Steel Building top floor supported by rafters 3

[Tezza91]

Just to make it clear that I am not going to start building anything without consulting a structural engineer first, I just wanted to make sure I'm not off my nut for suggesting it and that the idea is at least feasible.

I am a qualified and experienced welder and fabricator and have built and erected many standard steel frame buildings before so that side of it isn't a problem.

I am now designing my own building, it is to be 18mx18m 7m to the eves consisting of 3 6m bays. 1 bay is to consist of 3 floors of office, storage and assembly space.

Up to this point, quite simple, however, I've had the thought of extending the 2nd floor over the entire 3 bays, but I would like the 2 workshop bays free of pillars/columns. So the idea is to simply have 1 set of beams to be supported from columns attached to the rafters.

I'd appreciate it if some of you could look at the attached drawings and let me know if this is feasible depending on section sizes.

 

[Ron247]

In theory, what you are thinking of doing is not that much out of the ordinary but it would definitely require a structural engineer to design both the overall structure and the connections.

 

[JLNJ]

What is your lateral force system? I'm not seeing one.

 

[MIStructE_IRE]

Certainly doable. 356 UB’s won’t cut it though over an 18m span. But there’e nothing wrong with the principle of hanging from the roof once it’s designed accordingly by an engineer.

Personally I’d also consider roof trusses which might cater for the point loads better than a portal.

You’ll also need eaves beams, ties at top of hangers, roof plane bracing and perimeter wall bracing for lateral stability.

Looks interesting though.

 

[Tezza91]

Thanks for all of your responses, it's good to know the idea is do-able.

The initial design does have wall and roof bracing, eaves beams and ties at the top of the hangers, I just supplied a simplified drawing for clarity.

I'm more in the process of feasibility and budgeting at the moment, obviously adding the top floor with no under supports adds a significant cost to the frame.

Does anyone have any rough idea of what size UB sizes would have to be used?

Also, does anyone have any links or references to any similar projects or literature that will help me?

 

[HTURKAK]

The third portal frame will suffer ... I will suggest the use of full height ( in this case 3rd storey ht ) truss rather than hanging the 2 nd floor to roof beam with tension elements..

Similar truss can be seen at Ferries at passenger deck..

 

[BAretired]

Agree with HTURKAK. For economy, use a truss on the two middle frames. (See red diagonals in attached sketch)

BA

 

[Tezza91]

Thanks again for your input,

The truss design as suggested BAretired would limit the floor space far too much, the upper floor is to house an assembly line which requires the entire length of the building.

I can't see why any additional bracing would be required on portal 2 as the inner columns continue to the ground floor.

I could add bracing as shown here:

I think using larger or tapered columns and rafters on portal 3 is an option.

In the next couple of weeks, I will outsource the design to a structural engineer and post the results here

 

[r13]

The OP uses moment frame in one direction, and braced frame in another direction. I don't think truss is required under the roof, which may be intended for storage. Two secondary floor beams under the one level columns should work.

 

[MIStructE_IRE]

As a simpler alternative, how about a couple of floor beams spanning the 12m? Nice and easy.

 

[BAretired]

I can't see why any additional bracing would be required on portal 2 as the inner columns continue to the ground floor.

Sorry, I should have noticed that the columns on Portal #2 continued down. So the only problem is on Portal #3. I don't think knee braces would be enough. The beams shown by MIStructE_IRE may be the best option.

BA

 

[human909]

Does anyone have any rough idea of what size UB sizes would have to be used?

You would be approximately looking at a beam of about 800mm-1000mm deep. Though it would probably be cheaper to use a truss. You could potentially make the whole office floor a truss if you don't need open plan or don't mind diagonals running through it.

Yes it can be done, but you are talking very significant beam and column sizes. Your typical portal for building that spans 18m has vastly different loading requirements and deflection requirements.

Also, does anyone have any links or references to any similar projects or literature that will help me?

No and they likely won't do you any good. The above figure of the size of the beams will give you an idea of the scale of what you trying to achieve. It is completely possible, but not cheap.

 

[JLNJ]

Just hire an architect and have them help you with a schematic design.

 

[PROYECTOR]

As a simpler alternative, how about a couple of floor beams spanning the 12m? Nice and easy.

I am with MIStructE, that is the best and easiest solution. 12m beams @ 3m or 2m (depending on the steel deck) seem very feasible to me. If height limit is a problem, you can use joists or castellated beams to accommodate the duct work and increase the available space. Also, I would remove the columns above the second floor and let the rafters span 18m.

 

[r13]

This is the way I would do (bracings and ridge/edge beams not shown). Leave one open bay for overhead door.

 

[Tezza91]

Just hire an architect and have them help you with a schematic design.

Totally with you and that's the plan, I just like to be as fully informed as possible when talking to someone, as well as knowing why things are being done the way they are.

I am with MIStructE, that is the best and easiest solution. 12m beams @ 3m or 2m (depending on the steel deck) seem very feasible to me. If height limit is a problem, you can use joists or castellated beams to accommodate the ductwork and increase the available space. Also, I would remove the columns above the second floor and let the rafters span 18m.

I like this option very much which is probably what I will go with subject to proper calculations etc. Like I said I need the top floor to have an uninterrupted 18m length of floor space and the ground floor 18x12m workshop space to have no columns.

Another few points I'm probably being daft about:
Eaves beams: Are you suggesting I need UBs at eave level or just the typical 2mm cold rolled eave sections?
Using beams at 2-3m spacing to span 12m: I assume you mean to turn the floor c sections 90 degrees so they now span the 2-3m rather than 6?

 

[PROYECTOR]

Eaves beams: Are you suggesting I need UBs at eave level or just the typical 2mm cold rolled eave sections?

I just suggest that the top of the columns need to be properly braced. The elements used for that purpose (being a cold formed or a hot rolled section) need to be properly designed and connected to the roof diaphragm.

Using beams at 2-3m spacing to span 12m: I assume you mean to turn the floor c sections 90 degrees so they now span the 2-3m rather than 6?

Sorry, I didn't look at the floor system you are proposing, my recommendation is for a typical composite steel deck over beams with shear studs. You only need the blue beams at the column grid lines and turn the floor c sections 90 degrees. The c sections will still span 6m but in the perpendicular direction.

 

[Tezza91]

Yeah jut to be clear the flooring system is to be C Section purlins and 38mm chipboard

 

[r13]

Just a reminder, the floor system should be stiff enough to brace the floor beams in its weak axis. What is chipboard, is that checkered floor plate?

 

[12345abc6ttyui67]

Chipboard is a UK term form for what you folks over the atlantic call OSB as far as I know.