Span depth Ratio ACI vs BS

[SemiPE]

Hi All, I have been doing some comparison with ACI and BS code. The span depth ratio values given in both codes are not far off with each other. Having dealt more with ACI, I understood that the span depth ratio only satisfies the requirement of immediate deflections and is only applicable for beams that is not sensitive or critical for long-term deflection such as those not supporting brittle finishes or partitions likely to be damaged by deflection. This is clearly in ACI section 9.5.2.

The British code, on the other hand, stated it in a more different manner. As stated in BS 8110 sec 3.4.6.3 "The basic span/effective depth ratios for beams are given in Table 3.9. These are based on limiting the total deflection to span/250 and this should normally ensure that the part of the deflection occurring after construction of finishes and partitions will be limited to span/500 or 20 mm..."

This tells me that, under the BS code, using the span depth ratio is enough to control the long-term deflection of beams sensitive to this kind of deflection. On contrast to this, ACI requires a thorough analysis of deflection (use crack properties, shrinkage, creep coef, etc.) to ensure long-term deflection requirements of beams carrying deflection sensitive finishes.

Am I understanding this correctly? or am I missing something. Its quite odd that the british standard allowed the use of span depth ratio to ensure long-term deflection of beams carrying brittle finishes.

 

[KootK]

My understanding is the same as yours SemiPE. For what it's worth, I know that a lot of practitioners in North America treat the ACI limits as sufficient for brittle finishes.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SemiPE]

Thank you for the response kootk. From your comment about the practice in North America over the ACI S/D ratio, it seemed that I shouldn't be too worried about the brittle finishes and the whole long-term deflection thing (as long I used S/D ratio in sizing my beams), since you guys seemed to have no problem with your structures over there.

 

[hokie66]

I don't think that KootK meant that "you guys seemed to have no problem with your structures over there". Deflection/serviceability issues are as much of a concern in North America as anywhere else.

 

[KootK]

What Hokie said. I prefer to limit deflection by reinforcing less than 0.5 rho_b rather than span/depth, although I usually do both. Basing deflection performance on a formulation that doesn't include load has always creeped me out.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.