Temporary Rock Anchors

[geodoc]

The PTI Manual recommends a design load of 0.6 times the specified minimum tensile strength of the steel bar. Has anyone ever designed TEMPORARY anchors using a higher factor, and if so, what was your basis for the higher factor?

 

[PSlem]

You want to avoid a zipper failure. If one anchor should fail, the ones on either side would need to hold 1.5 times their design load or all anchors could fail. 0.6 x 1.5 = 0.9.

 

[PEinc]

No. Temporary anchors are designed for a maximum working (service) load of 0.6 x GUTS (guaranteed ultimate stress of Grade 150 KSI and Grade 270 KSI tendons. Grade 60 & 75 KSI bars are designed for a maximum working load of 0.6 x Fy (yield stress of bar). On highway jobs, the 0.6 is often lowered to 0.55 for permanent applications.

 

[born2drill]

Right. This also works neatly since anchor test loads are usually 133% of the design load. Designing for 0.6 x GUTS and testing to 133% DL gets you to just about 0.8 x GUTS, which is just under where the bar wants to yield. Some bar manufacturers will say in their literature that it's okay to design temporary anchors around 0.7 x GUTS, but I would not do that.

Also - if your specs need you to test to more than 133%, like say 150% DL, then you have to design around even lower than 0.6. This often happens on DOT jobs as PEinc mentioned.