AISC Bolted Capacity Discrepancy (?) - Table 10-10a vs Table 7-1

[rmcsqrd]

I am having trouble reconciling the differences between the shear strength of bolts (AISC table 7-1) and the published values for a single plate connection (table 10-10a). Assuming a single plate bolted connection with the following parameters:

- Grade 36 plate that is 7/16" thick
- (2) 7/8"Ø bolts with a spacing of 3" and a=2.5" per figure 10-11
- Group A (A325) bolts, with a standard hole type and excluding the threads

With the above in mind, for a single plate connection (table 10-10a) I get a value of 28.3k (ASD). In the table for connections with a plate thickness > 7/16" thick with other similar parameters, the max capacity is also 28.3k (ASD), leading me to believe that the bolted part of the connection is the limiting state.

When I look at table 7-1 for the shear strength of a bolt with similar properties, I find a max capacity of 20.4k (ASD) (single shear condition). After multiplying this by C=1.03 (interpolation between ex=2 and ex=3 for a 2 bolt connection with 3" spacing per table 7-6), I get a maximum shear capacity of 21.01k. This is much lower than the published value for the single plate connection per table 10-10a.

I'm looking for clarity on the link between the table 10-10a and table 7-1. I'm likely missing something obvious here and would appreciate any insight into why the single plate connection appears to have a much higher value than just checking the bolt shear capacity.

 

[Agent666]

Firstly I'd say I'm not familiar with AISC specifically, but check that the difference isn't attributable to the difference in capacity between the bolts being sheared through the shank, or the threads?

For thinner plates it is sometimes assumed in the connection design models that if the plates are thinner than the typical shank length, then the threads excluded state can be assumed for the design of the bolt capacity.

 

[sbisteel]

Check out table 10-9, the eccentricity is equal to a/2 in several circumstances.

 

[rmcsqrd]

Agent666
The AISC tables in question have different values for "threads included" and "threads excluded" conditions.

sbisteel
Are you allowed to use table 10-9 in conjunction with table 7-1? Maybe this is just my ignorance of the AISC manual but I am not sure if cross-chapter table usage is allowable and didn't see anything in chapter 7 explicitly stating to reference table 10-9.

I also noticed that the "a" dimension referenced in table 10-9 is depicted in fig. 10-11. The connection I was trying to describe in my original post had a distance = 3" from the weld line to the bolt line (a=3" if we reference fig 10-11). If we reference table 7-6 and assume that the eccentricity of the connection is from the weld line to the bolt line, the e=a/2 would yield e = 3"/2 = 1.5". Table 7-6 does not have an available ex value in the tables for this and the commentary on page 7-18 states that interpolation between adjacent values is acceptable. I'm not sure if this means that I am allowed to interpolate outside of two values.

 

[structSU10]

One thing to remember is the C value for eccentrically loaded bolts cannot be linearly interpolated - it is a non-linear value. Its best to use the most conservative value you have, or to calculate it yourself.

Next, the capacity in table 10-10 is for the entire connection. You are relating the strength of one bolt to two. For some reason, the single plate connections maxes out at .69 the capacity of (2) bolts. this is equal to a = 4" in table 7-6. They state a = 3 prior to the tables, so I don't know the reason for the difference.

 

[sbisteel]

Table 10-9 only pertains to very rigid dimensional requirements for single plate shear connections; which in turn allows you to use a smaller eccentricity in the tables of chapter 7.

As for interpolating, obviously for e=0", 'C' will equal the number of bolt rows 'n'. I think it's close enough to interpolate with those values. There is software out there that will run the actual calculation to compute 'C' if you really need that sort of accuracy.

 

[271828]

Page 7-19 of the 15th edition seems to indicate one can interpolate between values.

 

[Shu Jiang PEng SE]

Table 7-1 shows the bolt shaft shear capacity. Table 10-10a shows the bolts (connecting angle clip and the supporting element) capacity with specified eccentricity (2.5”) and bolt spacing (3”). The bolt group capacity is less than the sum of the shear shaft capacity of all the bolts because of the torsion effect on the bolt group.

For example if you have (2) 7/8” bolts without eccentric load, the shear capacity shall be 14.4x2=28.8 kips (ASD, bolt threads not excluded), however if single angle clip is used for connection (the force eccentricity is 2.5” and bolt spacing is 3”) then the shear capacity of the (2) bolts is 14.4x1.03=14.83kip.

Plate thickness is another governing factor of the connection strength.