jorton
Structural
- Sep 20, 2005
- 108
To start, I understand the need for minimum reinforcement requirements to prevent the failure of beams without warning. However, the requirements of AASHTO seem onerous. Using a stiffer section with ample prestressing increases the cracking moment, penalizing the design in the process. The requirement to use fr = 0.37 sqrt f'c (in ksi) amplifies the penalty. This shifts the bound to 1.33 Mu. In my opinion Mu is already a healthy number, especially for longer span bridges. Take the negative moment for a four lane bridge with 320' spans for example. The controlling live load alone consists of four lane loads with eight trucks (including a 33% impact factor) positioned for maximum moment. The result is multiplied by a load factor of 1.75 (and multiple presence factor of 0.65). This result is then multiplied by 1.33 to meet minimum steel requirements. It seems excessive. End rant and on to my question.
If the purpose of the minimum steel requirement is to promote cracking prior to failure, should it apply to structures that are not cast monolithic? Take a cast-in-place segmental bridge for example. Each segment joint is a built in crack. The joints will start opening before Mn is reached. In this case it seems wasteful to increase the PT to meet the 1.33Mu requirement or to increase the mild steel to meet the 1.2Mcr requirement. Any thoughts?
If the purpose of the minimum steel requirement is to promote cracking prior to failure, should it apply to structures that are not cast monolithic? Take a cast-in-place segmental bridge for example. Each segment joint is a built in crack. The joints will start opening before Mn is reached. In this case it seems wasteful to increase the PT to meet the 1.33Mu requirement or to increase the mild steel to meet the 1.2Mcr requirement. Any thoughts?