BigRy
Structural
- Jul 9, 2010
- 33
I am designing a 228ft. simple span bridge, with both abutments at 36° from roadway tangent / girder centerline. Intermediate bracing is colinear per state standards for bridges with this large of a skew. Two-stage construction, with 5 and 7 girder structures for stages 1 and 2, respectively. The roadway and deck curve at the far end of the bridge. We're not positive yet, but we think the contractor can do each stage in a single pour. We are stuck with tight constraints on superstructure depth due to railroad underclearance and large costs that would be associated with raising the profile. Right now we're working with about 6.5' from top of deck to bottom of bottom flange at mid-span. Girders are all spaced at 6.25'.
We have been working with MDX grid models for final and staged configurations. Due to the geometry, we decided to use bent plate diaphragms instead of cross frames. As you might have guessed, our bracing forces are blowing up. We're getting up to 2400k-ft in our diaphragms, which results in connections so massive that the connection plates are nearly touching. And the larger we make the bracing, the larger the forces they attract. I'm aware of limitations/errors in the way MDX analyzes bracing and flange lateral bending. I've read through NCHRP Report 725, but it doesn't seem that their instructions for a modified 2D analysis can really be applied in MDX. Those instructions are very general, and I do not see a way to input a modified torsional constant. They also do not go into great detail about how exactly this modification is carried through to other MDX analyses and output. We've tried a number of different bracing designs and layouts with the regular 2D analysis. The best case we can get is to essentially eliminate as much bracing as possible, which obviously isn't realistic from a constructability standpoint.
We are now looking to investigate the use of lean-on bracing, so we can effectively eliminate many bracing forces while still providing lateral support to the flanges. The state is reluctant to adopt this sort of "new technology", so we need to make a compelling case with this structure. The problem is that I can't find a way to model the lean-on bracing. From what I understand, it could be done in Larsa or another FE program, but we have neither the time nor the budget to begin a modelling task like that. I was hoping the relationship between diaphragm size and diaphragm force would translate to individual cross frame member sizes and forces. That is not the case. The forces are independent of the member size, even for Type E which is statically indeterminate. I then tried going back to the custom bracing input, using J, I, and Wt, except I used values for a pair of angles, one near each flange. For custom braces, MDX analyzes it like a beam and outputs moments and shears. My hope was that the shear would follow the path of the stiffest element, which would be following along the girder until it reaches a diaphragm. It did not do that either. So lastly, I minimized all bracing inputs for each "lean-on" brace (i.e., I set J, I, and Wt to 0.0001). That forced MDX to transfer all the bracing loads to the nearest diaphragm. But that also includes transferring the lateral forces that should be taken by the struts. So we end up with a couple diaphragms that still have sizeable loads, not nearly 2400k-ft but still large.
Has anyone had to design lean-on bracing? If so, how did you do it? I think I'm out of ideas here.
We have been working with MDX grid models for final and staged configurations. Due to the geometry, we decided to use bent plate diaphragms instead of cross frames. As you might have guessed, our bracing forces are blowing up. We're getting up to 2400k-ft in our diaphragms, which results in connections so massive that the connection plates are nearly touching. And the larger we make the bracing, the larger the forces they attract. I'm aware of limitations/errors in the way MDX analyzes bracing and flange lateral bending. I've read through NCHRP Report 725, but it doesn't seem that their instructions for a modified 2D analysis can really be applied in MDX. Those instructions are very general, and I do not see a way to input a modified torsional constant. They also do not go into great detail about how exactly this modification is carried through to other MDX analyses and output. We've tried a number of different bracing designs and layouts with the regular 2D analysis. The best case we can get is to essentially eliminate as much bracing as possible, which obviously isn't realistic from a constructability standpoint.
We are now looking to investigate the use of lean-on bracing, so we can effectively eliminate many bracing forces while still providing lateral support to the flanges. The state is reluctant to adopt this sort of "new technology", so we need to make a compelling case with this structure. The problem is that I can't find a way to model the lean-on bracing. From what I understand, it could be done in Larsa or another FE program, but we have neither the time nor the budget to begin a modelling task like that. I was hoping the relationship between diaphragm size and diaphragm force would translate to individual cross frame member sizes and forces. That is not the case. The forces are independent of the member size, even for Type E which is statically indeterminate. I then tried going back to the custom bracing input, using J, I, and Wt, except I used values for a pair of angles, one near each flange. For custom braces, MDX analyzes it like a beam and outputs moments and shears. My hope was that the shear would follow the path of the stiffest element, which would be following along the girder until it reaches a diaphragm. It did not do that either. So lastly, I minimized all bracing inputs for each "lean-on" brace (i.e., I set J, I, and Wt to 0.0001). That forced MDX to transfer all the bracing loads to the nearest diaphragm. But that also includes transferring the lateral forces that should be taken by the struts. So we end up with a couple diaphragms that still have sizeable loads, not nearly 2400k-ft but still large.
Has anyone had to design lean-on bracing? If so, how did you do it? I think I'm out of ideas here.