Critical Forces

[AJA14]

This is a general question. I am currently in the process of designing a steel factory. When i want to design a steel rafter beam, i have no way of getting the critical forces acting on the beam except by designing the beam using robot and getting the internal forces robot has got. These forces cannot be achieved any other way. The forces table only gives the end forces which may or may not be critical. The diagrams give all the loads, but one has to read all the force diagrams and try to manually judge which will be the largest case. Please see attached image. Is there a way of actually getting the critical forces without asking robot to design the member to get those forces.

[Artur.Kosakowski]

Having the forces table open press the right mouse button and select Table Columns. Then decide on number of points or run search of extreme values as shown on the attached picture.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[AJA14]

Thanks for the quick reply. However that did not answer my question. The values with the largest moment is not the critical moment in my case. Neither is the one with the largest compression, minor axis moment or even shear. It is a combination of these that occurs somewhere along the beam. If you look back at the image, you will find that the values of the internal forces are different from any extreme force acting on the member. if this was a case of a simple beam it would be easy to read the forces from the diagrams, but when biaxial moment and axial force are all combined together, it becomes hard to judge where the critical load occurs. I hope I explained myself better this time. Appreciate the help.

[Artur.Kosakowski]

Well, perhaps I'm missing the point entirely but in such case what kind of analysis would you like to run to find the most critical interaction between all the internal forces besides the steel design bearing in mind that this also depends on the restrains defined along a bar (e.g. buckling or LTB parameters)?

Assuming that there is no problem with the stability of a profile you can try to check the values of stresses in the stresses table but my personal feeling is that this is just what the steel design module is intended for.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[AJA14]

You are entirely correct. But to make myself clear, the design module says that with the LTB and buckling parameters inserted and with the load combinations present, the critical load combination occurs at x=0.9L of the beam. I was just wandering that if someone wanted to manually design the beam, how would he have known that the critical forces occur at x = 0.9 L of the beam. Moreover, if I were to manually design the beam, I would have never guessed that this is the location that would have the highest ratio. Thanks anyways.

   Good think Robot does the job for you. Thanks again.

[Artur.Kosakowski]

how would he have known that the critical forces occur at x = 0.9 L of the beam

Some name this experience I guess

   Good think Robot does the job for you

Be careful, it does what is asked for so make sure you decided on enough verification points especially when load distribution along the element changes or you have bar with different segments (design parameters) please.