Steel Frame Erectors:Temporary Bracing? 2

[IJR]

Consider a 4-storey building with glass cladding required by architects. No bracing is accepted, but shear wall is accepted around stairs. So the structural system defaults to steel columns with simply supported beams to carry trapezoidal deck topped with concrete serving as floor, and lateral stability is guaranteed by shear walls. Now here is the problem:

Steelwork will be erected first and concrete for shear wall and floors will be poured later. So how will lateral stability be guaranteed as concrete is poured? Some form of easy to remove temporary bracing?


Thanks in advance.

 

[Ginger]

Bracing will obviously be needed. For simplicity just bolt your bracing to the columns. But simplicity is not necessarily your primary concern as the holes left in the columns after you remove the bracing will be visible through your glass cladding.

So, why not make your beam/column connections into moment connections with sufficient capacity to carry construction based sway loads. The shear walls will be in place before the cladding is erected (presumably).

Otherwise, insist that the shear walls are constructed first.

Regards

Andy Machon
Andy@machona.freeserve.co.uk

 

[IJR]

Thanks Ginger for immediate response

 

[IJR]

Andy, how about connecting bracings to beams, again bolted but eccentrically?. I can hide the connections on top of beams with concrete topping and below beams with false ceiling. Should work, shouldnt it

Thanks

 

[Riz]

IJR,
Provided u have the scaffolding in place I do not anticipate any problem due to lateral forces. Have the scaffoldings in place until u finish all concrete works ie shear walls and slab.
I am supposing that u have a braced structure with shear walls taking all lateral forces. R they wind only? As I have understood the shear walls are located around the stairs. Is there appreciable eccentricity between shear walls and building centroid? U need to check torsional response as well against the minimum allowed. ( 2.5% width - BS )
I would suggest u design for a dual system with both frame and shear walls resisting lateral loads.
Hope this is not scary
Riz

 

[IJR]

Thanks Riz and thanks for the spreadsheet too.

I can take care of the torsion. No problem. And seems you and Ginger think of moment connections as sufficient. And I will probably do that.

Now how should I design the moment connections?. I am designing not for wind but for seismic effects of high intensity(eq to UBC Zone 4) and I was thinking using wind moment connections (angles attached to flanges) will stabilize my structure during pours of concrete. You think that would work. Design for wind.

Next, wont moment connections be more expensive than temporary scaffolds(bracings.

Thanks once more and again

 

[JAE]

IJR:

Are your shearwalls cast-in-place concrete? Usually, the concrete "shafts" that serve as lateral resisting systems in buildings such as yours are constructed first, prior to steel going up. The shaft serves as a lateral brace for all the framing during and after construction. The contractor simply jump-forms the shafts.

Once the shafts are constructed, the steel framing (Non-Self Supporting Steel Frame per AISC definition) is erected.

Another issue.....usually in the U.S., the means and methods of constructing a framed building are the domain of the contractor or erector (See AISC Code of Standard Practice, Section 7.9.3). Are you the engineer-of-record and/or are you working for the contractor/erector?

 

[IJR]

Thanks very much JAE.

I have no word against the contractor as to how he will pour concrete and he insists that he will cast shear walls in place together with concrete on floor decks only after steel erectors have finished(and may be quit the site)

In my opinion your reasoning is more logical. But this seems to be a difficult case.

Thanks once more and again. Nice weekend