Continuous beam on girder instead of column 2

[wilberz]

The reactions for continuous beam on column at middle with 2 point loads at midspan at either side has the following formula:

reactions at column or middle = 11P/16 where P is the point load at either side
reactions at either end = 5P/16

how about if a girder instead of column intersects the middle of the continuous beam (imagine forming a cross).. would the reactions be the same?

 

[wilberz]

If the edge is moment connected to the column with many bars.. or in other words.. not pinned but fixed connection.. can this affect the formula of 11P/16 and 5P/16?

 

[wilberz]

This is what is puzzling.. in Etabs why do you have to release the moments at the edge when in actual contruction, the edge can even be fixed and 5P/16 still satisfies as the earlier messages by Kootk and company shows?

 

[MacGruber22]

This thread is killing my soul...

"It is imperative Cunth doesn't get his hands on those codes."

 

[IDS]

This is what is puzzling.. in Etabs why do you have to release the moments at the edge when in actual contruction, the edge can even be fixed and 5P/16 still satisfies as the earlier messages by Kootk and company shows?

Which message are you talking about?

The 5P/16 factor only applies for pinned supports with no deflection at any of the supports, and no moment restraint. Any change from those conditions will change the reaction distribution.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[wilberz]

Which message are you talking about?

The 5P/16 factor only applies for pinned supports with no deflection at any of the supports, and no moment restraint. Any change from those conditions will change the reaction distribution.

Thanks for this important information. Before some structural engineers told me in continuous beams where the middle and ends support are columns.. they just assume it's pinned (since it's so difficult to tell how many percentage is the column-beam connections fixed or pinned). So what they did is put few moment bars in the ends assuming the middle column will take the major loads (close to 22P/16). Anyway. In your designs guys.. how many percentage do you think is your column-beam pinned or fixed? Do you design it as pinned or fixed?

 

[KootK]

If the edge is moment connected to the column with many bars.. or in other words.. not pinned but fixed connection.. can this affect the formula of 11P/16 and 5P/16?

You bet. Fixed end connections will draw more load to those ends.

in Etabs why do you have to release the moments at the edge when in actual contruction, the edge can even be fixed and 5P/16 still satisfies as the earlier messages by Kootk and company shows?

You don't have to release the ends of continuous beams. In fact, for most monolithic beam/column/girder framing systems, I'recommend against doing so. And, as I mentioned above, fixity will affect the reactions.

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.

 

[wilberz]

In monolithic beam/column/girder framing systems, would putting less moment bars in the connection makes it more pinned than fixed? instead of using deflection to control the loads, can you use moment bars to design it such that there is no (or very few) load at the end of continuous beams because you put it all in the center? How many do this here?

 

[KootK]

Yes, to some degree, reinforcing is a self fulfilling prophecy. If you reinforce it, the loads will come! The degree of "control" is not great, however, and designs should generally not deviate too far from anticipated elastic analysis results in my opinion. For example, even when faking a pin connection, any SE worthy of the title will provide significant negative moment capacity at the ends. The rule of thumb that I apply is that the negative moment capacity should be at least 1/3 the positive moment capacity.

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.

 

[wilberz]

Kootk, others

Please refer to the above. I'm learning how to design 2 interconnecting continuous beams. If the beam column connections at the top and bottom were made more pinned.. there would be more moments in the midspan where the beams holding the slab frame into.. this would cause more deflection that would distribute the slabs loads to the ends girder (rather than focusing all at the center). If I made the top and bottom column-beam more fixed.. there are less moments and less deflection at midspan.. this would attract more load from the slabs. In your design which do you prefer?

 

[wilberz]

About the above, isn't it always a good design to distribute the loads to different or more columns. So it's better to make the top and bottom columns connection to the girder beams as fixed as possible.. this involves extending the moment rebars one meter below the end column-beam joint? But then as I understand this (and wanted confirmation). When the top and bottom column and beams are more fixed, the moments in midspan would get lesser.. this would create lesser deflection and producing more load in the middle girder.. or is it like what Kootk said that during concrete pouring and hardness, the initial boundary conditions are set even if the secondary continuous ends are one meter higher as we originally discussed? So the deflections in the primary girder beam would have no effect on the load distribution of the secondary continuous beams?

 

[wilberz]

I guess you will say that depending on the tension bars at positive moments.. there is no problem with deflection if you choose either the pinned or fixed ends of the primary girder. Ok. And depending on the tension bars at negative moments at the continuous ends, you can control approximately the pinned and fixed condition. And these two is what can make the design different.

Well. I let 2 fellow structural engineers design the above. They come out with totally different designs. One guy modelled the ends as pinned while the other guy modelled it as fixed. Since there is no way to totally predict if it will be pinned or fixed and by how much amount, then you should design the center column to be stronger and the girder to avoid unexpected loads. Is this correct? If it is, the it summarizes this thread. Thank Koots for the very enlightening thoughts.

 

[Ron]

Go back up to your nice little shear diagram from AWS.....

You will note that V2 shading above and to the right of R2 is equal to the V2 shading that goes to the left and below R2. That line running thru R2 (the reaction at R2) is the sum of V2+V2.....exactly what SlideRuleEra told you long ago.....you seem to be arguing with a lot of people who have tremendously more experience than you.....why is that?