AISC-Steel Construction manual - Table 7.1 Bolt Pre-tension vs Available tension 1

[JJAV1983]

AISC- Interpretation
Hello All,

I was hoping you could shed some light on this question regarding bolt pre-tension in AISC-Steel Construction manual - Table 7.1

If the required minimum Bolt Pre-tension for Pre-Installation Verification per Table 7.1 AISC Steel Construction Manual is = 37 kips for a ¾” bolt

The values for bolt strength are as per Table J3.2 (Group B)= 113 ksi

Per Allowable Stress Design and Load and Resistance Factor Design we obtain the Available tension per bolt:

B(asd)= Syt * Bolt-tsa / 1.6 = 22600 lb (Available tension per bolt per Allowable Stress Design)

B(lrfd)= 0.9 * Syt*Bolt-tsa = 33967.8 lb (Available tension per bolt per Load and Resistance Factor Design )

Why is the allowable tension per bolt a lesser number than what it is suggested as the minimum bolt pre-tension for pre-installation verification (37 kips vs 22.6 kips for ASD and 37 kips vs 33.9 kips for LRFD)?

Notes
Bolt-tsa= Bolt Tensile Stress Area (in^2)

Cheers,

JA

 

[ProgrammingPE]

There's a Steelwise article about high-strength bolting that may provide some insight into this.

Laboratory tests confirm that the actual bolt pretension achieved by the turn-of-the-nut method can be substantially greater than the 0.70Fu specified minimum installed pretension. The installation process becomes uneconomical if the bolts break during pretensioning. Rotating the nut subjects the fastener to a combination of tension and torsion force during installation. Once the installation process is complete, the stress in the bolt decreases due to removal of the torsional force. Therefore, bolts that do not fail during installation are adequate for service from an installation perspective.

This suggests that the bolts that are able to survive the pretensioning process are acceptable since they did not fail under combined tension and torsion, and the torsion will be removed. The article also mentions that:

Measurements taken in laboratory tests confirm that the pretension that would be sustained if the applied load were removed is essentially zero before the bolts fails in shear (Kulak et al., 1987). Thus, the shear and tensile strengths of a bolt are not affected by the presence of an initial pretension in the bolt.”

So you don't use up any of your shear or tensile capacity by pretensioning.

(I should also point out there's a number of errors in your post. Pretensioning for a 3/4" Group B bolt is 35 kip (Table J3.1), and it has an ASD capacity of 25.0 kip and LRFD capacity of 37.4 kip (Table 7-2). This means the pretension does not exceed the LRFD capacity, which makes sense since pretension is 70% of the Tensile Strength and LRFD uses Φ = 0.75.)

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