Designing for a future addition on a roof (Steel Framed Building)

[MJC6125]

I have 3 story steel framed building where they want to have the potential to add a 4th floor addition in the future. One thing to muddy the waters a little is that the entire roof does not need to be designed for this addition, just a portion. See attached roof framing plan with the future addition area hatched.

I'm wondering if anyone has come across this condition in the past, and what you designed the roof framing in the addition area for. My specific questions are:

1. What do you think makes the most sense for the future 4th floor surface/structure? Would you use some sort of pedestal raised flooring system? Would you pour a concrete slab on metal deck in the future?

2. Would you slope the structure now and fix the slope in the future using a varying thickness slab, tapered insulation, or a pedestal system? Or would you keep the structure flat now?

3. If they are going to pour a slab in the future, would you automatically use a form deck now or is a roof deck (designed for the weight of the concrete) sufficient?

I'm trying to nail down how much dead load I should design the framing for, and whether I should slope the framing or not. Any other pointers if anyone has done this condition in the past are also appreciated.

 

[mtu1972]

I did something similar, but no where near as complicated, years ago. On a two story bank/office building about 75% into the design, the owner asked the architect to estimate the cost to plan for a future third floor. I provided the structural estimate costs related to the increased loads on the footings, columns, and the bracing systems. The proposed third floor would be above the roof level, so we left the roof slopes, etc. as they were already planned.

The owner approved the costs and the revisions were incorporated. I drive by a couple of times each year and the addition has never been added.

gjc

 

[JAE]

1. What do you think makes the most sense for the future 4th floor surface/structure? [red]concrete[/red]
Would you use some sort of pedestal raised flooring system? [red]no[/red]
Would you pour a concrete slab on metal deck in the future? [red]Yes - I'd use a composite metal deck with concrete topping on the beams at the floor area. In the roof area simply roof decking on the beams.[/red]

2. Would you slope the structure now and fix the slope in the future using a varying thickness slab, tapered insulation, or a pedestal system?
[red]We would usually construct the future floor deck flat and the architect would use tapered insulation in the floor areas. The non-floor areas could have beams sloped to drain but this would all have to be coordinated carefully - some sloped beams and some beams (with partial floor on them) being flat.[/red]
Or would you keep the structure flat now? [red]The entire floor-roof system could be flat and all roof drainage done with tapered insulation. You need to layout various concepts to see which one is simplest and less costly.[/red]

3. If they are going to pour a slab in the future, would you automatically use a form deck now or is a roof deck (designed for the weight of the concrete) sufficient? [red] I'd place the concrete now (much easier in the future) but if the decision was based on what mtu1972 suggests - that it might never happen - then perhaps placing ooncrete on the "roof deck" is a future option.[/red]

With a partial floor like that you also need to visit with your architect to see how they plan to flash up the roofing on the side of the "penthouse" that you are creating. It is generally easier to have your floor a bit higher than the roof to allow for cant strips or edge details typical of good roofing practice.

 

[MJC6125]

JAE, so let's say they don't want to pour the concrete now, but in the future instead. Would you opt to use a composite metal deck between grids 2 and 3, and raise that framing up by however far is needed for future flashing details?
Obviously the fact that the future addition cuts through the entire building complicates things somewhat, but for simplicity's sake let's assume the "penthouse" is only happening between grids 2&3.

Would you be worried about the stepping deck that would need to happen and how that will affect detailing or does that all seem do-able?

 

[JAE]

In general I'm not a fan of stepping the deck as a contiguous diaphragm is preferred.

If concrete is not desired at this time then simply finding a deck-slab design that works everywhere might be the way to go.
For the issue of the floor wanting to be up a bit from the roof - that is usually preferred when constructing it that way initially - but there is always the option of creating a perimeter concrete curb around that future floor perimeter. The curb would allow some distance for the roofing to flash up and ensure a water-tight separation between the interior floor and the exterior roof.

 

[phamENG]

I've only done this once, but the approach I used was to design standard roof deck to be flat across the entire building to create a nice, easy to analyze diaphragm. In the area of the addition, I designed the framing and deck to support the concrete floor and associated loads, but the concrete wasn't poured. The insulation was tapered to produce the necessary slopes for drainage. The assumption was that the expansion, if it were ever built, would use a concrete curb with waterstop.

As long as you don't design it for composite action, the concrete doesn't care what its form work is. Standard wide rib roof deck works fine, as long as the roof deck is designed to carry the wet concrete.

 

[MJC6125]

I think I will follow what phamENG said he (or she?, I don't assume) did. Flat structure everywhere with tapered insulation to simplify things. I'm going to plan to use the same roof deck everywhere and verify that it can act as a form for a worst case future slab pour (assuming 6" total thickness normal weight concrete). I will double check that the loading is also sufficient for a concrete curb along the perimeter of the addition like phamENG stated. Thanks for the help.

 

[Blackstar123]

I'm trying to nail down how much dead load I should design the framing for

Are you considering other loads such as seismic, wind or snow loads?

Would you use some sort of pedestal raised flooring system?
no

Wouldn't this create conflict with seismic requirement? The one, that says not to provide splice near the floor level. Isn't the minimum distance the solice should be provided is equal to depth of the column?