brick lintel design - torsional effects

huangyhg

brick lintel design - torsional effects
in sizing brick lintels, i have only been considering bending in the vertical leg (the "x-axis"). i started thinking and asked myself this question: at what point do i start considering torsional effects (or bending about the "z-axis") due to the eccentric loading of the brick? is this a legitimate concern or am i just crazy?  in school we learned about lateral torsional bending, but i haven't considered it in this application.  thanks in advance for any help.
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i don't think it's crazy at all.  i've never understood this either.  the angle is not loaded through its shear center, and is an open section, so should twist very easily between restraints.
just some random thoughts:
it's interesting that the 3rd ed. lrfd single angle spec. doesn't mention torsion.  it does mention angles with and without lateral-torsional restraint and gives some equations in 5.2.2, 5.2.3, and 5.3.
i guess one could calculate the pure torsion shear stress and add it to the shear stress from flexure.  there also seems to be warping, but i've never seen equations for how to calc the associated normal stress.  it could be approximated using mechanics, though.  imagine the section trying to twist--the legs would be bending out of their planes.
now the tough one: twisting angle.  i guess one could calculate the pure torsion part using dphi/dz=t/gj and then calc the warping part using the physical analogy of the legs bending out of their planes.
here's a weird one that i've been wondering about.  the 13th ed. manual lists cw properties for angles.  anybody here know how to calc that?!
it's not crazy, but it is not worth worrying about.  just stick to what has been proven to work.  the brick restrains the upstanding leg, and the brick is restrained by ties, assuming of course that the wall is built correctly.  and in the completed case, the load on the lintel usually reduces from the construction case because of arching.  so one for the use of standard lintel tables which have been successful.  no need to sweat over calculations.   
"the brick restrains the upstanding leg"
hokie66:
could you please explain what you mean here?
this topic has always interested me.  i would say 95% of the engineers i know neglect torsion when designing angle lintels and wide flange lintels with extended bearing plates that support brick and cmu.  sometimes i feel most engineers don't understand torsion design so they neglect it.
some comments about arching action and brick restraint.  often there are control joints at one or both ends of the lintel which eliminates or reduces arching action.  charles w. allen, m. asce indicated in an article, published  in civil engineering-asce march 1981, that cracking often occurs in the lintel area reducing the masonry's ability to brace the lintel.
his article provideds a lintel table and some design equations for sizing lintels.  i have used his table many times.  however when i did, my lintels were alway larger then what others were commonly using.
jike... for torsion to 'really' develop, it is necessary for the deformation causing it to occur... often, if something is restrained the anticipated forces cannot occur.  for compatibility, the wall has to rotate, too.  as soon as an angle starts to rotate, the load from the brick is applied closer to the fillet and if the upright leg wants to rotate, then then it presses against the masonry... strip windows for a mall are a different problem and i don't treat them as lintels.
dik
this has been discussed recently, check out this thread:
sorry that was
"jike... for torsion to 'really' develop, it is necessary for the deformation causing it to occur... often, if something is restrained the anticipated forces cannot occur.  for compatibility, the wall has to rotate, too. "
this is the exact argument i made (and was castigated for by many lolol) for cmu walls restraining torsion of a steel beam lintel below.  gotta watch those bcs in both cases.  one big difference between this and the cmu is that the cmu has a good connection to the steel beam, lol.
what 271828 says is basically right.
when the angle starts to rotate under the 'torsion', the brickwork will tend to stay vertical. the result is that the center of load moves closer to the inside face of the brick.
in the thread csd72 mentioned, jae said that depending on the brick to brace the lintel torsionally was like two drunks holding each other upright.  he didn't like the idea, but i do.  maybe i have had more experience with that analogy.