TNX Tower Arbitrary Sections 1

[Antnyt23]

I am having trouble using the arbitrary sections in the database for TNX Tower.

The example I'm trying to look at is a self-supporting tower. A 20 foot section I'm looking at has be strengthened with the following modification:

The original leg was a solid round 1 1/4" leg.
2- L1 1/2x1 1/2 x 3/16" angles (bolted together to form a T) have been U-bolted to each leg (The U-bolt is around the Solid round leg bolted through the one remaining leg to each of the angles).

When calculating the properties of this modified shape for each leg I took the cross section of the solid round with the two angles and came up with the following properties:

Area = 2.281
Ix = 0.99
Sx(top) = 0.61
Sy(bot) = 0.88
Iy = 0.55
Sy(left) = 0.37 = Sy(right)
rx = 0.658
ry = 0.490

with the centroid located at
Xc(left) = 1.5
Xc(right) = 1.5
Yc(top) = 1.631
Yc(bottom) = 1.119

I created and arbitrary section, input these values and assigned the shape as the leg at that location and then tried to analyze it but I'm coming up with major problems (ie weaker than the original shape). I assumed that the arbitrary shape was just suppose to be the cross section of whatever the combined shape would be then you assigned it to a member type (leg, diagonal, etc.) and it assigned the length and such.

If anyone has some experience with this and could help me get a better understanding of this I would greatly appreciate it.

 

[JoshPlumSE]

You should contact the Tnx technical support. RISA-3D doesn't do code checks on arbitrary members. It's tricky to do so, because code checks are frequently based on values that are unknown for arbitrary members. Plastic section modulus, local buckling Q values, LTB torsional values (rt, Cw, et cetera). I'm sure that Tnx is making somem assumptions about these values, but what those are I cannot remember anymore.... at one point I probably knew, but it's been long enough that it's just not coming to me.

 

[Antnyt23]

Okay thank you. I have contacted Peter and will try to get it figured out

 

[msquared48]

Personally, I use another program, Shape Builder, to calculate the revised section properties, which will also handle grouted column legs, and import the values into the TNX Tower database as a new shape. Works just fine for me.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Antnyt23]

I think I figured out simple shapes. Have you ever done modified truss legs in TNX tower?

I will check out that Shape Builder trial version and possibly purchase it.

 

[msquared48]

Yes I have. Have to do it all the time as not all available sections and conditions, particularly for tower upgrades, are in the TNX Tower database.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Antnyt23]

I was trying to figure out how to accurately do this. Say I add angles to the solid rounds of the legs of the truss legs. When you do this do you input the leg member as a truss leg or as an arbitrary shape section that would represent the truss leg? If you do the arbitrary shape, do you do the cross section of the 3 legs of the truss leg and neglect the diagonals/top girts since they are not a continuous part of the cross section as it travels over the length of the leg?

I attached something I put together in shape builder to represent three legs of a truss leg with two added angles on each leg as upgrades. Was curious if this is how it is commonly done.