Deflection limits for a composite transfer beam

[csd72]

What limits would people be looking to design a transfer beam that supports a concrete column and post tensioned flat slab at the level above and a reinforced concrete slab at its wown level. I have a consultant that has designed a transfer beam that is no stiffer than your average floor beam and I am trying to find something down on paper to back up my professional opinion.

Are there any references that suggest a limit for these? I am struggling to find anything. European references would be excellent but any reference would be very much appreciated.

 

[WillisV]

I do not have a reference but would generally design a transfer situation like that for a max. deflection of about 3/4" max in 30 feet (L/500ish) for the superimposed dead+live loads from floor(s) above.

 

[structSU10]

I design these very stiff, about what WillisV said. Its deflection affects upper floors, so the compounding affects should be considered.

 

[msquared48]

I would design to a 1/2" max deflection for the dead load of the column and slabs on the non-composite section, and a total of 3/4" with the live load on the composite section.

Mike McCann
MMC Engineering

 

[csd72]

Thanks,

That is about where I am coming from.

The transfer beam that I am looking at has a 2.25 inch total deflection in a 42foot span with the coluumns supporting the middle of the PT slab above. The design consultants just dont get my concerns, they keep stating that it is 'to code' but the code does not specifically cover this situation.

I am amazed that there seems to be no guidance on these. Maybe I will write a paper.

 

[steellion]

You may not be able to point to anything specific in the code, but I think your judgment is correct: way too much deflection. Code-mandated deflection requirements did not even appear in the IBC until 2003, I believe. Stick to your guns on this one.

 

[rapt]

Stricter deflection limits should definitely be applied. The latest version of the Australian concrete code limits transfer member deflections to
L/500 if support settlement effects are allowed for in the design of the supported structure
L/1000 if support settlement is ignopred in the design of the supported structure.

 

[asixth]

I use the same deflection criteria as rapt for transfer beams. I think that the 2.25 inch is well over the limit.

 

[rapt]

csd72,

Technically for the transfer beam, according to the code, it is ok. The problem is for the design of the floors above which must account for the support settlement. This will require bottom reinforcement at the columns and more reinforcement in the bottom at midspan! And also take into account the settlement in calculating the slopes and deflections of the floors above for deflection control(L/D limits are actually a slope limit) and also for the slopes on the roof for runoff.

They are simply ignoring the need to design the floors above for all of the action effects on them.

 

[csd72]

Rapt,

Thanks, that was the direction that I was thinking and was concerned about, particularly for a pt slab above.

We have resolved the main issue of the moments by letting the transfer slab sag under the weight of the wet concrete. This is not perfect but I had three structural consultants swearing blind that it was fine and as my responsibility is for temporary works then I had to concede that it was their risk. Not 100% happy with the compromise but I had a heavy handed director push it in that direction.

I will try and get my hands on those pages from AS3600 as they may be useful in future.

Thanks everyone for the info.

 

[rapt]

csd72,

I did a report on a similar case a few years ago in northern UK.

16m RC transfer beam supporting a few floors of 8m span PT flat slab. Transfer beam deflection at about 6 months was about 65mm and rising. Unfortunately the transfer beams were left propped during constructiuon of the slabs which made the situation even worse. But it still would have been a major problem without this as the initial transfer beam deflection was relatively small (compared to the 65mm) under the beams own weigth as it was uncracked under that condition and no long term effects.

Cracks in the bottom of the PT slabs at the supports and meandering throughout the slab were well over 1mm and increasing with time, all due to support settlement caused by the transfer beam deflection combined with the balanced load from the prestress negating the effects on the slab stresses from the slab self weight (situation is not as bad for RC slabs).

Repairs cost a lot and cracking will probably increase with time and extend to midspan as loading is applied causing more transfer beam deflection and increased +ve moments significantly above the design values!

Hope they have a lot of fully lapped bottom reinforcement throughout the slab!