50 ft long truss beam with a 300 kip point load
50 ft long truss beam with a 300 kip point load
(OP)
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.
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.






RE: 50 ft long truss beam with a 300 kip point load
Are you getting the expected axial forces in the diagonals and verticals and what are you getting in the the bottom chord?
RE: 50 ft long truss beam with a 300 kip point load
RE: 50 ft long truss beam with a 300 kip point load
1) Where on the span is the point load located?
2) Is the point load located at a panel point as one would expect?
3) parallel chord truss shape or something else?
4) What size, shape, and orientation of chord is being used?
5) Linear elastic modelling so far?
6) How deep is your truss?
My guess is that your truss is simply not deep enough. Any parallel chord truss with continuous chords will have the following three mechanisms acting concurrently and drawing load in proportion to their relative stiffness:
1) Top chord acting as a beam.
2) Bottom chord acting as a beam.
3) Truss acting as a beam.
If three isn't substantially stiffer than one and two, you'll get beam-ish results in the chords (moments and shears). Check the vertical shears in your chords near your supports and add them together. That's about how much load is not going through your truss, acting like a 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.
RE: 50 ft long truss beam with a 300 kip point load
Dik
RE: 50 ft long truss beam with a 300 kip point load
hokie66: I have the chords oriented in strong axis. You are onto something here because I tried to play with different chord members. When I made them more flexible, I was able to reduce the sizes and the moment went down at both chords. Unfortunately, this resulted in L/180 which isn't good enough. To get the deflection limit to L/360, I needed stiffer member, i.e. wide flange, and it increased the moment somehow. As a result, my combined axial and bending check exceeded unity as my bending demand went up while axial load demand was relatively similar for either member choices.
Hope this clarifies a little. Thanks.
RE: 50 ft long truss beam with a 300 kip point load
RE: 50 ft long truss beam with a 300 kip point load
RE: 50 ft long truss beam with a 300 kip point load
1) Where on the span is the point load located? 50 ft span and point load is at 40 ft. (we have 10 feet on the other side (2 bays) that can fit 2 diagonals to reach to the support at the right)
2) Is the point load located at a panel point as one would expect? It is located at where the diagonal is.
3) parallel chord truss shape or something else? Truss looks like this. Assume that the angles are actually HSS members. http://quicksilver.be.washington.edu/courses/arch4...
4) What size, shape, and orientation of chord is being used? W18x87 at top and bottom chords. HSS8x8 at diagonal where the point load applied and smaller HSS at other locations where there are no point loads.
5) Linear elastic modelling so far? I am using reduced stiffness option in RISA that is adjusting stiffness iteratively.
6) How deep is your truss? Truss is 5 feet deep.
RE: 50 ft long truss beam with a 300 kip point load
RE: 50 ft long truss beam with a 300 kip point load
RE: 50 ft long truss beam with a 300 kip point load
Another thing to play with is your web sizes. Up the sizes well beyond that required for strength and see if that makes a difference. Sometime, if webs are strong enough but not very stiff, you'll get high shear deflection across the truss that will draw more moment to the chords. I wonder about it here because, with that monster load only 20% from the support, I'd expect your truss response to be dominated by shear more than flexure.
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.
RE: 50 ft long truss beam with a 300 kip point load
If you're top chord hung, is that where the big moments are showing up?
How about a screen capture of the truss with it's moments plotted?
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.
RE: 50 ft long truss beam with a 300 kip point load
RE: 50 ft long truss beam with a 300 kip point load
RE: 50 ft long truss beam with a 300 kip point load
RE: 50 ft long truss beam with a 300 kip point load
RE: 50 ft long truss beam with a 300 kip point load
Dik
RE: 50 ft long truss beam with a 300 kip point load
- give consideration the the possibility that it may only be the deflection immediately below the 300 kip load that matters here rather than the midspan truss deflection.
- I've suggested post-tensioning the bottom chord. The big advantage of that is that you can tune the tensioning rods in the field to drive the truss upwards and offset the deflection which seems to governing your design. That may get you some meaningful efficiency improvements.
- You'd have some extra compression in your top chord of course but also less bending.
- In terms of pure material quantities, it would probably be more efficient to harp the post tensioned rods up towards the supports. Doing so would probably introduce some costly connections at the direction change, however. Doing the straight PT shouldn't add all that much fabrication cost. Some erection cost though.
- Like my colleagues here, I'm a big fan of the weak axis WF chords. In addition to the praises already sung, it might facilitate the easy delivery of the monster load into into the truss. If you're interested in taking it that far, let me know and I'll elaborate.
- Considering clearances, it may be more constructable to put the tensioned rids outboard of the bottom chord flanges. I'll leave that decision in your capable hands.
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.
RE: 50 ft long truss beam with a 300 kip point load
RE: 50 ft long truss beam with a 300 kip point load
BA
RE: 50 ft long truss beam with a 300 kip point load
RE: 50 ft long truss beam with a 300 kip point load
Seems like it would be cheaper to go the I-beam route for the other scenarios.
RE: 50 ft long truss beam with a 300 kip point load
RE: 50 ft long truss beam with a 300 kip point load
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
RE: 50 ft long truss beam with a 300 kip point load
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.
RE: 50 ft long truss beam with a 300 kip point load
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?
RE: 50 ft long truss beam with a 300 kip point load
@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.
RE: 50 ft long truss beam with a 300 kip point load
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.
RE: 50 ft long truss beam with a 300 kip point load
RE: 50 ft long truss beam with a 300 kip point load
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.
RE: 50 ft long truss beam with a 300 kip point load
BA
RE: 50 ft long truss beam with a 300 kip point load
RE: 50 ft long truss beam with a 300 kip point load
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.