Added Bar Lengths in Two Way Slabs

[KrisM]

I am a contractor looking for some help with regard to two way slab estimating. In particular verifying lengths of added steel for both column and middle strip areas. No one that I have been talking with has been able to give me a straight answer and when I compare an old shop drawing to calculated lengths based on the chart from ACI Code 13.4.8 I do not get the same number that the detailer used. Thanks in advance.

KrisM

 

[hokie66]

To get a straight answer, ask the design engineer.

 

[asixth]

Lengths of added steel?? What is this referring to?

Top steel over the supports in both the middle and column strips should be a minimum length of 0.5 x span length (staggered 0.3 and 0.2L either side of the support).

The column strip will have additional bars that account for approximatley 25% of the total steel in the column strip. This accounts for the concentration of moment that occurs at the face of the support.

 

[kslee1000]

hokie66 is correct, it is the designers responsibility to provide information on partial span length bars by either marking the length/range on the drawing, or cover it under notes for erection. Because only he/she knows the exact intent. You may guess now, but to be rejected during the shop drawing review process, or site inspection stage. Worst of all, you may interpret the code in a different manner than the designer, and make mistakes. You can challenge his design once you know his/her intent, if it is worth the fight.

 

[mudflaps]

Also keep in mind that there may be some bays if different size than others. Most designers will not chase after a perfect design for every odd bay, electing to design three or four bays and use one for similar smaller bays. In those bays there may well indeed be too much rebar. The more complex the design, the more complex the drawings, the better chance for a screw up all along the way.

Kiss!

Old CA SE

 

[kslee1000]

mudflaps:

Fully agree with you, at least it was my practice - to minimize complicate designs that causing high labor costs and fibraction/construction errors. I think the labor cost alone can fully compensate (in the US) the added reinforcement for most of the cases.

 

[civilperson]

Rebar schedules with bar labels and total lengths/weight are normally provided for structural concrete.

 

[hokie66]

Like structural shop drawings, yes. But not on the design drawings for buildings, which I believe the OP is asking about.

 

[neillydavies]

Yes, I agree with "mudflaps", I avoid complecated detailing and especially the distances are within a short range we provide the same design/detailing.

Neilly Davies Consulting Engineers

http://www.neillydavies.com.au

 

[asixth]

I lap the top steel midspan for short bays, this takes care of any negative moment that may occur due to the irregularites of the span.

 

[jheidt2543]

My understanding of the question is that you are trying to compare ESTIMATED rebar quantities to ACTUAL.

During my time as a construction estimater, we took off the rebar shown on the structural drawings and added 10% to our take-off quantities to cover rebar lap and other contingencies. We checked quite regularly and worked it very well for us. It does make some difference on the larger sizes of rebar, so theoretically you could work out different percentages for each size of bar.