Torsion of continuous beam 1

[dccd]

Say that I have a 2 span of continuous beam subjected to torsion, is the torsion value smaller than those 2 single span beam subjected to torsion of same value ??

I think the torsion of continuous beam shall be lower, correct me if I am wrong...

Anyone can correct and explain the situation???

 

[Lomarandil]

I've always assumed that torsion at the interior support of a continuous beam could be larger than that for a simple span (similar to shear). But I've never had a source for that beyond intuition.

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just call me Lo.

 

[dccd]

Weird enough, my beam torsion is smaller when the beam is discontinuous(break into 2 span of simply supported beam), is my structural model wrong ???

 

[HTURKAK]

Will you post the model with support conditions for 3D Model? Is your simply supported beam modelled 2D ?

 

[rb1957]

that doesn't make much (any?) sense. Torsion (or any load) on the beam is dependent on the loads and the reactions. The difference between a continuous beam and 2 spans is only how this external load is distributed between the 3 supports.

another day in paradise, or is paradise one day closer ?

 

[jayrod12]

A continuous beam would have more torsion at the central support, assuming all supports resist torsion, just like udl loads on a continuous beam. So it is not surprising to be that you would have more torsion at the central support with continuous. However the total torsion between all supports should be the same regardless of continuous or simple span.

 

[dccd]

for continuous beam, one of the midspan of beam subjected to torsion only, The support reaction will be much higher than those simply supported beam, does it mean that the torsion of the continuous beam will be much higher in this case?? I knew that the torsion reaction of support will be greatest (torsion reaction= support reaction * ecc)

Correct me if I am wrong...

 

[jayrod12]

Yes. Draw your torsion diagram. At each side of the interior support it will be 0.625*L worth of torsion instead of 0.5*L.

 

[dccd]

Yes, we concluded that for continuous beam (UDL on both span), only one beam subjected to torsion, the torsion of support will be much higher than those simply supported beam.

How if continuous beam, both equal span, same UDL subjected to torsion on both span ? Does it give the same effect? Which the torsion at support will be higher??


Surprisingly, mu model show that when both span subjected to torsion, the continuous beam making the torsion value near the support become lower…I just don’t understand, can you explain it ?Thanks

 

[patswfc]

For a 2 span beam with both spans subject to an eccentric udl causing torsion and 3 fully fixed torsional supports, then each beam will have the same torsion moment each end of the 2 spans (+/-). If the inner support is pinned then the 2 span beam will act as if it were a single span w.r.t the distribution of the torsion moment. If the 3 supports have partial fixity, say from members connected at 90 degrees with appropriate connections, then the distribution of the torsion moment along the 2 spans will be dependent on the degree of partial fixity offered by each support. If the internal partial torsional support has twice the stiffness as the outer supports then the distribution will be as per the case with 3 fully fixed supports. If all 3 supports have the same stiffness then the torsions at the ends of the spans at the internal support will be smaller than those at the opposite ends at the outer supports.

 

[dccd]

If the internal partial torsional support has twice the stiffness as the outer supports then the distribution will be as per the case with 3 fully fixed supports. If all 3 supports have the same stiffness then the torsions at the ends of the spans at the internal support will be smaller than those at the opposite ends at the outer supports.

Mind to explain why ?

 

[dccd]

I have built a FE model. It's a 2 span beam with equal span, same UDL subjected to torsion on both span.....

However, my model show that when both span subjected to torsion, the continuous beam making the torsion value near the support become lower...

I think I have assigned almost simlar stiffness to all the 3 supports , i think it satisfied with your statment above that "If all 3 supports have the same stiffness then the torsions at the ends of the spans at the internal support will be smaller than those at the opposite ends at the outer supports."

I just dont understand why , can you elaborate further?

 

[jayrod12]

Near which support is lower? The outside two supports, or the interior one? Or both?

If both, then something is wrong with your model. If the outside supports, then that would be expected as the central support attracts extra load away from the outer supports.

 

[patswfc]

dccd, it's due to the stiffness of the torsional support and the amount of rotational restraint it provides. For example if you had a single span with different rotational support stiffnesses at either end then the distribution of torsion in the beam won't be symmetric, there will be higher torsion at the end of the beam with the stiffer support. That's why for a 2 span beam the central support needs to be twice as stiff to get equal torsions at the end of both beams. If the central support has the same stiffness as the outer supports it will rotate more due to the moment from both beams, hence its like the single span beam with different torsional support stiffnesses.

 

[dccd]

For my case, the torsion near the interior of support is lower , does it make sense ?

 

[BAretired]

Say that I have a 2 span of continuous beam subjected to torsion, is the torsion value smaller than those 2 single span beam subjected to torsion of same value ??

I think the torsion of continuous beam shall be lower, correct me if I am wrong...

Anyone can correct and explain the situation???

It depends on the supports. If torsion is applied uniformly along the span, and each support is fixed against torsional rotation, the torsion at each end of the beam is half of the total applied torsion whether the beam is continuous or not.

You have told us nothing about the supports, so your question cannot be answered.



BA

 

[dccd]

Can I say that the concept of torsion is somewhat like shear force / support reaction ? the central support near to take the shear from both parts of beam , with at least 0.5x of whole torsion of simply supported beam ? (In fact , it's more than 0.5x due to load distribution...) , The stiffness of the support determine the reaction it can sustain...

If I assigned fixed condition for all 3 supports, the torsion value of interior support will be somewhat higher than the other exterior support? However, I am assigning all support to have same stiffness. Hence, the interior suppprt not able to sustain torsion from both span of the beam, the torsion get redistributed to the exterior support ? (When the beam is continuous, it nned to take torsion from span A and also span B , due to limited stiffness, it cannot take the torsion value as in fixed-fixed-fixed condition)... The exterior support gt higher torsion value ?

Is my statement correct ? Please correct me ... Thanks

 

[jayrod12]

I agree with that statement. Although if all supports had the same torsional stiffness, less than infinitely rigid, I would expect the results to be eerily close to simple span.

 

[BAretired]

ba[/color]]If I assigned fixed condition for all 3 supports, the torsion value of interior support will be somewhat higher than the other exterior support?
Not somewhat higher, but twice as high because it would be resisting equal torsion from two sides.

However, I am assigning all support to have same stiffness. Hence, the interior support not able to sustain torsion from both span of the beam, the torsion get redistributed to the exterior support ? (When the beam is continuous, it needs to take torsion from span A and also span B , due to limited stiffness, it cannot take the torsion value as in fixed-fixed-fixed condition)... The exterior support gets higher torsion value ?
Yes, reducing the torsional stiffness of the interior support causes some torsion to migrate to the exterior supports.

Is my statement correct ? Please correct me ... Thanks

Note: If the interior support is twice as stiff as each exterior support, the torsion values are identical to that of the fixed-fixed-fixed condition. In that case, equal torsional rotation occurs at each support.

BA

 

[dccd]

Weird enough, when I assign all support to become pinned, the torsion increase noticably, why ? Shouldnt the torsion value decrease ? Because the support is unable to take torsion, so tosion shall decrease, right ??