volumetric changes on building 1

[newbuilder]

I have come across a situation where an engineer has undertaken a detailed analysis of the bending moments induced in the columns of a tall building caused by shrinkage. I have been able to find a small reference on this kind of effect in one text but not much.

My question is this: in a tall reinforced concrete building, the floors would be expected to shrink at roughly the same rate and time. Would any forces theoretically induced in one subframe by the restraint (fixity) between it and the floors above and below be of serious consideration or would they cancel?

The building in question comprises a post tensioned floor scheme; however, would this make a difference on the quantum frame forces. Understandably the shrinkage affects the PT losses but i think this is a separate issue.

 

[RiBeneke]

I cannot give you a definite answer on multi-storey buildings, except to say that the shrinkage forces are always constrained to some extent by the amount of reinforcing steel used.

However, a related example I came across recently was the case of a 2-storey large-area parking garage with post-tensioned slabs. The designers had spaced the expansion joints up to 50 m apart, presumably to reduce post-tensioning operations. There were bending moment cracks at the tops of many of the columns where they joined the upper slab. This was presumably caused by the slab shortening after post-tensioning. The garage was under construction at the time and the cracks were being plastered closed .. not an elegant solution.

Regards
Richard Beneke

 

[newbuilder]

Wow, I can't believe anyone actually replied after all this time!

Thanks for the case study. 50m joints are normal for regular shaped rc buildings
and perhaps the frame analysis is specific to PT type structures. I'm sure our brothers in the bridge business may know.

Was it actually possible to determine if the column cracks were due to PT?

nb

 

[RiBeneke]

I was just visiting the site and did not get a clear answer when I asked questions.

However, I did notice that the cracking at the column heads occurred mostly near the perimeter of the slab.

Richard Beneke

 

[abdelmageed]

dear newbuilder,,
Going through the case u raised i would like to comment as follows:-
1/In general the shortening of slabs causes stresses at the columns connections inducing moments which need to be considered in columns design,,,(this is why nominal moments are to be considered in columns axially loaded only,,,caring of construction imperfection and unforecast cases),,,Besides the crack is to be injected if required,,especially shrinkages crack will be encountered alot during exposed construction,,,
2/This cracking is to be addressed seriously if it is deep,but as it happened during construction, this may be due to stressed concrete as it is immature,,,,
3/Main topic to be considered is the effects of (shrinkage, creep and elastic shortening of columns) as they shorten by time causing stressed tilting slabs, this is crucial as the system is PT-system,,,,
abdelmageed

 

[rapt]

The effects of shortening due to shrinkage will normally only be significant on the columns at the footing level. At this level, the slabs will shorten and the footings stay where they were so significant moments could be induced in the columns. At other levels in the building the slabs will tend to move the approximately same and not induce large stresses in the columns.

Shortening due to temperature difference/change could be more significant. Temperatures could vary wildly between floors and also vary significantly from the construction temperature. Effects such as snow, high temperatures, air-conditioning could induce stresses higher then the shrinkage ones in the columns.

Effects of elastic shortening due to prestress will affect, to different degrees, all of the columns but the amount of shortening is significantly less than that for shrinkage and temperature effects in the bottom lifts of columns.

If the shringage effects are calculated based on an elastic analysis, the moments in the columns will probably be over-estimated. Effects of creep and cracking in the columns over time will relieve the moments in the columns. The final amount of moment in the columns is impossible to calculate. A very old PCI manual I have nominates figures for this based on rebar percentages in the columns. I presume the latest one still does.

 

[Ingenuity]

rapt,

i cannot seem to find that column data vs rebar % in the 6th edition of the PCI manual - can you tell what section of your copy it is in?

thanks

 

[abdelmageed]

dear,,,NEW BUILDER AND APT
As far as volumetric changes in braced high-rise building with post-tensioningare concerned,i do re-recommend the following:-
1/Columns design is to address the moments caused by the lateral effects due to post-tensioning effects(simplified conjugate beam method helps)
2/Columns shortening due to (elastic shortening+shrinkage+creep) is accumulative value as u go up, not as many think it is higher at foundation level columns,,,the issue is the differential movement of columns relative to the core concrete walls ,,,this causes the stresses in the slab,,,
3/As building is under construction, u need to address the volumetric changes with great care, but as the building gets finished many other factors are considered,,,so crtically,,,it is when under construction...
4/Building robustness is of high concern for hi-rise buildings,horizontally, at periphery, between columns and vertically,,,consult british code bs8110 part1&2,,,
,,,,hope u the best

 

[rapt]

Ingenuity,

It is in section 4.4.2 Equivalent Volumn Change of the 2nd edition(I told you it was old).

The Kl term which varies from 3 (heavily reinforced) to 5 (lightly reinforced)allows for this.

 

[Ingenuity]

rapt,

found it...thanks...for those who have the 5th edition of PCI it is section 3.8.5.1 Equivalent Volume Change.