Shear in AASHTO LRFD code 1

yakpol,

I am familar with AASHTO LRFD 2nd Edition which states that bv (effective web width) is equal to min. web width less diameter of UNgrouted ducts, OR 1/2 of diameter of grouted ducts.

Check out section 5.8.2.7 of AASHTO.

So for your example (GROUTED) bv = 12 - 1/2(4) = 10"

This is the same as the Australian code, that i am more familar with. If you have mulitple duct passing through a horizontal plane (which AASHTO does NOT seem to address directly) you would sum up the 1/2 duct diameters and deduct this summation from the web width.

I am not familar with AASHTO LRFD "latest interim"...what is the date of this document?

HTH

Thanks Ingenuity,
I found the same wording in the guide specifications for design and construction of segmental concrete bridges. AASHTO produced 2002 interims instead of the new edition... The shear design has changed in 2000 interims, they changed some formulas and added more comments...

Just thinking out loud here, but if you are considering vertical shear, then reducing the width of the web by the thickness of the duct seems overly conservative since obviously the thickness of the web is only lessened at the duct. Shear on a finite element does not occur only in the vertical direction however, it has to occur horizontally as well. If you think of the horizontal shear that is occurring, the section is compromised for the full length of the duct.

Kramer,

The crushing of the web is what the code is part checking against with a reduced/effective web width - and this, if it occurs, would happen prior to yield of the shear reinforement and is very, very brittle, so very undesirable...

I believe this is from work/theory by Leonhardt back in the '80's, and supported by actual tests.

Horinontal shear across critical planes does need to be checked too, as you stated.


HTH