50 ft long truss beam with a 300 kip point load 4

[davab]

Hi guys,

I am designing a 50 feet long steel girder that has one point load that is around 300 kips. This resulted in a huge steel beam that is W40x372. Due to cost implications, we are considering to design a truss beam at that location. I have been using RISA to analyze this truss member and I am not able to optimize the steel tonnage as much as our team expects. I have looked into every possible resources to figure out whether something is wrong with my model. I can't seem to find anything wrong with the boundary conditions or the model setup.

Here are some general info:

Top and bottom chords are continuous Wide Flange.
Diagonal and vertical members are "pinned" to take only axial loads and each bay is spaced at 5 feet o/c.

Note that the top and bottom chords are modeled continuously, which is more realistic representation of a real life truss.

Contrary to what I was expecting for a truss, I am getting a huge moment at the top chord. I believe that it is resulted from "secondary moment" that is being created by a huge point load.

My question is:

1. Has anyone modeled a truss that had a substantially asymmetric point load? (meaning that 300 kips is located at one side only and not symmetrically)
2. Does anyone know if trusses are not economical when handling such asymmetric loading?

Any thoughts or concerns would be helpful.

Thanks guys.

 

[BAretired]

Maybe you have selected the wrong shape of truss. What happens if you build a truss in the shape of the bending moment diagram? Top chord is flat with constant compression throughout. Bottom chord slopes from 0 at each end to d at the 300k load with uniform tension throughout; d can be selected for best economy. One vertical is needed under the 300k load. Additional web members may be used to brace the top chord but they carry no load.

BA

 

[Deker]

I highly recommend watching this video if you haven't already: Link. Lots of great info about truss configuration, connections, economy, etc.

 

[XR250]

I was just thinking about this and then saw BA's post. That looks like the most economical way to go (see attached)
Seems like it would be cheaper to go the I-beam route for the other scenarios.

 

[SteelPE]

BA isn't that how metal building manufacturers used to build their frames? This is what I was told by an old metal building engineer.

 

[BAretired]

BA isn't that how metal building manufacturers used to build their frames? This is what I was told by an old metal building engineer.

I'm not familiar with that example, but when supporting a single large concentrated load such as we have in this problem, it seems to make sense to configure the bottom chord of the truss in this way. It is the same shape a cable would assume when carrying a point load. It has to be efficient because every part of the top and bottom chord is stressed to its maximum and web members are not required to carry shear.

BA

 

[KootK]

When it comes to saving material, it generally pays to have as close to a pure tension system as possible. BA's truss gets pretty close to that. It's only a beam and slab world out there because of the economics of floor to floor heights etc.

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.

 

[bowlingdanish]

Interesting ideas

Hi KootK, sorry to keep calling you out but you come up with some obscure things that pique my interest. Is the idea to precamber the truss, or to reduce the tension in the bottom chord that the high point load transmits? If 1) why not just precamber, if 2) just nump up steel? Is this something that has been done with regular steel structures before for various reasons?

 

[KootK]

@Deker: I watched that video for the first time yesterday. It was excellent, thanks for the recommendation.

@Bowling:

1) No sweat. Tech debate is usually fun for me.

2) I read a book on post tensioning steel things a while back and have been obsessed with trying it ever since. So there's a bias at play.

3) Bumping up the steel is definitely an option but I took OP's intent as being to try to reduce steel weight.

4) In many ways, precamber would do the same thing. Precamber has two disadvantages however:

A) Given the load, I'm envisioning a structure above being transferred out. Some folks, including me, don't like precamber for controlling deflection at transfered elements. The lack of precision and predictability in the camber can cause issues for the erectability of the steel above. With the post tensioning, one can dial up the prestress incrementally as the structure above is constructed and control the result with some precision by measuring it.

B) Precamber affects the final position of the truss but not the amount that the truss actually deflects. Thus, camber wouldn't decrease the moments in the chords where I feel that the post-tensioningg would as the post tensioning could be used to lower the deflection experienced by the truss.



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.

 

[KootK]

Oh, and people have certainly post tensioned steel before but it really isn't very common.

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.

 

[davab]

Thank you for your recommendations. I didn't expect to have such great discussions among everyone here. I am going to try to digest all of the suggestions above and keep you posted.

 

[davab]

When it comes to figuring out the unbraced length of the chords, what do you guys use?
Currently, I have my unbraced length in weak-axis (call it yy) as 5 ft and in strong-axis (call it zz) as 55 ft. Now, because I am assuming that there is no bracing point vertically (in strong axis direction), KL/R is being governed in the strong-axis. This governing factor is dropping my compression capacity significantly, which is the reason why my chord sizes kept on increasing.

Do you guys have any suggestion what to use for unbraced length? Weak-axis unbraced length is simple and it is just a matter of adding perpendicular beams or kickers along the top and bottom chords. However, what is the unbraced length in strong-axis? Can I assume that the diagonal members are "bracing" the top and bottom chords vertically or is it wrong because there isn't any "restraining mechanism" from those diagonals as the entire truss moves together?

Thanks.

 

[BAretired]

I agree that the unbraced length for weak axis is 5'. The unbraced length for strong axis is the length of the truss unless you can brace it to something else such as a roof or floor system. If not, it may be more economical to consider a boxed shape where two trusses are tied together by a cross braced system. In that case, there may be no particular advantage in using a shape with a strong axis and a weak axis; an HSS could be used with unbraced length of 5' in both directions.

BA

 

[hokie66]

Or, as an extension of BA's suggestion, perhaps a 3 chord truss, where the cross section is a triangle.

 

[KootK]

Can I assume that the diagonal members are "bracing" the top and bottom chords vertically

Yes. In fact most trusses would be terribly uneconomical otherwise. It's essentially relative bracing as described in the AISC section on bracing.

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.