Proof Load vs. Ultimate Load? 1

[JJOO]

Could anyone enlighten me on the difference between proof load and ultimate load?

I have a contractor claiming that the allowable bolt tension capacity as per ASCE48-05 is 0.83Fu*Ab. I've found this specification, and here is the Section 9.3.1:

The tensile, proof-load, and yield stresses shall be specified minimum values determined according to the ASTM specification for the material involved.
Bolts shall be designed so that the sum of the tensile stresses caused by the applied external load and any tensile stress resulting from prying action does not exceed the tensile stress permitted, Ft, as follows:
1. For bolts having a specified proof-load stress, Ft, shall equal the lowest value of ASTM proof-load stress by the length-measurement method or 0.83Fu, where Fu is the specified minimum tensile stress of the bolt.
2. For bolts with no specified proof-load stress but a specified yield stress, Ft, shall equal the lowest value of Fy, where Fy is the specified yield stress, or 0.83Fu, where Fu is the specified minimum tensile stress of the bolt.
3. For bolts with no specified proof-load stress or yield stress, Ft shall equal 0.60Fu.

Based on this, this contractor believes that the allowable tension capacity of 2" dia. F1554 Gr.105 anchor rod would be (0.83)*(125ksi)*(3.14 sq.in.)=326 kips!! Well, that seems really high to me.

The nominal tensile stress of 2" dia. F1554 Gr.105 anchor rod is 0.75Fu as per AISC Design Guide 1 Table 2.2, and the tension capacity would be (0.75)*(125ksi)*(3.14 sq.in.)/(omega=2)=147 kips [ASD], (phi=0.75)*(0.75)*(125ksi)*(3.14 sq.in.)=221 kips [LRFD].

What is "proof load"? I think I need to understand better of what "proof load" means to convince these guys that a 2" dia. anchor rod (F1554 Gr.105) does not have that much allowable tension capacity. Anyone familiar with transmission tower design? Please help!

 

[MintJulep]

From ASME B18.12-2001 Glossary of Terms for Mechanical Fasteners:

Proof Load:a tensile load that the fastener must support without evidence of permanent deformation. Proof load is an absolute value, not a maximum or minimum value. For most carbon or alloy steel fastener strength grades or property classes, proof loads are established at approximately 90 to 93% of the expected minimum yield strength. Proof loads are frequently used as design values in joint analysis and fastener selection.

 

[Tmoose]

I think this makes a fun topic over coffee, but not usually a significant part of design. Lots of anchors (maybe most) are more limited by conditions other than the anchor's "strength," like the design of the embedding feature.

If the nature of the load is cyclic (rotating machinery, etc) then best efforts to make "strong" fasteners are going to include preload, as covered in many other threads.

 

[AHaddad1]

I believe there is a confusion between Fu and Ft in your post. Fu should be the Ultimate Failure Stress and Ft as the equated yield stress which in your case is 105 ksi

Available Properties for F1554 Anchor bolts

F1554 Grade 36 Low carbon, 36 ksi yield steel anchor bolts
F1554 Grade 55 Low alloy, 55 ksi yield steel anchor bolts
F1554 Grade 105 Alloy, heat treated, high strength 105 ksi yield steel anchor bolts

The allowable tensile stress on the rod material is what you have calculated based on the Ultimate Stress:
Quote
The nominal tensile stress of 2" dia. F1554 Gr.105 anchor rod is 0.75Fu as per AISC Design Guide 1 Table 2.2, and the tension capacity would be (0.75)*(125ksi)*(3.14 sq.in.)/(omega=2)=147 kips [ASD], (phi=0.75)*(0.75)*(125ksi)*(3.14 sq.in.)=221 kips [LRFD].
Unquote

Or the following Service (allowable, ASD) based on the 105 ksi yield stress
0.60*Fy*3.14 = 197 kips without allowing the 4/3 stress increase due to wind loads

Notice that with your 147 kips that allows 4/3 stress increase is 147*4/3 = 196 kips which is close to 197 kips.

All the above deals with the Rod Material itself without dealing with the Rock Anchor Grout Bonded Length and Unbonded Length that you may have.

Rock Anchors are usually proof tested and locked to 120% of the service design load which is 147 kips x 120% = 176 kips

You also need to check the Local requirements in the state you are designing the transmission tower. There are some States in the US that do not allow the 4/3 stress increase due to wind load in which case your service design load including wind load cannot exceed 147 kips, I believe that your lockoff load for the anchor in this case will be 147 kips and not 176 kips.

The lockoff load of these anchors is very important to eleminate rod elongations during the service life of the tower. Otherwise, if your unbonded length say is 20 feet, then you can imagine how much deformation the tower would have to experience before the 147 kips load in the anchor is achieved.

Hope this helps

 

[AHaddad1]

More references to calculating the allowable tension stress

http://www.modernsteel.com/steelinterchange_details.php?id=108

http://www.aisc.org/WorkArea/showcontent.aspx?id=17640

See page 162 Table J3.2 which indicates a Nominal Tension Stress Fnt = 0.75Fu (where Fu = Ultimate Stress)

From Page 166:

Rn = Fnt x Ab
The allowable Load = Rn/(Omega=2)

Again, the above yields an allowable tension stress = 147 kips on a 2" diameter F1554 Grade 105

Note that the Fu (ultimate) range on the F1554 Grade 105 is 125 to 150 ksi, 125 ksi is usually used

 

[JJOO]

If proof load is "a tensile load that the fastener must support without evidence of permanent deformation," and ultimate load is a tensile load when the fastener breaks, the ultimate load must be greater than the proof load. Am I correct?

Why is the tensile stress permitted, Ft, as per ASCE48-05 greater than not only the allowable tensile stress but also the ultimate tensile stress based on steel manual?

Ft=0.83Fu=0.83*(125ksi)=103.75ksi.
vs.
Fnt/(OMEGA)=0.75Fu/(OMEGA)=0.75*(125ksi)/2=46.875ksi
(phi)Fnt=(0.75)*(0.75)*(125ksi)=70.3125ksi

I have a case where the maximum tension at anchor bolt from LRFD design analysis done by a pole manufacturer is 300 kips, which exceeds the ultimate load capacity (221 kips) but within the limit of tension load permitted as per ASCE48-05 (326 kips). Our project-specific design criteria happened to refer to ASCE48-05. However, the material for anchor bolt is F1554 Gr.105, and I know for a fact that 2" dia. does not have that much capacity. The pole manufacturer claims that their design is fine, but I don't want to compromise yet; I just don't know how to interpret this ASCE48 Section 9.3.1 though.

More inputs on my thought would be greatly appreciated.


AHaddad1:
Yes; the 4/3 stress increase due to wind load is not allowed in my project. I think we agree that the tension capacity of 2" dia. F1554 Gr.105 anchor bolt can't be reaching 326 kips.

Tmoose:
Embed is not the issue in this case. I'll have the drilled shaft type of foundation, and even the full-length anchors bolts could be used depending on the foundation loads and soil conditions as well.

I have attached, "Technical Manual 1: Design of Monopole Bases" by Daniel Horn, which has good info regarding tower base anchorage.

 

[Adalius]

I believe you are correct. If memory serves, it's fairly common that:

Rated Load < Proof Load < Ultimate Load

Which is to say, you test it at a less-than-deformation load (proof load) which is less than the breaking point (ultimate load), yet is more than the rated load typically due to safety factors.

 

[AHaddad1]

JJOO,

I am not familiar with ASCE48 nor do I have a copy right now to check it for you; however, I do offer the following insight.

A factor of safety of 2 from the nominal tension stress should always be used for the Anchor Bolts unless these safety factors are compensated for in the load application.

1) A dynamic load analysis can easily prove that the static applied load should be multiplied by certain factor usually 2 in many cases.

2) Fatigue due to cyclical loading can also prove that the applied stresses should be reduced below the nominal.

3) The elastic range for many steel materials do vary with temperature variations or cycles. Where there is no redundancy in the system, deaming the bolt as a fracture critical element leading to more attention to safety factors.

If your contractor addressed all the above then may be there is chance that he can use the nominal tension stress that I thought was 0.75Fu for the F1554 Grade 105 bolts.

GLTU

 

[JJOO]

Well, I have found one thing:

As per ASCE48, the tensile stress permitted, Ft=0.83Fu with net area of bolt. So, the proof load tension capacity of 2" dia. F1554 Gr.105 anchor rod would be (0.83)*(125ksi)*(2.50 sq.in.)=259 kips.

This still is beyond the ultimate capacity of 2" bolt based on steel manual, but maybe ASCE48 is allowing a little more because the overload factors for transmission towers are bigger than the usual 1.2 for DL and 1.6 for LL for building design. That's just my guess..
(NESC specifies that, 1.5 for DL, 1.0 for ice DL, 2.5 for transverse wind, 1.65 for dead-end wire tension, etc..)