How to achieve large open span/area on residential 1st story. 1

[massengineer]

I am considering a home expansion and would like a large open area on the 1st floor (actually 2nd floor over an extension of garage due to grade). Say 24' x 24'to32'.

Considering modular construction, dropping in rooms by crane to form the upstairs with bedrooms, it seems to me such a module should be rigid enough to span the distance. Construct long walls as a truss.

A truss 24' long and 8' deep is only in length three times its depth. This seems to me (albeit an electrical engineer) like a piece of cake. And so much more elegant and efficient than the brute-force massive beam approach.

Why do I not see, hear of, or read of construction like this to achieve large open spans on a lower story? Why can't the 2nd story just hold itself up?

I imagine a truss with wood lam beams top and bottom, sandwiching cross members to make a classic bridge-like truss. I saw a pic on web of something like this where the diagonal cross members were tubular steel, bolted at the ends between two parallel lam beams.

Perhaps such a truss could be dropped on a completed lower foundation, and modular rooms could then be dropped in (and somehow supported by truss, maybe joists rest on top of beam?).

Any comments from those more knowledgeable in this field?
Would it be easy for a P.E. structural engineer to design such a scheme and sign off on it?

And what about suspending a story from the apex of the roof? Old firehouses used to commonly suspend the 2nd story by iron rods from the roof to achieve the necessary large open span on the 1st floor.

thank for any comments anyone could provide.

 

[RARWOOD]

Based on experience I think a draw back to your idea would be the expense of the trusses. In addition I believe your engineering cost would be high in comparsion to the cost of your materials.

As far as designing your system it would not be a difficult problem for an experienced structural engineer. However your system would probably require an engineered design for almost all the compoents. While with a typical house a lot of the elements are just constructed following common practice.

 

[MiketheEngineer]

RARSWC is right on concerning the money. Not hard to do - but the engineering costs and truss costs might be a bit high. Watch your deflections.

Also - trusses don't like windows and especially doors. HVAC and plumbing might be a bit dicey.

I once did a retail building where trusses were used as bearing walls between the bays....still standing after 30+ years!!

Good Luck

 

[dik]

I've used parallel chord wood trusses, open web steel joists, and 8" HC slabs... all work well.

Dik

 

[civilperson]

I will do the structural engineering for $3/sq. ft. You get someone else for the plumbing, HVAC, windows, doors, hardware schedules, drafting of site plan, floor plan, elevations, sections and details.

 

[massengineer]

Thanks for the replies. I see the way this works I can't send individual emails to respondents.
It appears that I am not missing something here, my idea that a truss is very strong if it is only a few times longer than its depth is sound. The only drawback seems to be that doing something non-standard runs into engineering expense. I need to sell some property to have the cash to implement this addition, and in the meantime I will do more research about the cost implications and what I could do to minimize that.
Thanks for your replies.

 

[dik]

In these environs, lumber yards or truss manufacturers supplying the materials will often provide framing information. or a local contractor can be approached...

Dik

 

[boo1]

dik has a good point, sketch your design and bring it in to your local truss shop and ask them for there design ideas.

 

[Tmoose]

If the open area is to be 24' X 24' on the second floor won't you be faced with spanning 24 feet between the wall-trusses?