The beam will carry the reaction to its supports as shears. Obviously most of the shear will go to the end of the beam closest to this point load.
During typical analysis the support is assumed to be an infinitely small pin, with no rotational resistance. In reality in reality we are attaching to a column with an angle. Thereby introducing both eccentricity and stiffness.
The eccentricity introduced by attaching to the face of the column is, in my experience, typically neglected in the design of both the connection and the column. This is based on historical practice and performance and also that columns have a fair amount of strength in the strong axis, where the eccentricity is larger, and both significantly less strength and eccentricity in the weak axis, so there is some parity between the demand and resistance.
It also needs to be noted that the inherent stiffness of the connection, while adding some moment will also add resistance to the column. If the column is assumed pinned between floors (k=1.0), this extra, unaccounted for stiffness will increase the buckling capacity, as will the fact that the column runs continuously through the floors instead of being broken and pinned.
The following questions/answers provide some thoughts on when eccentricity must be accounted for in the design of some standard shear connection types - and more importantly why. In addition to the reasons provided below, it should also be noted that connections tabulated in the Manual are configured to be ductility with regard to end rotation and can therefore accommodate the simple beam end rotation without rupture.
When must eccentricity be considered in a connection? The effects of eccentricity must be considered for every connection. However, there are instances where the effects of eccentricity are considered, deemed to be irrelevant and then neglected during design. This may seem like an exercise in semantics, but it is an important point. Eccentricity has been considered in all the connections shown in the AISC Manual but has been neglected in some cases during design, since the effects have been deemed to be insignificant relative to the assumptions made in the Specification.
For all-bolted double angle connections, with one row of bolts in the web, the AISC Manual neglects eccentricity for web bolts. For the connection to the supporting member, eccentricity is always neglected. With 2 rows of bolts in the beam web, eccentricity is considered. Replacing either set of bolts with welds requires that the welds consider eccentricity. Why is this?
The reason the bolts and angles are not designed for an eccentricity when a single column of bolts is used is largely historical practice and acceptable performance. However, there are some other more rational reasons that can be given to explain the practice.
First, the design of the welds does consider eccentricity. Why? The welds are generally stiffer and less ductile than the bolts. Also, tests indicated that there could be a problem if the eccentricity was not accounted for. Also since the welds are stiffer, the model that includes the moment is more accurate for the welded case.
Additionally the actual eccentricity from the bolt line will be less than the eccentricity from the center of the weld group. Again, this will tend to reduce the effect of the eccentricity on the bolt group relative to that on the weld.
Further, there is a reduction in bolt strength inherent in the Specification to account for uneven force distribution in end-loaded connections. This is discussed in AISC Design Guide 17 (
Shear connections are not end-loaded but are subject to this reduction in strength anyway. This adds some additional conservatism to the design of the bolt group that may not be apparent.
All of these factors taken together can be used to justify the design procedures contained in the Manual.
For end plate connections, no eccentricity is considered. Why? Since the bolts are essentially in-plane with the support there is no eccentricity in this direction. In the other direction, measured from the bolt line to the supported beam web, the eccentricity is assumed to be taken by the plate at the beam web and not the bolts. It can be shown that for normal configurations, the bending in the plate will not govern the design, so it is not checked.
For shear tab connections, for a < 3.5 in., no eccentricity need be considered for the bolts - does eccentricity need to be considered for the welds? The decision to allow the eccentricity to be largely (it must be considered for n=10-12 with STD hole) neglected during design of a single plate shear connection is based on a couple of factors. First, all the nominal bolt values (strengths) in the Specification are reduced by 20% to account for uneven stress distributions that occur in end-loaded connections such as lap splices. Shear connections are not end-loaded but are still subject to this reduction. This inherent conservatism is used to offset some of the effects of the eccentricity. Second, the ductility requirements for the single plate shear connection are very explicit in the design procedure. The limitation on plate thickness relative to bolt diameter is intended to allow the bolts to plow through the plate in order to accommodate the simple beam end rotation without fracturing. This reduces the secondary effects that might occur if these ductility checks were not in place. Finally the validity of neglecting the eccentricity has been verified through testing and the geometrical limitations of the use of the design procedure are calibrated to these tests. Eccentricity is considered in the design of the weld, which is required to be 5/8tp. The weld is sized such that the plate will theoretically develop a plastic flexural hinge prior to the weld fracturing. A derivation of this can be found here:
Is there any general guidance on when eccentricity must be considered? Eccentricity must always be considered but can sometimes be neglected in the design checks. I would as a general rule include the eccentricity in the design checks unless the Manual specifically states it can be neglected for a particular case and all the underlying assumptions provided in the Manual are met.
