Cable members in truss

[MIStructE_IRE]

Hi,

I’m looking at a simple floor truss and considering using Macalloy cables or tension rods (with turnbuckle) for the tension only members.

My question is, can the cables/rods be installed and simply hand tightened or do they need to be tensioned to a specific value? If so, then how is this force calculated?

Thanks

 

[BridgeSmith]

I would assume the purpose of having the adjust would be to counteract the deflection of the truss under load. If that assumption is correct, then the pretension in the cables would be determined by analyzing the truss with the applicable loads applied, determining the tension in the cables, calculating the deformation of the cables due to the tension, and tightening up the turnbuckles beyond hand tight enough to compensate for the amount that each cable is expected to stretch. This assumes that the deformation of the other truss members is negligible; if it is not, then the analysis becomes far more complicated, unless adjustments are made in-place under load.

 

[dhengr]

MIStructE_IRE:
Several things to consider here…. End threaded solid rods may be easier to work with because their elongation and the force they apply is more strictly an elastic situation. Whereas, cables have both an elastic elongation component of the individual wires and strands, and a cable lay consolidation or manufacture elongation component with an applied force. And, the two sum for the total wire rope stretch. They do pre-stretch some heavy structural cables to pre-set most of the mechanical stretch component, but you and I probably wouldn’t pay for this on a small job. The certain (most accurate) way to determine the solid rod force and elongation would be with strain gages, but again, this might be more time and cost than we want to spend. If you know the thread pitch on the solid rod and the turnbuckle, a single turn of the nut means a certain rod longitudinal end movement, but maybe not the rod elongation/force. This takes some experience and engineering judgement, because at first you might just be tightening up truss joints, or seeing some end bearing movement at the rod end connections. You might mark a couple widely spaced gage marks on the rods and try to measure the elongation to a fine accuracy. You might also consider two harped rods, one each side of the truss, for symmetry and ease of detailing and use. These can apply some of the tension you want to the system and they also apply one or a couple upward reactions at the bot. chord at the harp points. These reduce/change the loads/forces in the truss members due to the gravity loads, and you can measure the deflection change they cause. Then do some calcs. to bracket the truss forces and deflections with various rod loads which you have assumed or measured.

 

[slickdeals]

Rods are better than cables. You need the area to keep deflections down. Cables stretch out more than rods even though they are stronger. Plus rods with turnbuckles give you more adjustment capabilities.

 

[klaus.]

No...basically do not need to be prestressed
Cable not and rod not

 

[MIStructE_IRE]

Macalloy say that their cable will stretch by a length of NL/EA (Load x Length / (Youngs Mod x Area).

This is an initial permanenet stretch. I think to post tension this to counter this effect before it happens would overstress other members within the truss. Perhaps its best tosimply adjust the cables as needed as the system deflects?

 

[Agent666]

Certainly if you were post tensioning then you'd need to allow for the forces on the other members depending on the construction sequencing (manage when and by how much, and in what sequence you tensioned the rods).

A better option to overcome any initial deflection would be to fabricate the truss with some preset to offset the initial/final deflections.

 

[BridgeSmith]

"Perhaps its best to simply adjust the cables as needed as the system deflects?"

If feasible, this is the most accurate way.