Strut and Tie Models - 3D with moment applied on Diagonal

[fredPE]

So, I am just learning strut and tie and had a three dimension footing over (4) drilled pier footing that is supporting a tower crane. The worse case load case for the tower crane is on the diagonal, where I will get tension on one pier, a big compression on the opposite pier, and two smaller compressions on the piers closest to the axis of bending. So I set up a strut and tie model in risa 3D, and have only found one configuration of struts and ties that give me the actual global reactions at the supports. My problem is to get the global reactions, I have two diagonal ties with quite a bit of tension force running through the footings. This diagonal tie goes from the tower crane legs that are nearest the axis of bending at the top of the footing, and extend down to the top of the pier that has the most compression at the bottom of the footing. My problem is that I don't have rebar that will be in line with this diagonal tie in my model, and I am not sure how to handle that. I have tried countless ways of adjusting the model trying to get rid of these random tension forces, and have found no other way to do so. Is it OK to sum the top mat rebar and reduce it by the difference in the angle?

 

[KootK]

Firstly, kudos to you for attempting a 3D STM design. I've abandoned a large number of the ones that I've attempted in frustration. While your proposal surely has some practical merit, A complete STM model requires anchorage of all ties within crossing struts. I'm sure how you'd reconcile that with your rebar layout. If you post a sketch of your situation and STM model, there's a much better chance that we'll be able to help you find a solution.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[asixth]

Sounds like it would be more easily done as a 2-dimensional strut and tie model if you have an overturning moment from a tower crane being resisted by 2-piers, one in tension and the other in compression.

An important part of strut and tie design is to model the actually behaviour of the structure. If you have "diagonal ties with quite a bit of tension force", then you will need to provide anchored vertical reinforcement for this tension.

If I were to do a STM of this footing then I would be modelling diagonal compression struts only. If the distance from the tower crane leg to pier is long enough that you can't achieve a 45-deg diagonal compression strut then I would have a vertical tension tie. If the tension force is significant enough to warrant reinforcement than it should be detailed with some ligs.

 

[KootK]

These two articles are about all that I've got on the subject:

Link

Link



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[fredPE]

Thanks for the responses! I slept on it over night, and I was able to get it to work. Basically, I grouped the three compression load legs of the tower crane into a single node at the centroid of the load, and it greatly simplified the model. Now all of my diagonal struts are compression only. I was also using Chapter 8 of this link as an example for creating the model.

http://www.utexas.edu/research/ctr/pdf_reports/5_5253_01_1.pdf

Before, when I tried to model all three legs, the diagonal struts that went to the two lightly loaded legs had big tensions, but they were in the same general direction of the main leg diagonal strut that was a much bigger compression force.

Thanks for the help! I will study the links that you gave me KootK! They look quite a bit simpler than the 300 page .pdf that I was using before.