10 Replies Latest reply on Jan 21, 2014 7:17 PM by Mike Pogue

    Analyzing Bolt Hole Stress

    Nathan Cassou

      On the attached assembly I have performed a static analysis on several bolt holes of a flange. The load is coming off of the beam structure putting stress on the flange bolts as shown. For the flange bolts I used a split line round profile over each hole and added a simple static fixture to each face of the split lines to simulate the bolt head diameter. All other connections use the Bolt Connection feature. The material is Al 6061-T6: Yield = 40 ksi, Shear = 37.7 x 10^5 psi. The green plot shows a stress of around 8000 psi while the blue stress plot shows a range of 1600-4000 psi. These bolts are going into concrete anchors which from what I've found can only hold about 4000 psi "pull-out" load. How should I go about analyzing this?

       

      Thanks

      Nate

        • Re: Analyzing Bolt Hole Stress
          Mike Pogue

          BOlted connectors can be connected to the ground. It's pretty much exactly for this situation.

            • Re: Analyzing Bolt Hole Stress
              Nathan Cassou

              Mike,

               

              I tried the Foundation Bolt feature but the simulation failed due to the large displacement analyzer failing. What stress plot around the holes would be more accurate to analyze?

                • Re: Analyzing Bolt Hole Stress
                  Mike Pogue

                  My first thought is that this plot is not likely to be accurate at all. Large deformation is a sure-fire sign that your linear analysis is jacked. Also, stress in the neighborhood of a discontinuous load is not going to be reliable. The reason the surface is blue is that you’ve fixed it—you’ve defined the strain in x-y to be zero. So to the extent that you should believe anything, you should believe the higher stress (but see above).

                   

                  I’d troubleshoot the large deformation. It may be trying to tell you your structure is failing. Barring that, you know the bolt preload and can read the reactions at the bolts from the fixtures you are doing. From that you can do a pretty straight forward classical bolted joint analysis.

              • Re: Analyzing Bolt Hole Stress
                Peter Biggert

                Using stress results directly under a fixed restraint is usually not recommended. When you used a fixed restraint it does not properly account for the bolt preload in the joint.

                 

                When you used the foundation bolt feature did you also define a virtual wall that represents the concrete? Did you use friction or constrain your part from moving sideways? If not that could be why you had large displacments, you possibly had Free Body movement of your part.

                 

                Also be careful about using "Foundation Bolt" elements when you have an anchor bolt embedded in the concrete. The bolt length gets defined as the distance from the nut head to the virtual wall, not how far the bolt is embedded. The force in the bolt is a function of the stiffness of the bolt and the stiffness of the joint. When the bolt length is wrong the stiffness is wrong and the force in the bolt is wrong. You also have to consider the concrete in the joint stiffness.

                 

                I think of a pull out load as a force not a pressure. Are you sure you are looking at the right results?

                  • Re: Analyzing Bolt Hole Stress
                    Nathan Cassou

                    Thanks for tips fellas,

                     

                    I think tensile stress plot around the holes is an adequate gauge to find a suitable contrete anchor. I can easily do a calc to find the axial force in the area of interest but since the anchor specs use stress it is easy for me.

                     

                    I will try and rerun the simulation using foundation bolts and make sure I have values in the contraints. Which I did not..

                      • Re: Analyzing Bolt Hole Stress
                        Jared Conway

                        large displacement warning and accept the results without re running with large displacement = likely junk results

                         

                        and for bolts, they sacrifice the stress near to the bolt connector in trade for accurate bolt forces. if you have a high level of mesh refinement around the bolted area you can get better stresses but only modeling the bolt gives you the correct and ideal results.

                          • Re: Analyzing Bolt Hole Stress
                            Nathan Cassou

                            Jared,

                             

                            If I go ahead and insert the six bolts as models, how would I constrain them? Fix their shanks to the holes?

                              • Re: Analyzing Bolt Hole Stress
                                Jared Conway

                                starting with everything bonded is a good start

                                when you connect to the ground, fixing the shank would be a good fit

                                  • Re: Analyzing Bolt Hole Stress
                                    Mike Pogue

                                    This is the standard way to get bolt stress. I'd be less willing to trust this method to get the stress under the bolt head. The bond between metals with dramatically different Young's modulus is going to cause divergent stress. I can't find the paper, but it was recommended to average the stress within .01 - .03 inches from the discontinuity (2 answers, 2 papers). Of course, this would likely depend on a lot of things and is pretty difficult to do in Solidworks. the upshot is that I just would not trust the stress near bonded dissimilar materials unless I really knew what I was doing.

                                     

                                    It works for the bolt stress, because the bolt stress is in some sense 'far' from the discontinuity.

                          • Re: Analyzing Bolt Hole Stress
                            Jaspreet Hothi

                            See the highlighted text below from bolted connection definition. I would expect this effect to get amplified if you fix the area under the bolt head.

                             

                            bolts.jpg