steel truss web member

shei · Jul 2, 2003

The original span of the truss was 17.45meters. During construction stage there was revision on the roof framing which required the truss span to be 23m. Before the revision was final we had the trusses fabricated already. To avoid having the fabricated truss go to waste we tried to redesign the existing truss to meet the required sections of the 23m span.
Here are the methods used:
1) addition of flat bar for the top and bottom chord
2) addition of 2 angles for the web members which make the web members to 4 angles.

Questions:
a) should the 4 angles in the web member be placed back to back? why?

b)the web member of the fabricated truss is placed back to back. If the additional 2 angles be placed such that it will form a box on each side (thus having larger Ix than back to back), is this advisable? Why?

c) what other methods can be done beside those i mentioned in 1 & 2?

thanks and best regards.

dlew · Jul 2, 2003

shei,

Some thoughts about your problem:

If the load on the truss remained the same, the axial force in the web members would increase 32%. The added back-to-back angles, if they are of the same size would double the capacity of the member, that is more than enough.

Another way to reinforce the double angles would be to weld a flat plate to the toes of the adjacent legs, making a I-section.

You should consider also what changes need to be done to the end connections of the web members.

Hope this will help

AEF

flamby · Jul 2, 2003

I had one similar problem in the past. The span didn't change but the loading got changed and we were halfway through our fabrication work.
We simply incresed the number of trusses in the bay by reducing the spacing. The reduced tributary width allowed us to use the same sections.

In a truss if you club the angles to a box section, compression members can take larger load due to incresed l/r. Tension members do not care whether they are box or star. I would give more priority to ease of fabrication than any effect due to change in I.

dlew-
Shouldn't it be 74%?

Ron · Jul 3, 2003

Your fix should be appropriate, at least at first glance.

The reason for placing angle back-to-back is to allow easier connection between them for buckling considerations under compression. As dlew mentioned, you can add a flat plate across the opposing legs or you can put a small piece of square or round stock between adjacent legs and weld them together.

It's a truss, thus a linear relationship...32%. If it were a beam, the moment would be increased by 74%.

trainguy · Jul 3, 2003

In my experience, the issue that will drive the design of your web members will be the layout of the connections at the extremities. You need to look at it from that viewpoint, and pretty soon, you will be able to select the best approach.

Remember, all your members must have sufficient connection strengths at the extremities, and you have to make sure that the adjacent structure can support the increased web member loads.

my 2 cents...

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