Cable Tension Tie for Preengineered Metal Building

[DaveAtkins]

I have done a lot of foundation designs for preengineered metal buildings, but our client has requested something I have not heard of before. I designed a tension tie through the slab-on-grade using #6 bars (the thrust at the column bases was too large for hairpins). The client wants me to consider using a CABLE instead. How is the connection to the pier done? Does the cable need concrete protection, or can it be buried under the slab-on-grade?

DaveAtkins

 

[Taro]

Maybe the client is referring to a post-tensioning strand (sometimes called a cable). The strand would be embedded in the concrete and connected with a standard anchor plate at each end.

 

[DaveAtkins]

I am sure he is referring to "wire rope" as would be used for temporary bracing of structures.

DaveAtkins

 

[Lutfi]

I agree with Dave. I done a similar design where the owner was a steel fabricator and they did not want a concrete slab. To handle the thrust at the rigid frame base, I added a bar with turnbuckle and tied the rigid frame footing and buried the rod below the dirt.

I would use the cable and would sleeve it in a pvc pipe to keep it from bonding and transmitting loads to other structural members (i.e slabs).

Regarding the end, be creating as long as the connection can take the tensile and any bending. Check on the pier and footing for new induced possible overturning moment and the soil bearing pressure as well.

Good luck

 

[PEinc]

I have used post-tensioning strands as Taro has mentioned. The strands were installed in small trenches just beneath the slab. The strands were connected to the column pedestals at each end with standard hardware for mono-strand tendons. The stressing hardware (bearing plates and stressing wedges) were locked off against the concrete in boxed-out recesses which were then concreted for corrosion protection. I used Polystrand from Lang Tendons, Inc. in PA and CA (Chris or Ken, 610-268-2221,

http://www.langtendons.com).

Polystrand is is a 270 KSI, 7 wire strand with a grease filled extruded polymer sheath. The maximum working load of a 0.6 inch diameter strand is 35.2 kips.

Remember, for the strand to carry the load it must stretch. You can let it stretch when the load is applied or you can pre-stress the tendon. If you let a long strand stretch (PL/AE) when the load building load develops, you could get significant structure movement. This can also happen with other types of cross ties such as threadbars and coupled rebars. If you pre-stress the strands, you will be pulling on the bases of the columns with nothing to prevent inward lateral column movement except the backfill around the columns. If, in the future, there is a need to excavate inside the building for other construction such as a new pit, foundation, or utility, the tied, opposing columns could possibly move toward each other when the restraining dirt is excavated.

Sometimes, you just can't use hairpins.

 

[LPPE]

Would the (not post tensioned) cables relaxation under load be a problem?

 

[jheidt2543]

I've used a solid steel bar with turnbuckle connectors too.

The problem of corrosion of the strands that make up the wire rope or prestress strand is a critical one. A few wires can corrode pretty quickly compared to a solid rebar or rod, with really bad consequences.

And, you can always design the column piers and footings to take the applied moment of the horizontal thrust, but it leads to big footings. But, it does solve the problem of future remodeling that may cut the ties between columns.

 

[PEinc]

The strand tendons are the same type of low relaxation tendons used in post-tensioned slabs and tanks and for tieback and tiedown anchors. The Polystrands are coated with a corrosion resisting grease and are encapsulated in an extruded plastic sheath. The strands should have better corrosoion resistance than a bare steel cable or threadbar.

See

http://www.langtendons.com

for additional information.