Questions on AISC Manual Part 7 & Part 10

[Nick6781]

Per Table 7-1, a ¾” diameter A325 bolt has a shear capacity of φ * rₙ = 17.9 k when the threads are included in the shear plane. Page 10-89 indicates that bolt shear must be checked with the assumed eccentricity shown in Table 10-9, where e is determined to be a/2 for a 2-bolt connection. Page 10-91 states that the tabular values (in Table 10-10) are based on a = 3”. Therefore, for a 2-bolt connection, e = 3”/2 = 1.5”. Table 10-10a lists the capacity of a 2-bolt connection as φ * Rₙ = 25.1 k when the threads are included in the shear plane.

For eccentrically loaded bolt groups, page 7-19 indicates that the C value given in Table 7-6 can be linearly interpolated. Thus, C = (1.63 + 1.18) / 2 = 1.405. Therefore, φ * Rₙ = C * (φ * rₙ) = 1.405 * 17.9 = 25.1 k.

We know that the bolt shear capacity governs the connection design, and the tables in the manual are consistent across chapters.

Based on the above, I have the following questions:

1. The total moment due to the eccentricity a is P * a. If half of it is transferred to the bolt group, does the other half go to the weld connection based on equilibrium?

1. When the number of bolts is between 6 and 12, the considered eccentricity for the bolt group design is taken as a instead of a/2 for standard holes. What is the rationale behind this? What is the moment on the weld group in this case?
2. Is the assumed eccentricity in the table based on test results or engineering mechanics principles?
3. Table 10-12 lists the bolt and angle strength for a bolted-welded single angle connection. For a 2-bolt connection, the capacity is 35 k. How is this value derived? Why does it appear that eccentricity is not considered in this case?

 

[HDStructural]

Regarding your question about the weld taking the other half of the moment, the answer is yes, kind of.
That is what happens in theory, but the beam, column, connection etc.. are an indeterminate structure so we can't confidently say where the point of zero moment occurs. AISC requires that the weld from the shear tab to the support be sized to fully develop the moment capacity of the shear tab (5/8* tab thickness fillet weld on each side of the plate).
There is also another requirement giving a maximum plate thickness to make sure that the plate will fail (ductile yielding) prior to the bolts (shear failure), i.e. the moment capacity of the plate is to be less than the moment capacity of the bolt group.

 

[DanKile]

The 13th edition of the manual did not have the eccentricity requirement for the conventional single shear plate. In that manual the single plate and single angle had the same maximum load. The 14th added the eccentricity requirement with guidance from this paper Muir and Thornton (2011). It doesn't appear that single angles were considered, this could be an oversight.