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Cantilevered Beam with Through Beam Detail

Cantilevered Beam with Through Beam Detail

Cantilevered Beam with Through Beam Detail

(OP)
First time poster to the forum and recent graduate.

I am still in the phase of questioning typical details in our office even though they have used for a while and nothing bad has happened.
We have a typical detail for cantilevered beam conditions that has girder interrupting the cantilevered beam. The goal of the detail is to engage some backspan and use the thru beam as a pinned support through which the cantilevered out beam rotates.






In my mind, this detail induces a non-neglible torsion about the girder. After discussion in the office, it was said that there is some torsion induced, but it is ignored. Is this a good/correct assumption? When can something like this not be ignored.

Thanks in advance,
S&T

RE: Cantilevered Beam with Through Beam Detail

When an "I" shaped beam like a wide flange has some small amount of twist induced the resulting torsion is usually very small because the girder has very little torsional stiffness.

With small stiffness the amount of torson (per unit of twist) is small. So the girder would tend to behave like a true pinned support for the cantilevered beam.

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RE: Cantilevered Beam with Through Beam Detail

(OP)
That makes sense.
Is there any concern for the connection of the through girder or beam back to it's supporting members?
Will a simple shear connection be able to "twist" the beam back to an upright position at the supports?

RE: Cantilevered Beam with Through Beam Detail

Expanding a little on what JAE said, torsion is often ignored if it is not required to support the load. So a beam supporting an eccentric load must be designed for torsion as there is no other way to provide support.

A beam which supports secondary beams or joists may feel torsion from the rotation of those members but does not need to be designed for torsion provided they are capable of carrying the load without reliance on rotational restraint from the carrying beam. In special cases, it may be wise to limit deflection, hence end rotation of the supported members.

BA

RE: Cantilevered Beam with Through Beam Detail

One thing I am somewhat unsure of is not using flange plates for the moment connection. You are putting a tension perpendicular the a main bending compression on the thru beam, that should probably be considered together to some extent - the use of a flange plate eliminates that condition. It also could allow for a simpler down-hand fillet weld or bolting.

RE: Cantilevered Beam with Through Beam Detail

You've identified an important issue OP. Many common girder connection details will not facilitate large twists without generating large beam torsions. For this reason, when I set out my beams, I'll try to land a beam right at the columns and have a full infill beam spacing adjacent to the girder connection. That way, hopefully, some girder torsional flexibility will kick in before the next applied torsional load is applied down the line.

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: Cantilevered Beam with Through Beam Detail

(OP)
All very helpful comments, thanks!!
StructSU10 I share your concern. If that top flange of the thru beam or girder is close to buckling already from bending, it would seem like this tensile perpendicular load would be the icing on the cake to make it buckle.

S&T

RE: Cantilevered Beam with Through Beam Detail

(OP)
Never mind, I take my comment back StrcutSU10, that tension is on both sides of the connection. Maybe not as concerned.

S&T

RE: Cantilevered Beam with Through Beam Detail

Yea, its more a combined stress thing - your stress component when combining the effects in both direction will be sqrt(comp^2+ten^2). What happens with the case in which the thru beam is designed for its plastic moment as it is fully braced - compressive stresses will be high, and you are now also inducing a relatively high tensile stress perpendicular to that. In some cases, I'm sure its no problem, but in others it seems like a potential problem.

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