Dead Load Deformation for a Tall Building

[NewImhotep]

Dear all
I'm working in a high rise building (about 70 stories Residential Building)
The system is post tensioned flat slab with RC shear walls & Columns.
We got a high lateral deformation in the suspended slabs due to dead load besides the lateral forces drift.
The lateral drift satisfied H/650

The problems are:
-these deformations (from dead load only) making a high sagging and hogging moment near the slabs supports ( higher than deformations due to lateral forces)
-each story deform in a different shape which cases a different moment for each floor which is very difficult to study for each floor.
- the post tensioning design will be done by another firm so what we have to do is estimate the differences between the moment due to gravity and from the deformation shape, because the other firm well design tendons for gravity loads only(the ultimate one).
I argued with my senior about the load combination which we have to consider in this case, he provide ultimate one but I suggest to take the service one because the deformation should be a serviceability issue but he think we can't compare between ultimate case and service case.

Does any body familiar with this issue? Is it common to get the high deformation due to dead load? And how u investigates it?

Best Regards
Ahmed

 

[MotorCity]

I'm not sure I understand, you have large lateral deformation due to gravity loads? This situation seems unusual to me. It also seems unusual that these lateral deformations would be greater than those caused by lateral forces. If you have these problems with only dead load, one would expect that these deformations would only amplify with live load. Something else to consider would be why the deformations due to gravity loads are different at each floor (assuming that the geometry and load patterns are similar). If anything, I would expect the dead load to counteract the effects of lateral deformation. If you are coming up with this model using software, you may want to double check your input. Good luck.

 

[jike]

I guess that you are talking about the additional slab moment in the upper floors due to the differential axial shortening of the interior columns versus the exterior columns. Is this correct?

 

[theonlynamenottaken]

I also don't understand how you're getting any lateral displacements due to dead load that aren't negligible. Is the structure very antisymmetrical? Have you taken column shortening into effect and sequentially analyzed the structure?

 

[jike]

Lack of geometrical symmetry or lack of loading symmetry can cause lateral displacement.

 

[NewImhotep]

MotorCity
Yes u understood right the problem, More lateral deformation from dead load
actually what happened that the building has about 15 different levels and there are some changes in the columns lay out like floated columns on a beam girder that duo to the architectural shape.
but about 70 % of the stiffness in the core which is definitely in his place in all the floors.
the building has been modeled by ETABS with not by me cuz i joined there recently
stiffness of all members has been reduced (70% for walls & 35% for Slabs & beams)
, and all the column ,Walls , building members and stability check already done. only some parts unfinished like the slabs action .
now I'm exporting the floors from etabs to safe and i getting a very high sagging moment (about 350 KN.m for 260mm slab!) in the moment near the supports and the moment diagram be very difference than the case with out deformation.

the deformation due to seismic also significant but it's less than the dead load and it's increasing gradually when u go up in contrast with dead load Deform which irregularly varying between floors.

ETABS model has been done by Experts and a bout 4 Engineers
so i don't think there is problem in the model.

 

[NewImhotep]

u can consider the building as unsymmetrical
Etabs Results should be combinations of axial shortening and the distortion of supports.
also axial shortening cases deformation in x,y & z direction due to irregularity of building.

may be I have to ask a new questions


are the etabs results due to this matter should be taken as it?

what about the effects of creep and construction sequences ? is it going to give different results?
and with which case we have to investigate this ultimate or service ?

 

[MotorCity]

A couple of things to look at: why such a large reduction in the stiffness of structural elements? This definitely will contribute to your deformation dilemma. Also,the deflections should be checked at service loads as this is a service, not a strength criteria. As for the software, I have found that most of the time when things don't look right they probably are'nt. You may want to check that all of your units and other design parameters are consistent when exporting from one software to another. Review the model generated by the "experts", they make mistakes too. I assume that this 70 story building is a steel frame?

 

[J1D]

It seems to be a RC structure with post-tensioning in flat slab as per the first message.

AhmedR, how about check the lateral deflection diagram (due to DL) along a maijor column and a shear wall and check the base reactions to see the reasons. Or the shear walls are very eccentrically located, then the shortening may cause the problem.

I don't know how can you design member strength without post-tension info and how can the other company design the post-tensioning for gravity load only.

 

[AUCE98]

Just an item of intrest, if the structure is post-tensioned, your assumption of the beams and slabs having an applied cracked factor of 0.35 is incorrect. I would assume the slabs and beams, if post-tensioned, would have a crack factor of 0.90 depending on the stress limit being designed to. Additionally, ACI permits an increase in moment of inertia of 1.43x values indicated for various members (10.11.1 Commentary, ACI 318-99. These may help in the lateral deflection of the building.

Regards,

AUCE98