Two direction concrete slab supported on HSS columns 3

[smcp4blo]

Hello, I have a question, if anyone helps i'd be really glad.

I've been asked to designed a small house. This house has a two way slab (concrete) without beams and the architect wants to support it on HSS columns of 4". I don't know if this is possible, since I'm used to designing steel frames (columns and beams) and then on top of it putting the concrete slab. However the architect doesn't want any beams, just the columns and the slab, is it possible? And if it is, how would the connection be?

Thanks for the comments. I'm in a hurry to make this design so any answers would be greatly appreciated.

 

[hokie66]

Yes, it is possible. Use cap plates on the columns, design slab for punching shear to suit size of cap plates. Connect cap plates to slab, maybe with welded studs.

Otherwise, just like any other flat plate design. Assume supports pinned. For lateral loading, use shear walls or similar.

 

[smcp4blo]

Thanks!

Yet, if I do it the 2nd way "any other flat plate design" would I just sit the slab upon the cap plates without anything joining them?

 

[hokie66]

I didn't mean that there were two ways of doing it. A flat plate it is. Just not supported on concrete columns. Connect the slab and columns.

 

[grantstructure]

You'll probably have to use stud rails to get punching to work. Also, that cap plate has to be very stiff.

Is there some means of providing lateral stability other than the 4" columns?

 

[smcp4blo]

How would I connect the Steel HSS column to the concrete slab so I can transfer moments? Any paper?

 

[hokie66]

What moments? The slab will be much stiffer than the little columns, so just design the slab with pinned supports.

 

[frv]

Hokie is right. Use X-Bracing for LFRS.

 

[tony1851]

But has the architect included any bracing, X or otherwise?
If he wants to support a house on 4" HSS, wouldn't be at all surprised if the need for bracing never entered the architect's head

 

[shobroco]

Maybe not, but I've never seen a house without some walls; even if the exterior is all glass there must be some partitions. Even though you guys might like a transparent house (sorry for the sexist comment) someone wants to obscure the toilet & shower from somewhere. I've done some pretty far-out houses & always found a spot for a shearwall or a few braces.

 

[abusementpark]

Sounds fine for gravity load but what about lateral? You're not going to be able to put bending into 4" columns.

 

[smcp4blo]

Thank you all for the responses.

The thing is that this is a remodeling... so, the old part already has several walls and lateral stiffness, so I'm not that worried for lateral loads. Nevertheless i'll check it of course, and try to solve it just with the rigidity of the columns (maybe HSS4x0.25). I'm putting several columns, like ever 1.2 meters (4 feet), besides it's a really low seismic and wind zone.

The complicated things are now 2 things:
1. The architect wants a waffle slab, however this slab (10x5 meters) is supported over 1 wall, 1 beam and then HSS on the other 2 sides. However, this HSS aren't on the perimeter exactly, the slab overhangs 1.75 meters on one side, and 2.4 meters on the other side. So... you can imagine this being a little complicated. I'm proposing the use of flat beams since he doesn't want the beam to show (my slab is 30 cm = 1 feet). I may be able to solve it using flat beams, but my question is regarding analysis...All of these are solved by simply supporting, but I guess this is more like a flexible slab with flexible beams so I'm worried that in reallity my beams will be transferring moments to my HSS Columns. Any comments?

2. The connection of my concrete slab/flat beams to the HSS columns can be solved by putting a cap plate, stiffeners and some shear connectors, but what if I have moments?

I attach a drawing just in case I'm not being clear.

 

[BAretired]

Assume North is at top of sketch.

What is the crossed area to the West of the N-S beam? Is it an opening in the floor?

What does the solid turquoise square represent?

Why are the HSS columns positioned as shown? Doesn't seem to make sense.

Should there be an edge member on the South and East edges of the floor area?



BA

 

[smcp4blo]

The crossed area is an opening for stairs. The solid square is a concrete column. The HSS are positioned as that since that is the principal request of the architect. I'm not obligated to use them all as structural elements of course, but some may help to transfer loads into the foundations. On the south and east edges I can put some flat beams also, but i'm trying not to.

 

[BAretired]

The waffle slab will behave pretty much as a one way joist system spanning N-S. The E-W ribs will do very little except possibly those south of the E-W beam.

The E-W beam will behave as a span and cantilever.

The only HSS column which carries any load is the one at the junction of the two beams. The remainder are useless, in fact if they are attached, some probably act in tension and thus increase the load on the solitary working column.

A depth of 300 mm is not sufficient to prevent excessive deflection of a 2750mm cantilever.

So far as moment is concerned, I would assume that the rotation of the top of the HSS is equal to that of the beam pair as the contribution of column to stiffness is negligible.

BA

 

[tony1851]

What supports the slab to the east of the 4 columns against the stairwell? Am I missing something?

 

[smcp4blo]

BA - Yeah that's what I'm thinking about, the slab will behave in one direction so that other way can be just as aesthethics or temperature ribs. About the HSS I understand the one that intersects with the beams will receive more loads, the other ones can be neglected or I could take some to consideration, that's no problem. I'll consider your comments regarding the moment, thanks.

Tony - Nothing supports it, that is why i'm considering putting those 2 turqouise / cyan beams to the right so slab can be supported on the beams, if not I'd have to consider the beam next to those 4 HSS (next to the opening) to take torsion.

 

[BAretired]

What supports the slab to the east of the 4 columns against the stairwell? Am I missing something?

The only thing that can, namely the one way action of the waffle slap spanning north to south from beam to beam.

The structural system is not good, but it does work.


BA

 

[smcp4blo]

Actually I was thinking making the design as a one way slab (N-S) with 'temperature ribs' on the other way (W-E). Perhaps designing the structural ribs of 15cm of width and the other ones just 10cm.

 

[BAretired]

Your main problem, is going to be deflection. The system is going to be too flexible, particularly at the S-E corner.

BA