Columns braced or not - Massive cost influence

[sybie99]

I have the following scenario, which I will try to describe as best I can:

I have a concrete frame structure, on 2 sides the structure is built into an embankment with retaining walls. Its a flat slab structure, and at the 2 sides where the retaining walls are, the detail is for the slab to key into the top of the retaining wall, but no structural connection. Only friction as we do not want moment transfer between the wall and slab will result in some cracking in the slab which cannot be designed for this moment.

So, the big question is, do I design the columns as braced or unbraced? The British Standards, BS8110, states that "a column may be considered braced in a certain plane if lateral stability is given by walls or bracing designed to resist all lateral loads in that plane"

Now, windload on this struture is only from two sides as it is not exposed on the other 2 faces. The windward sides are opposite the embanked/retained side, so any windload will push the building into the retaining walls, so it is stable in those 2 directions.

Now, can I design the columns as braced seeing that there are no lateral loads in the 2 unbraced directions? Or should I allow for some lateral force other than wind? This is not a seismic area. Where the slab rests on the key joints at the top of the retaining walls, a large frictional force can be created which would assist in countering any force in the unbraced direction.

The reason for me not just designing as unbraced, is that there is a huge difference in cost, basically the column areas double and so too reinforcment.

Thanks guys

 

[BAretired]

If it's not too late, I suggest you review your decision not to tie the wall and slab together with reinforcement. It strikes me as the wrong decision.

BA

 

[ron9876]

BA wouldn't recommend tieing the slab into the walls because you may get restraint cracking.

You mention windward wind load but how about leeward? What are you using to resist lateral loads on the overall structure? I don't think you should count on the retaining walls to brace the columns.

 

[hokie66]

I agree with BA. As the slab bears on the wall and "a large frictional force can be created", the slab will be restrained, and crack control reinforcement will be required. I would therefore bend the wall reinforcement into the slab, creating a reliable diaphragm connection parallel to the wall sides.

 

[sybie99]

Thanks for your replies gents, I think the safe option is to design it as a sway frame, I.e. unbraced.

 

[tclat]

Wind directions can change, even if rare and it can act as a pressure or suction.