The title is too obvious right? Well not really.
Imagine you are into this situation:
A variation order just came from the owner requiring new lift and stair core walls and piling is already done and you’re required to give feedback immediately regarding the possibility of adding piles adjacent to existing ones which are originally just single piles. If you’re a newbie structural designer or still a few years from professional infancy, you would think instantly (right?) that you just need to add piles, go to the client and tell him and the issue is resolved.
Right?
Well I made that mistake. Not to the extent that I told the client myself but by thinking that just adding piles to existing ones for new corewalls is necessary to make the old piles work. But based on the results of SAFE, this required more thought than what I presumed was enough.
Tension, Tension, Tension
When, before adding new corewalls, and you experienced piles in tension, with 99% probability the corewalls and the supporting piles you added will likewise experience tension.
One important thing to note also is that for single piles which usually supports one column, the maximum reaction will come from gravity loads. Lateral loads might add a few kiloNewtons of reaction but it is nothing like how lateral load affects corewalls. Sometimes, load combinations including lateral loads do not give the maximum pile reactions compared to the gravity load combination. Go ahead and try to prove this one.
So what’s the connection you say?
When we prepare the pile reaction table, you might have provided minimal if not zero maximum tensile loads to the single piles because they are not expected to resist tension anyway since the corewalls should carry the bulk of the sway due to the lateral load. So if you provide new corewalls on top of these originally compression-only piles, how are they going to resist tension when they were already detailed as compression-only and the piling work is done and gone?
Of course you can always ask the geotechnical guy on what other fat can he let go just to have some tensile values. But like in my case where the maximum tension induced is 4300 kN from formerly 0 kN tension capacity (maximum compressive load that the pile can resist is 6000 kN) do you know think geo guy can give that much? I don’t think so either.
What Now?
In my situation, we might just remove the proposed additional piles and provide a beams spanning from pile cap to pile cap of existing piles to support the block works enclosing the stairs and the lift in order to avoid this tension. In that way, the loads are limited to compression loads and the effects will be localized only and I won’t be needing to check the other corewalls already in place, at rest and in peace. Or you can propose a new location for the corewalls which will not use the already casted piles…
– or, like what we structural engineers constantly face, wait for more challenging updates from the owner/architects.
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