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I am asking this because some of my engineer friends said that they won't want to analyze beam as continuous beam for RC building structures, because it is "unsafe" to do so, whether it has anything to do with less deflection or no, I don't know.

His logic is that continuous beam predicts less deflection ( and hence gives more economical design), is his claim valid?

I wonder how does practicing engineers view the modelling and analyzing/designing beam spans as continuous beams ( instead of beams with many supports) when it comes to reinforced concrete building structures? Do they generally feel that it is OK to design beam spans as continuous beam ( and hence more economical because the reinforcement bars used will be less), or do they share my friend's opinion?

What does the building code of practice ( or common design practice) across the world say about continuous beam design, do they discourage or encourage it? Any reason not to design beam spans as continuous beam?

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Graviton
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  • I don't understand what you're asking. What makes you think that perhaps less deflection = unsafe? And given a [previous question](http://engineering.stackexchange.com/q/8923/1832) of yours, you've already got some opinions stating that continuous beams are perfectly fine, if not better than multiple simply-supported beams. And building codes don't care, so long as the structure is safe. – Wasabi Aug 18 '16 at 02:01
  • @Wasabi, I am not sure that less deflection=unsafe, but I do heard from some engineers that continuous beam is dangerous. So I want to know: whether continuous beam is dangerous, according to the opinion of other majority engineers? – Graviton Aug 18 '16 at 02:27
  • @Wasabi, question updated with clarification – Graviton Aug 18 '16 at 02:29
  • Why didn't you ask your friends why they think continuous beams are unsafe? – Wasabi Aug 18 '16 at 02:31
  • @Wasabi , I will. But in the meantime, I want to know what others think – Graviton Aug 18 '16 at 02:41

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You question starts with (emphasis mine):

Given that continuous beam predicts less deflection

This is not correct. A continuous beam does not predict less deflection, a continuous beam results in less deflection (than a pair of simply supported beams).

Similarly to this question, you seem confused between how a structure behaves in real life and how it behaves in a structural analysis model. With reference to my answer there: if you design it as continuous in your analysis model, then you need to detail the reinforcement to achieve this. Hence analysing a beam as continuous is safe as long as you detail it to be continuous.

For additional safety in design, you could design a beam as both pin-ended and continuous. i.e. you could carry out a pin-ended analysis, then carry out a continuous analysis, and then envelope the results. This makes some sense: if a continuous beam fails in hogging, it will then behave as a pin-ended beam for further load (provided it has rotational capacity at the joint). However, this would generally be thought of as overdesign: you're designing in extra safety but at extra cost, so unless you absolutely need a paranoid level of safety then you're wasting money.

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AndyT
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Continuous beams are safer than simply supported ones in general. They have greater degree of static undetermination. If one section fails, it will form a plastic hinge and will redistribute to the other sections. Then it will form another hinge and another until the whole beam fails. For simply supported beams, we need just one section to fail and it is done. This is in case you design both as needed but something unpredicted happens later.

On the other hand, you can make any beam safer than any other, regardless the static scheme, by simply increasing the size and reinforcement.