Tiebeam Loading/Freebody Diagram Question

[zrck99]

We are having a little debate about how tension forces would resolve themselves when they are occurring simulataneously on the same member. In my attached sketch, I show two Pre Engineered Metal Building Columns with outward 10k forces applied to them.The building is used for soccer fields so there is no slab present. If you were to wrap hairpins around the columns then run a tiebeam between the two, is there any point in the middle of the tiebeam where the two outward forces meet each other and result in the bars seeing 20k of force? Our general feel is that if you cut a section along the tiebeam at any given location then you can resolve the 10k force with an opposite reaction so at most the tiebeam bars need to be designed to handle the 10k force in tension. I still have a lingering feeling that somehow the bars may need to be designed for the overall 20k force. I was just hoping for some confirmation that we are thinking about this correctly or what other peoples thoughts are.

A similar but opposite situation would be if you had two retaining walls supported with a pipe brace between them. If each pipe brace receives a 10k load from each wall that it is supporting then would the brace need to be designed for 10k or 20k? you could cut a section along the pipe brace at any given location and resolve the fbd with 10k but again, I worry that the column is seeing 2x that force.

https://files.engineering.com/getfi...b78-8af7-d888260e9597&file=tie_beam_loads.pdf

 

[zrck99]

I suppose if you had a foundation with a dead load = 10k and a column with 10k of uplift from wind load, It isn't like you have to design your anchor bolts for 20k of load to tie the two together. I've just never really worried about it until seeing this tie beam example.

 

[Celt83]

If you called this a building column with axial load P bearing on a foundation so the total soil reaction is P are you designing the column for an axial force of 2P or P. With equal and opposite forces you just have a system in static equilibrium.


Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[KootK]

Definitely the 10 kip everywhere and not the 20 kip. Trust the FBD. Newtonian physics is still pretty reliable.

 

[mmarlow]

10 kip for sure.

Back in undergrad we had a similar question on a steel final. It was for shoring for the big dig in boston. It was basically horizontal columns that took 50kip (might be making that number up)from either side, and we had to run some buckling checks... A lot of people were thrown off by the 50 kips being from either side...



-MMARLOW EIT

 

[Ron247]

It would be the 10 kips. As already stated, the FBD would be correct. I assume you do trust a FBD of a tension member with the same 10 kip forces on it. I see why on the surface, it makes sense to add the two and get 20 kips. Other than the FBD, I think the following may explain why you don't.

If you just have 10 kips on the left column, you pull the right foundation towards you in effect. The 10 kip reaction is the right column fdtn in passive bearing and sliding. As the 10 kips starts appearing on the right column, the passive and sliding are replaced by the new 10 kip tension force. So you do not add them, what happens is the source of the 10 kip reaction changes its make-up. I think that is what is happening, but would like comments about the correctness of what I am thinking.

 

[BridgeSmith]

If there was no friction between the tie beam and the material around it, there would be 10 kips tension everywhere along the beam. With friction, the tension is potentially less at some points along the beam, but never more.

 

[BAretired]

It is a very important element of structural understanding to see why the force in the hairpin is 10k and not 20k. If one of the applied 10k forces is removed, the structure is not in equilibrium. To achieve equilibrium, statics requires that an equal and opposite reaction force is required somewhere along the length of hairpin. In this case, it is available from an applied 10k force in the opposite direction at the other end of the tie.

Suppose that, under some loading condition, there is an applied 10k force at one end and a 5k force in the opposite direction at the other end. What then?

The tie must carry a 10k tension in order to satisfy the larger applied load. Since 5k is not enough to maintain equilibrium, an additional 5k is needed; this must come from the anchorage of the column to the earth.

BA