mcox
Structural
- Aug 14, 2002
- 4
I am currently designing a post-tensioned hammerhead pier cap and have run into a design issue with the wording in AASHTO and I am looking for some help. My cantilever cap is 10 feet wide and 12.5 feet deep at the pier shaft. The load is placed only 7 feet from the edge of the pier shaft. In reality I have a deep beam in two directions. This pier cap is also approximately twice as stiff as the hollow stem section just below it.
Per 5.8.1.1 I meet the deep beam criteria and therefore I can design using the strut and tie model of 5.6.3 and the detailing requirements of 5.13.2.3. No in-depth shear calcs are then needed. The detailing requirements lead to a required 6 inches^2/ft or less.
When I check my torsion criteria of 5.8.2.1 I am above the limit for neglecting torsion (0.29*theta*Tcr in lieu of 0.25). According to that section shear is then to be in accordance with 5.8.2.1-2. When I do my in-depth shear and torsion comps I then need a tremendous amount of shear reinforcement with a minor increase for torsion requirements. I would then need 19.8 inches^2/ft for shear and 1.5 inches^2/ft for torsion. I would need to utilize not only the detailing steel but the bursting steel from my post-tensioning to make this work.
My question is whether I can just take the small amount of additional steel necessary for torsion and add it to the detailing requirements of 5.13.2.3. The code does not appear to be clear on this matter. If not is it acceptable to utilize the bursting steel as a part of the shear steel?
Per 5.8.1.1 I meet the deep beam criteria and therefore I can design using the strut and tie model of 5.6.3 and the detailing requirements of 5.13.2.3. No in-depth shear calcs are then needed. The detailing requirements lead to a required 6 inches^2/ft or less.
When I check my torsion criteria of 5.8.2.1 I am above the limit for neglecting torsion (0.29*theta*Tcr in lieu of 0.25). According to that section shear is then to be in accordance with 5.8.2.1-2. When I do my in-depth shear and torsion comps I then need a tremendous amount of shear reinforcement with a minor increase for torsion requirements. I would then need 19.8 inches^2/ft for shear and 1.5 inches^2/ft for torsion. I would need to utilize not only the detailing steel but the bursting steel from my post-tensioning to make this work.
My question is whether I can just take the small amount of additional steel necessary for torsion and add it to the detailing requirements of 5.13.2.3. The code does not appear to be clear on this matter. If not is it acceptable to utilize the bursting steel as a part of the shear steel?