steel truss web member
steel truss web member
(OP)
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.
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.






RE: steel truss web member
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
RE: steel truss web member
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%?
RE: steel truss web member
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%.
RE: steel truss web member
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...