Erection load case for steel frame

[Contraflexure74]

Hi,

I've been asked to design a steel frame for sway during the erection load case. In the permanent load case the frame will be braced by rc cores. My issue is the builder wants to erect the frame before the cores are built.

My question is what load do I apply to the frames to design bracing members during erection and is there a rule of thumb in terms of the distances between braced bays as my building is 150m long?

John.

 

[XR250]

Will the cladding be on the building before the cores are constructed?

 

[Contraflexure74]

No cladding won't be on.

 

[Lomarandil]

I don't know about the building world per se, but in temporary structures for construction we often apply 2% (short exposure period of a few days, low consequence of failure, non-seismic) or 5% (seismic region, high risk, or longer exposure period) of DL as minimum lateral load thresholds to ensure stability.

Depending on the structure and exposure period, you might also consider wind load with a lower design return period (per ASCE 37 or local experience)

 

[oldestguy]

Is there any possibility that a model can be tested?

 

[Contraflexure74]

Yeah 2% of DL sounds reasonable + whatever wind the frame catches. Thanks.

 

[WillisV]

ASCE 37 + AISC DG10 cover applicable loading to use on buildings under construction. You will likely find that the base shear for the open structure is higher than that of the final clad building resulting in a ridiculous amount of cabling and temporary bracing required by the numbers and a pissed off erector. Enjoy!

 

[Contraflexure74]

I wouldn't be familiar with these codes on this side of the globe (UK) but thanks.

 

[calvinandhobbes10]

Sounds like you have a steel frame minus its lateral system (RC core-shearwall)---is this correct?
Have never heard of the shearcore being built after steel framing in this order. Interesting challenge.

A few factors would influence my decision on this:
---Is the RC core indeed the ONLY permanent LRFS element?
---How many stories? RC core implies 4 or more.
---Will the floor decks (presumably composite steel) be poured in advance of the RC core?

Assuming the worst (4+ stories), decks poured, no other lateral---you need to create a lateral system without the core. Knee brace kickers and/or temporary braces, or possibly X-braced cables, are some options that come to mind. The braces should be added prior to concrete floor pours, which cause the potential for racking (as well as adding seismic load).
As Willis pointed out, the GC will quickly learn why most buildings don't go up in this order.

Use 70% of wind for temporary loading (1 year or less). I'd just use your Equivalent Lateral Force Cs value for seismic.

 

[KootK]

I would not dismiss AISC DG10. It's not British code based but those wise rules of thumb that you seek are probably there nonetheless. If your design case is sans decking, you've probably got cable bracing on all of your framing lines due to the absence of a diaphragm. If your design case includes decking, then you can get away with less. Connecting your diapgragm to the cores may pose challenges if the walls will go in after floor deck concrete is poured etc.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Contraflexure74]

Thanks guys very informative and good points which I will discuss with contractor. Thanks again.