I have created a STAAD model of a simply supported bridge superstructure. In the model I have used steel girder and diaphragms along with concrete plate elements for the deck. A few things I have noticed in particular:
1) There are bending moments at the the supports (which I can eliminate by using the offset command and releasing them at the supports)
2) The exterior girder away from the load shows negative moment when it should be possitive, as if the entire bridge is in torsion lifting up on the girder
3) Axial forces are present in the members (likely due to the vertical member offsets)
Does anyone know why I might be getting negative moments in the far exterior girder under vertical loading? What can I do to fix this? Also, the girders are broken down into 8 elements but I've noticed that the values of the moments do not equate when looking at the same node from two beam elements. Can I eliminate the axial forces in the girders so it doesn't create additional bending moments? I've attached the file. Any help would be greatly appreciated. Thanks.
LET ME TRY. YOUR 1ST QUESTION WHY IT'S NEGATIVE MOMENT WHEN IT SHOULD BE POSITIVE BECAUSE IT'S SIMPLY SUPPORTED AND THE CONCENTRATED LOADS ARE AT CENTER. THE ANSWER IS, IN STAAD IT FOLLOWS I BELIEVE THE GLOBAL DIRECTION MEANING THE NEGATIVE SIGN MEANS THE POSITIVE BENDING MOMENT OF THE BEAM AS WHAT WE HAD BEEN TAUGHT IN THE BOOK. IT REALLY AMAZED ME AT FIRST BECAUSE IT'S THE OPPOSITE FROM THE BOOK. ON THE OTHER HAND THE POSITIVE SIGN IS THE NEGATIVE BENDING MOMENT, IT'S REALLY THE OPPOSITE, BUT THERE'S NO PROBLEM WITH IT SO LONG AS YOU KNOW LIKE FOR A CONCRETE BEAM WHEN THE MOMENT IS AT BELOW ( NEGATIVE) SO THE REBAR IS NEEDED THERE AT BELOW SPECIALLY AT MIDSPAN. IF THE MOMENT IS AT TOP ( POSITIVE ) THEN THE REBAR IS NEEDED THERE AS TOP REINFORCEMENT SPECIALLY AT SUPPORTS. IT FOLLOWS I BELIEVE THE GLOBAL DIRECTION. YOU MAY ASK ALSO THIS FROM BENTLEY ABOUT . BUT THIS IS I BELIEVED. YOU CAN CHECK THEIR EXAMPLES ALSO ( OF STAAD U.S. EXAMPLE ) AND SEE IT'S THE OPPOSITE.
HOPE THIS EXPLAINS,
Thanks so much for your response. I believe you are correct about the sign convention in STAAD. I did a quick single simply supported beam with a concentrated load in the center and it does appear that the sign convention is reversed. I was wondering though why the far exterior girder away from the loading on my model was experiencing a negative and not positive bending moment? I have a 5 girder bridge and the loading is concentrated on one side of the bridge (i.e over two girders). The far exterior girder on the opposite side shows a negative moment while the remaining 4 are positive moment as expected. If you look at the attached model "west lane" load case has 4 concentrated loads representing a vehicle. The exteior girder on the far east is under negative moment while the remaining 4 are positive as expected. I tried looking at just the steel frame of the superstructure (without the deck) applying two concentrated loads at the midspan of two girders on one side of the bridge and it does the same thing (negative moment at the far exteior girder on the opposite side). I feel like there is something I need to release or something but I'm not sure what that would be. It appears that there is significant torsion on the superstructure. Any thoughts on this? Thanks again for your help, I appreciate it.
Hi Mike, the problem is because you don´t have sufficient rigidity, You can fix this by increasing the thickness of the slab or the section of the transverse girders ( or number ). I make some changes to your original model because the shape of your plates ( the optimum shape for a quadrilateral plate is a square, the best results are obteined when the ratio of the element´s longest side to its shortest side is no grather than 2:1).
Thanks for your time in looking at my model. There are curbs running along both sides of the bridge which I didn't account for yet. If I increase the slab thickness over that area (which includes over the exterior girders) will that suffice. I will also change my mesh size to see what happens. I couldn't open the .rar file that you attached. Is there anyway that you can zip the .std file and attach that? Thanks again for your help Atilio!
hi mike I attached file in Word format, copy and paste in the STAAD editor.
Thank you again for your response. I uploaded the file into STAAD and the continuity at the joints is now there from the new mesh. What did you do to generate the "X" meshing in some locations? I still have the problem of a negative moment on the exterior girder farthest from the load. I increased the slab thickness in that area as you can see on the attached file. There is a curb which is 8 inches thick over each of the exterior girders and overhangs. I used the "move" command to move the plate upward so it was at the top of the flange of the exterior girder, I hope this is correct. Any ideas as to why I may be getting a negative moment on the exterior girder? Is there something I can do to fix that? Thanks again for all of your help. It has been a big help and a great learning experience. I know I have a lot to learn about STAAD, since I am just getting started, but thanks for your time and patience.
1_ About the x meshing, I generated a density point using 3 node plates, because I need a point there to apply the load. Would be optimal to use only square elements and densify so, that my load match at points of the plates.
2 _ When you used the move command you generated independent structures that are not interacting with each other. To interacting the nodes must be share and that is not happend with the exterior and central plates ( there will be continuity or interaction betwen the elements only if there are shearing the nodes ). You can easily check this if you go to TOOLS (in the bar menu) --> Check multiple structures. When you do this you can see 3 independient structures in the box.
Dont use the move command you will have only a litle exentricity, that is not important.
I attach a file
I will have to look into the x meshing a little more, I am not familiar with that. I had the point load applied on the corner node of a plate I think before. The model seems to be working much better, I have a little refining to do I think. Thanks again Atilio and thank you as well to William. I appreciate the time and assistance both of you gave me. I'm sure I will be back again with a few questions later on down the line. Until then, take care.