UBC Section 3402.2 (Existing Structures)

[wale01]

I have situation where I am adding 4' of new parapet to an existing building (only about a length of 20'). This will cause some drifting. I seem to be confused everytime I do something with an existing structure. At this time it is not known the size of the roof bar joists. I do have spans and spacing and so forth. The way I see section 3402.2 is that I shall calculate the load on the joists according to loads specified by the building code when the building was built. Then add the snow drift according to the governing code today. If the increase force due to the drift is less than 5% then I'm good to go.

Thanks for the help

 

[sundale]

That's what I would do. Worst case scenario is having to weld in new web reinforcement (angles or A706 rebar) on the existing bar joist for the extra end shear due to drifting. Flexure might be a problem in the top chord.

Look very carefully at all the bar joists at the ends. Often times, the old bar joists have an metal "dog tag" that calls out the type and series of the joist.

SJI publishes a historical book of all the old load tables for old joist callouts. If you can find out what size joist you are dealing with, you may be able to infer the shear capacity of the joist directly in lieu of capping it at 105% of what it was carrying.

If you can't find a dog tag, then you might could tell what kind of bar joist you are dealing with using caliper and a machinist's ruler.

 

[wale01]

Thanks sundale

Thats the process I normally go through. So I'm familar with the tags and SJI and so forth. This building had a remodel about 4 years ago a another parapet was added to the building adjacent to where I'm adding a parapet. All the joists and spans are the same. And no additional reinforcing of those josts had to be done. I just wanted something quick and easy that I could fall back onto instead of running out to the site and taking measurements and so forth. Not that I'm taking the easy way out. I'm want to take the logical way out.

Thanks

 

[sundale]

No worries wale01.

The last time I had to do this sort of thing, we added a quite tall parapet to an existing building so that it retained all the drifted snow from our modified roof and did not load the neighbor's existing roof with a new snow drift load. Not an elegant solution, but the building department agreed with the concept.

Reinforcing the bar joists was not easy to design and was even worse to construct in the field given an existing building (hat factory) with plumbing, mechanical, etc. all in the way of the welders. The fire danger was an issue too if I recall.

 

[Taro]

wale01, can you limit the length of the parapet to 15 feet instead of 20 feet? Then you would not be required to design for drifting snow at all.

sundale, adding a parapet does not necessarily eliminate snow drifting considerations for the adjacent building. The parapet can act as a projection that traps snow if the adjacent building is upwind and deposit it on the adjacent building. Was this considered in your case?

 

[sundale]

The adjacent building was many feet lower with a horizontal separation of something like 5'-0". This was 10 years or so ago...

The idea was to create a tall projection on our higher roof to prevent snow from our windward roof depositing onto the neighbor's lower and leeward roof.

Our projection would not have created a windward drift from the their roof.

 

[geoffdale]

We have done hundreds of buildings where we reinforce the existing structure. The easiest way that we have found is welding new rods into place in the top and bottom chords, and web members as required. We have also substituted a plate on the bottom chord (half the welding). Don't forget to also check the joist shoes, easiest way we have found to fix them is to jam them tight with grout.