additional rebar for lateral loads

[mathcadboy]

is it possible to design a column in a concrete moment frame by just designing it with Dead load and live load then just add rebars at column-beam joints for lateral loads, i.e. seismic loads? why or why not?

 

[msquared48]

You design joints and members for the worst load combination for that particular joint or member. These load combinations include all the forces you have described, plus others, and need to be combined, not separated, and multiplied by their appropriate load factors to get the correct design figures.

Unless you are doing an unfactored design, such as WSD, why would you want to do it differently?

Mike McCann
MMC Engineering

 

[mathcadboy]

hello, i dont think you got my question. but of course you need to design with the worst load combinations. if you are going to checkout the effect of lateral loads to a moment frame, you will notice that moments will be maximum at the top and also at the bottom of the columns. at the middle you will see that there is minimal moment since there will be your point of inflection. my question is, why not just add rebars at the column ends? making your rebars continuous from top to bottom is a waste since moments at the middle will be minimal.

 

[JAE]

It is much more difficult and cumbersome to do partial height bars in columns - takes more coordination and labor to tie the special bars. The small cost savings you would gain with partial bars would be more than offset in extra labor $$.

 

[apsix]

And adding to JAE's reply; by the time development length is taken into account the length that can be omitted would be relatively short.

 

[mathcadboy]

but is it acceptable? that style of design?

 

[hokie66]

It is reinforced concrete design. If the reinforcement is in the right place and developed, it is acceptable.

But design is not just about forces and stresses. It is about practicality of construction, so your suggestion has little merit.

There are a lot of traps for young players in designing the joints in concrete moment frames. For that reason, most of us old players avoid moment frames.

 

[mathcadboy]

very well said. thanks hokie.