Defn of Minimum Gauge Length 2

[cvengr]

One of the recent changes to ASCE 7-10 (15.7.5 Seismic Anchorage) and ACI 318(App D Eqn D-3) require that anchor bolts have a minimum gauge length of 8 diameters.

I haven't been able to find the explicit definition for "Minimum Gauge Length" in the context of concrete anchor bolts, either in the ASCE 7 or ACI 318.

Surely this is a simple question, but I haven't found the simple answer.

Is it simply the minimum depth of the anchor in the concrete, below the threads? ????

(I've seen other past comments indicate a minimum anchor depth of 6 diameters was preferable, but never to exceed 8 diameters. I still am not explicitly clear on the intention.)

Any help is appreciated.

 

[BAretired]

Gauge length for a bolt usually refers to the spacing of lines of bolts, so I'm guessing that is what they mean.

BA

 

[DST148]

Definition of gauge length (books on mechanics) - It's the length over which percentage elongation of a specimen is measured.

 

[wannabeSE]

The code is not very clear. The term could mean several things (bolt spacing or length of unthreaded portion). However, the answer can be found enclosed in parenthesis on page 158 of FEMA P-750 NEHRP Recommended Seismic Provisions for New Buildings and Other Structures, 2009 Edition, Part 2 Commentary to ASCE 7-05, section C13.4.2 on anchors in concrete and masonry:

"Ductile bolts should only be relied upon as the primary ductile mechanism of a system if the bolts are designed to have adequate gauge length (unbonded strained length of the bolt) to accommodate the anticipated nonlinear displacements of the system at the design earthquake."

 

[cvengr]

I perceive the purpose of a gauge length is in reference as a basis of strain. I find the term used with respect to minimum tendon length in post-tensioned concrete (being 10'), so it makes sense for it to represent a min length of anchor bolt in which tensile stresses are allowed to operate and known to behave per specification, minimally 8 times the diameter of the bolt.

On the other hand, I also read commentary elsewhere placing a guide of 6 diameters, and never to exceed 8 diameters. The ASCE 7-10 revision, though, states minimum of 8 diameters, so I'm not fully clear as to why this minimum condition is being made.

Would this imply the bolt needs to be sleeved, so it's ability to strain isn't influenced by adjacent concrete bonding or friction? ...to insure the bolt is able to handle the pure tensile forces in the design?

The ACI AppD Eqn D-3 is actually in AppD Section 5, Design Requirements for Tensile Loading. I suspect other commentary recommending no greater than 8dia are looking to avoid lengths which might bend or distort, as the material strength of a homogenous material is approached from a different geometry, loading shear stresses along with the tensile design.

Generally the bolt or bolt group will also have a moment being transferred to the concrete foundation, but the code section is only focusing on tensile design, so I suspect there is something practical in mind with this constraint.

 

[amec2004]

Gauge Length in ASCE 7-10 15.7.5 = Stretch Length in ACI 318-11,D.3.3.4.3 (a) -3

See my presentation at

http://civilbay.com/download/eng/ASCE-Anchor-Bolt-Gauge-Length.pdf

anchor bolt design per ACI 318-11 crane beam design

http://www.civilbay.com