## MULTI-SPAN (CONTINUOUS) CONCRETE BEAM DESIGN

## MULTI-SPAN (CONTINUOUS) CONCRETE BEAM DESIGN

(OP)

I usually compute moment of continuous concrete beams using MDM (moment distribution method). This gives larger moments at supports (neg. moments)and smaller moments at mid-spans(pos. moments). While this may be a common practice to many, a senior designer advised me to consider it as simple beams where max. moment is at mid-span for safety purpose.

He argues that it is economical to consider as continuous beam but, in reality, if one span gives up due to high earthquake magnitude, others capacity become insufficient to carry redistributed loads.

Please share your advice based on some theoretical and practical information.

He argues that it is economical to consider as continuous beam but, in reality, if one span gives up due to high earthquake magnitude, others capacity become insufficient to carry redistributed loads.

Please share your advice based on some theoretical and practical information.

## RE: MULTI-SPAN (CONTINUOUS) CONCRETE BEAM DESIGN

1) most codes include patterned live load cases that will reduce dependency on the presence of adjacent span live loads.

2) vertical seismic loads on gravity beams are relatively small. They, and the seismic drift, can be designed for so as to greatly minimize the risk of adjacent span damage.

3) Continuity is a fundamental characteristic of cast in place concrete construction. Disregarding the benefits of that continuity altogether is unacceptably wasteful in my opinion.

4) Most codes include integrity reinforcement detailing provisions that will increase the robustness of all spans within a continuous beam and reduce dependence on adjacent span live loads and capacity.

I myself practice a much less conservative version of your colleague's strategy. I often redistribute a portion of the beam negative moment to the positive moment region and I never design for a positive moment less than wL^2/20 no matter what value is computed in the analysis process.

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.

## RE: MULTI-SPAN (CONTINUOUS) CONCRETE BEAM DESIGN

Patterned loading is the key here and typically for any series of continuous spans you will have six different pattern combinations:

Full Live Load

Live load on Odd spans

Live load on Even spans

Live load on adjacent spans (pattern 1) Load/Load/zero/load/load/zero/load/...

Live load on adjacent spans (pattern 2) Zero/Load/load/zero/load/load/zero/...

Live load on adjacent spans (pattern 3) Load/zero/load/load/zero/load/load/...

Also with patterned loads you need to consider whether the "zero" loaded spans above are more correctly some percent of the live load (i.e. some of the live load is sustained)

This might make the zero load more like 25%LL.

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## RE: MULTI-SPAN (CONTINUOUS) CONCRETE BEAM DESIGN

## RE: MULTI-SPAN (CONTINUOUS) CONCRETE BEAM DESIGN

I seem to have been more confident with my design after reading both 1st and 3rd points. I am curious about wl^2/20. I think this could have a relation to ACI code provision: "The pos. moment strength at the joint faces should not be less than half of the neg. moment. Neither the pos. or neg. moment strength at any section be less than one-fourth of the max. moment strength provided".

By the way, what is the common practice to compute moments for steel frame building (SMRF). My colleague uses wl^2/8 (simply supported) for all steel beams where no moments are transferred to steel columns. This definitely results in larger steel beam section but will economize column section and footing pad as they only carry seismic moment.

BUGGAR, I take note of what you shared.

Sorry for asking many questions. I am a newbie in design practice.

## RE: MULTI-SPAN (CONTINUOUS) CONCRETE BEAM DESIGN

I think you are too conservative regarding this subject. Just think about that steel and concrete strenght are divided by safety factors. The real strength is the medium one and for steel is 1.15*value in desing and for concrete + 8 mpa. So only from steel you have 15% bigger bending capacity. The second point is in loading big values from codes. That live load for example is very very big and has 95 % probability in reality to be less. So the values that we work with in design are very very not reality like, ultra conservative so in my opinion you dont have to more conservative....

## RE: MULTI-SPAN (CONTINUOUS) CONCRETE BEAM DESIGN

The real material strength is not the medium one, it is accepted as the lower 5 percentile figure as you could get the material with the lower percentile value in your building, so the medium one will not help keep your building standing if it is not there.

Load factors and material factors have been developed over many years to provide the necessary risk/safety factors that are considered to be acceptable for structures of different importance levels. And buildings still fall down. We are not being overly conservative.

Yes, Buggar's concept of designing for the worst possible case of continuity is not required for most structures. But may be logical for structures that are likely to face damage from severe accidental actions e.g. a very large ship colliding with it and causing something that is continuous to all of a sudden become simply supported. It is a method of providing Robustness for a special structure that requires it as the port could be out of action for years if badly damaged.

All of the other comments above simply suggested following defined Design Code requirements. They are not overly conservative.

## RE: MULTI-SPAN (CONTINUOUS) CONCRETE BEAM DESIGN

## RE: MULTI-SPAN (CONTINUOUS) CONCRETE BEAM DESIGN

My colleague uses 'continuous beam' analysis for such frames but I am not convinced since those girders or beams are monolitic with lower and upper columns.

## RE: MULTI-SPAN (CONTINUOUS) CONCRETE BEAM DESIGN

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.