345]

Bubba1:

I have seen what you are describing done. I'm not sure I agree with the theary.

One solution to a joist and joist girder roof that needs to be upgraded to handle new snow drifting loads is to build a secondary roof system over the area in question. therefore by passing the existing joists and joist girders which are very difficult to strengthen.

Come out from 1 bay deep over your existing wall, and set new small post directly on top of the existing columns (poking up through the existing roof), and span a beam between them, and then frame bewteen this beam and your new structure with joists, beams, light gage studs, etc.

This has the effect of putting most of the new drift load into the new structure, where you can control the size of memebrs. The existing interior columns only see a small bit of the increase load, and if they need streghtening, it is usually easier to streghten a column that a joist or joist girder.

usually have to talk yourself into that fact the footings will be OK, since the drift load is only a short duration loading.

Just some thoughts.

 

[COEngineeer]

Bubba, sorry of my lack of engineering terms. I thought all you had was a steel beam supporting your steel joists.

Your original post makes sense. If you put a sloped structure to match the anticipated drift snow load then theoratically you wont have the leeward and winward drift snow. Then you have to make sure the josts and the girder can take the extra DL of the new structure. Also, the addition, Does it have a sloped roof going toward the existing lower roof? You need to look at the sliding snow. Good luck! I must say I always hate remodels.

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[COEngineeer]

lkjh345, why dont you agree with the theory? If you do that I dont see why you cant assume the whole lower roof will have ground snow load?

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[lkjh345]

Coengineer:

I worded my first reply in a way that did not reflect my opinion on this subject very well.

I agree that this works if you can make the 'high' end of the new roof 'over framing' at the same (or nearly the same) elevation as the height of the new wall/roof that is creating the drift. The low end of the new 'over framing' is at the height of the balanced snow load elevation on the existing roof. Then, yes, I agree no drift is formed.

However, I have seen other engineers trying to justify using a sloped roof that was substainatially below the level of the new wall and roof that casued the drift problem. ( like 10 feet below), and trying to justify that becasue the new 'over framing' roof was sloped, no drift could form. This is the part I disagree with.

 

[BubbaJ]

I can see how this theory is valid for leeward drifting. What if the larger of the two drifing scenarios is the windward drift?

Is the idea then that the wind would blow the snow up the new "ramp" and onto the proposed roof and no drift would form?

 

[COEngineeer]

I think the shape of the ramp will control the angle of the wind so that the snow on the ramp will not be any higher than the snow on the lower roof that is not on the ramp. But that is my personal oppinion. This is only valid if you make the ramp like the anticipated drift snow (not steeper, not flatter).

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[JAE]

Really the BEST way to deal with this is to tell the owner that the first NEW bay adjacent to the existing building should be built at the lower elevation.

Thus, the first new bay will take all the drift and you can design for that directly.

If this is not an option, then you are back to doing what all the great comments above suggest.

But it is usually less expensive to come in and strengthen the existing framing as opposed to building a whole "cover" roof over the existing framing.

 

[COEngineeer]

JAE has really good comment there or just tell them the firs few feet of the existing has to be redone. If he ask why.. tell him " Never, but never question the engineer's judgement)

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[BubbaJ]

Unfortuntely, I believe the pre-engr building has been fabricated. They need that height for their manufacturing process.

I appreciate all your input and suggestions. Something will work out.

 

[DaveAtkins]

I agree with all of the comments above--brainstorming is fun

I have also upgraded lower roofs with new drift load by adding more joists (you can get each joist in two pieces that are bolted together in the field--you can't install a one piece joist into an existing roof easily). Or, you can get the original joist supplier to check the existing joists for the new load, and design reinforcement as required--I have done this, and the original joist supplier provided drawings that showed where to add in angles, etc.

All of this assumes that the joist girders, columns, and footings are OK--and my experience is that they usually are.

Don't forget that the deck itself may not be OK, and so joists would need to be added just to make the deck work.

DaveAtkins