Raising Portion of Existing Bar Joist Roof?

[MJC6125]

I have a project where the client wants to raise a portion of the roof on an existing one story retail space. The roof structure is metal deck, steel joist, joist girders, and HSS columns. Pretty typical construction. See below for where they want to "Raise the Roof".

Has anyone ever done this before with success? I have a few areas of concern, but I'm not familiar enough with steel joist design or the construction methods to do this work to know how big of a concern they are.

1. We will be taking the load off from one side of the existing joist girders at the roof step. The joist girder will see less load however it will be eccentric all to one side. Does this cause too much torsion on the joist girder? Can the torsion be addressed with kickers to the bottom chord of the JG?

2. How plausible is the construction of demoing/re-using the existing roof joists and girders in a new higher location?

 

[KootK]

Has anyone ever done this before with success?

I've not done this but can't resist jumping in anyhow because it's interesting. I'll happily defer to anyone with actual experience.

Can the torsion be addressed with kickers to the bottom chord of the JG?

I don't see why not. This is what I would do.

How plausible is the construction of demoing/re-using the existing roof joists and girders in a new higher location?

Quite I would think. If it's all welded, somebody will just have to carefully grind of the welds without wreaking havoc on the parent materials.

Questions for you:

A) do you plan to add extensions on to the perimeter columns, cantilevered up from the existing tops of those columns?

B) what's your plan for diaphragm shear transfer up and over the popup?

C) do you plan to reuse any of the deck from the popup area? If so, tying it back into the roof step edge member might warrant some consideration.

 

[KootK]

D) I'd guess this would markedly increase snow drift on some of your joists and girders. Are you thinking that they'll be okay as is or are you expecting to reinforce?

 

[r13]

Not to recommend anything, but help to see what challenges you are facing. This project is liking open up a big hole on the roof framing, and build something above it. I think all members within this opening are to be vacated, as a result, the immediate concern is how to modify/strengthen the boundary elements to handle and passing the lateral load. The edge columns are to be extended, are you going to keep the middle column or not? The wind load is more complicate, and drift of snow is to be considered. The utilization of the existing structural elements largely depends on the method of existing construction (simple span joist vs joist continuous over spans) and strength demand. As KootK pointed out, this is an interesting project, tedious though, expecting busy days ahead.

 

[KootK]

How high will the step be?

 

[MJC6125]

How high will the step be?

+/- 6'-0"

A) do you plan to add extensions on to the perimeter columns, cantilevered up from the existing tops of those columns?

I was thinking a new column built on the top of the existing columns at the roof step. There's two columns that will only be attached to the high roof (not at step). I'm thinking new full height columns for those because we don't know the wall thickness of the existing columns, so it's possible those taller ones wouldn't calc out at the new KL/r.

B) what's your plan for diaphragm shear transfer up and over the popup?

Probably a short steel brace, but connecting this to the joists and joist giders at the low side of the step could be a little funky. Any better ideas?

C) do you plan to reuse any of the deck from the popup area? If so, tying it back into the roof step edge member might warrant some consideration.

I was thinking it would be all new deck.

D) I'd guess this would markedly increase snow drift on some of your joists and girders. Are you thinking that they'll be okay as is or are you expecting to reinforce?

We're going to check if they are OK as is. If not, it will be bigger deal because the joist that sees the most new snow drift is is over a different tenant's space.


To R13's comments, everything is simple span that's why it seems like re-using the existing is just as good as bringing new members in so long as they don't damage them during construction. Also, this will affect some diaphragm chord and collector stuff that I haven't fully figured out yet. We have boundary members at the step which are continuous through the rest of the roof so I feel like a splice could be done at those corners. The diaphragm chord along the exterior wall will now have a step in it, so I'll have to address that.

 

[XR250]

Might be able to add/verify some chords on the lower roof and use some sub-diaphrgams to make the lateral workout without having to use the upper diaphragm for the overall lateral

 

[jayrod12]

Raising that amount of area, that far, there will be some fairly substantial build-up at the step. I can't imagine the existing joists just magically having that much excess capacity when anytime I add even a small rooftop unit that triggers the smallest of snow build-up it requires reinforcement.

I do think that re-using the removed joists is possible and is a great way to save some cash. As long as the contractor removing them is extremely careful.

 

[KootK]

Any better ideas?

Workin' on it...

What kind of top chord to we have on the girders? Double angle? It'll have a big impact on the details that I've got rolling around in my head.

 

[KootK]

Conversation starter.

 

[r13]

Is the column on 46-E.3 to go or stay?

 

[MJC6125]

Is the column on 46-E.3 to go or stay?

I'm initially thinking the column on 46-E.3 and 47-E.3 are replaced with new full height columns

What kind of top chord to we have on the girders? Double angle?

Double angle. See below for a picture of the roof framing.

I like the stacked truss idea. Seems like that could be an efficient way to detail this.

 

[KootK]

I like the stacked truss idea. Seems like that could be an efficient way to detail this.

Well, if you do go that way, report back. While I don't want to waste my own time needlessly, I definitely would want to be involved in the detailing challenge.

 

[r13]

Make sure the column can handle the load after increase in length. Column 46-E.3 will not have intermediate support in both directions.

 

[KootK]

'm initially thinking the column on 46-E.3 and 47-E.3 are replaced with new full height columns

Another option would be to weld on an extension using a similar cross section and, then, reinforcing the works with a pair of channels if needed. It won't be especially pretty but it would save on foundation work if the column replacement would require SOG demo & restore. I suppose that another option might be to cut the existing column just above the SOG and weld on a new column above it. Just make sure that your contractor would be comfortable with the process of plumbing the column while welding etc.

 

[bones206]

I'm thinking all new columns. You could install the new columns pretty close to the existing columns, threading the needle through the roof joists and joist girders. Plus, exposing the foundations to install the new columns provides an opportunity to enlarge the footings for the new drift load (if required).

This also relieves load off the existing joist girders, which should leave enough capacity for the new drift load.

I think this approach could be advantageous from a winter weather standpoint, since you could keep the roof largely intact while framing new above it. Just need to cut some holes to drop in the new columns. Then once you've installed the new girders, proceed with cutting out the old joists and reinstalling one-by-one up on the new level.