www.sefindia.org

[sukanta.adhikari]

Dear All,

When it comes to deflection check for a reinforced concrete beam we have total deflection as sum of short term and long term(creep and shrinkage) deflection..the calculation of short term deflection makes use of effective moment of inertia of the section...

When it comes to commercial software does any of the software calculates short term and long term deflection...in my knowledge STAAD does not calculates the  short term and long term deflection...what about other software.

Regards,
S Adhikari

[Dr. N. Subramanian]

Dear Er Adhikari,

Most of the software check only short term deflection. Deflection of concrete members is also a subject under constant research. The monumental work done by Branson resulted in the excellent text book published in 1977 by McGraw Hill (     Branson, D.E., Deformation of Concrete Structures, McGraw-Hill, New York, 1977, 546 pp.) His research is still the basis of ACI code.

Several other researchers like Prof. Gilbert of Australia, Prof. Bischoff of Canada and Prof. A. Scanlon have done recent research on deflection and based on that Canadian and Australian code formlae have changed. You may read the following references:

• Gardner N.J., Span/thickness limits for deflection control, ACI Structural Journal, Vol. 108, No.4, July-Aug. 2011, pp. 453- 460.
  
• Gilbert, R.I., Shrinkage, cracking and deflection-the serviceability of concrete structures, Electronic Journal of Structural Engineering, Vol. 1, No. 1, 2001, pp. 2-14 (http://www.ejse.org/Archives/Fulltext/200101/02/20010102.htm)
  
• Gilbert, R.I., and A. Kilpatrick, Improved predictions of the long-term deflections of RC flexural members, Proceedings of the FIB Symposium, Prague, 2011, pp. 187-190 (http://ww2.integer.it/Web_1/database_locale/Fib%20Praga_2011/pdf/030%20596%20065%201%20Gilbert%20SE.pdf)
  
• Gilbert, R.I., and G. Ranzi, Time Dependent Behaviour of Concrete Structures, Taylor & Francis, London, 2010, 426 pp  
  
• Bischoff, P.H., Re-evaluation of deflection prediction for concrete beams reinforced with steel and fiber reinforced polymer bars, Journal of Structural Engineering, ASCE, May 2005, pp. 752–767, and Discussion by Gilbert, R.I., Vol. 132, No. 8, Aug. 2006, pp. 1328-1330.
  
• 
  
• Bischoff, P. H. and A. Scanlon, Span-depth ratios for one-way members based on ACI 318 deflection limits, ACI Structural Journal, Vol. 106, No. 5, Sept.-Oct. 2009, pp. 617-626.

I feel that the serviceability limit states like deflection, cracking, vibration, etc are not given enough importance, especially in India!

Regards,
Subramanian

[sukanta.adhikari]

Respected NS,

I agree with you that serviceability limit state is not given much importance here.
But even short term deflection is not calculated by many softwares..

Regards,
S Adhikari

[Dr. N. Subramanian]

Dear Er Adhikari,

As you are aware if the L/d ratio is within the code permissible limits (Canadian and ACI codes have better equations), there is no need to check short term deflection.

Having said that, I want to emphasize that in long Cantilever beams, say > 3 m, it is better to check long term deflection- as due to creep of sustained loading the long term deflection may be 2 to 3 times the short term deflection.

Regards,
NS

[es_jayakumar]

Sir,

The attached drawing is that of a portion of a multistoreyed building RCC frame, in plan. The internal transverse beam is just below 10m in length and two secondary beams frame into it as a grid. The size of the beam is 450mmx650mm. What method should be adopted to check deflection of this beam ? The deflection value obtained from STAAD is within 40mm (L/250). Is it necessary to calculate long term deflection also, in this case ?

Regards,
E S Jayakumar

[Dr. N. Subramanian]

[mohanarbat26]

[saikiran gone]