Software Safe

[Soco]

Dear all,

I am currently working on the design of a raft piled foundations of a tower in Singapour:
The design office uses the software safe to model the raft and puts springs to take into account the piles:

The questions are the following:

1) With Safe, Can we take into account the direct transmission of the loads into the piles? (to reduce the effect of bending into the raft)

2) Does the software safe take into account the rigidity of the superstructure that tends to reduce the bending moment in the raft.

Thank you in advance for your help.

Alan

 

[gwolf]

"safe" is an uncommon FE package, If you don't get any posts it's because no-one has used it rather than ignoring you.

 

[rapt]

I have not personally used safe but would make the following comments from an engineering logic perspective for your 2 points

1 The piles, soil and raft will act together. You cannot asssume that some load will go directly into the piles as, when the piles shorten under load, some of the load will be trasnferred to the soil through the interaction of the 3 elements. That is what the finite elementanalysis is attempting to model for you.
The only way for load to go to piles only would be for the column and piles to be independent of the raft.

2 To allow for the rigidity of the superstructure you would need software that models the construction sequence with the loads being applied as the structure is constructed and then live loads added to the final structure. I do not think any of the CSI or many FEM products worldwide will do this for you. Doesn't safe only model the floor being designed? If so, it definitely cannot do this.

This actually shows the inaccuracy inherent in any analysis software for concrete buildings. FEM software is only accurate for the model that is defined. It cannot allow for inaccuracies in the model compared to real life (and these differences can be enormous) and cannot allow for inaccuracies in the modelling of the materials used, eg reinfoprced concrete cracks shrinks and creeps etc and the effects of these can be more than the vertical load effects and completely change the stiffness of the members in the model and consequently the results

 

[Soco]

Thank you very much for the answer.
I precise that indeed my intention is to assume that the loads inside the critical shear perimeter just go into the piles.
This doesn't seem yto be implicitly writen in the codes. I am currently working on BS.
The fact is that, they explain the cross section where critical shear has to be calculated but this point of direct transmission doesn't seem to be implicit in the codes.
If someone has informations on this point, that would be very nice.
I have received a demonstration of Safe, so I can see the features of Safes. But now, I am thinking about the concepts. We define critical shear at a certain distance of the periphery of the column, so What is inside just go directly to the piles. Am I wrong?

Thank you very much in advance.

 

[rapt]

Soco,

See answer 1 above.

The piles will axially shorten due to the compression in them. Especially in a country like Singapore where the piles will be very long to reach stable foundation.

Also, the pliles will probably not be resting on solid rock so will settle further under compression load.

Because the piles and raft are connected and must settle together, these 2 effects above will cause some of the load to be carried by the piles and some by the raft. So you cannot assume all of the load from columns directly over or within the shear perimenter of piles will go into those piles.

If you are designing a piled raft, you have to determine the loads being carried by piles and raft based on the interaction of the 2 using a proper piled raft analysis and design each accordingly.