Supporting New Penthouse Column on Existing Roof Joists

[Eng_Struct]

Hi All,

For one of my projects, I am adding a new penthouse on top of the existing roof. The new penthouse columns can only be arranged such that they fall directly above the existing roof joists. Connection detail to support the column on the joists is not something I have seen before. Needless to say, the joists will have to reinforce for the gravity load as well as take the lateral load from the column base.

I am thinking a much better solution would be to replace the existing joists with wide flange beams. Does anyone have dealt with this situation before? I will like to use the existing joist considering the economic benefits but I do not see how I would support the column on the joists.

The column is W6x16 and the joist top chord is 2-L3.5x3.5 with a total depth of 20".

Thanks

 

[JedClampett]

You are correct, but will it make much difference?
You: "Adding beams will make a better design, plus joists are of unknown properties."
They: "But those joists are already there, plus will new wide flange beams be free?"

 

[KootK]

I'm sure that we could dream up a connection. That said, do these joists, even heavily reinforced, have any real chance of working with the penthouse added? What's the penthouse floor system going to be? Concrete deck slab? Plywood laid over the existing roof deck?

 

[JLNJ]

A few observations:

- Those are super heavy joists. Two 3.5" angles with a 20" total depth? Wow.
- This must be a very small penthouse to even have a prayer of working.
- If you have any appreciable lateral load you might have issues resolving those forces into joists and deck.
- I don't know what climate you are in, but if it's a snowy one, you will have snow drift issues all around.

 

[Eng_Struct]

The existing roof decking is timber. However, within the new penthouse, I will be replacing the wood deck with a metal deck with a checkered plate on top to provide a flat surface.

I also need to think of a detail of the existing timber roof decking and new metal decking interface at the periphery of the penthouse. A wide flange is ideal to pick up both floor systems on either side given that the flange is wide enough. However, the challenge with the wide flange beam is its interface with the existing girder that is picking the joists and sits 5 inches below the underside of the deck (space for the joist shoe. The beam may need to have a dapped end to be supported.

The joists are capable of taking the new penthouse floor loads but they will also need to carry a portion of the gravity loads coming from the penthouse roof including lateral loads. I am unclear on the implications of the joists being subjected to axial force. Also, in the middle of the penthouse that is going to be an 18ft by 10ft opening that will cut through some of the existing joists. I will need to provide new framing members (likely wide flange beams) around the opening and think that it will be easier to replace all the joists around the opening with beams as there will complications in terms beam to joists connections, joists to be beam connections if both need to stay at the same elevation, etc.

Note that except for the opening location and the perimeter of the new penthouse, I will be utilizing existing joists for the new penthouse floor loads.

 

[jerseyshore]

For a lot of penthouses I end up just building a floor above the roof. If it is wood framed I just run TJI's and if steel is needed then metal stud joists are an option. This helps eliminate dealing with too much of the existing framing (just post down onto the existing column grid), and also removes the existing roof slope out of the equation. Depends on the size and height limitations obviously, but would this be an option?

 

[dhengr]

Eng_Struct:
Why not use a small bottom sill beam (curb beam) as the base for the two new bearing walls in the penthouse? This would not only tie the stl. jsts. together laterally, but it would/could distribute the penthouse column loads to several of the bar jsts., not just one bar jst. Maybe the new wall is just a bearing wall now without discrete heavy col loads. The curb beam would be a beam on springs (the bar jsts.). The curb beam would also be something to flash the new roofing into.