Single Plate Shear Connections with New Eccentricity

The following post contains several ideas that will form a future article in a magazine (by Erik Nelson and Doug Seymour),  therefore comments are welcome! ... In the 13th Edition AISC Steel Construction Manual, eccentricity was neglected for most conventional single-plate connections (shear tabs). Some of this practice was based upon the 20% reduction in bolt strength for end loading (a condition that doesn’t apply to shear connections).  As a result, the 14th Edition AISC Manual contains recommendations for accounting of the eccentricity in conventional single-plate connections. Now bolt shear calculations must include an eccentricity (typically half of the distance from the bolt line to the weld line).  This is an important change, since the reduction in bolt shear (and possibly bolt bearing strength) can be significant even for small eccentricities.

Consider the conventional single-plate connection with SSL holes illustrated in Figure 1.


The ICR method is used to account for the effect of the eccentric load on the bolt shear strength. For a number of common connection types and eccentricities, including the configuration of the conventional single-plate connection, the effective number of bolts for that connection, C, is given in the AISC Manual in Tables 7-6 to 7-13. The values published in AISC Manual Table 7-6 are useful, but the small eccentricities common in single-plate connections often fall below the entries in the table.

Since many conventional single-plate connections have a = 2 in. or 2½ in. (and a/2 values of 1 in. or 1¼ in.), we have prepared C values for these eccentricities for the convenience. The data is given in Table 2 with the additional information beyond that given already in Table 7-6 shown shaded.


The C values given in AISC Manual Tables 7-6 to 7-13 are determined using the strength predicted for each bolt group by that method, divided by the strength of one bolt under concentric load. The result is an equivalent number of bolts for the given bolt pattern, with the given load eccentricity, to be used in calculations.

In addition to bolt shear, this same C/N approach can be used to account for the effect of eccentricity on the nominal bolt bearing strength. Using C and N, the total number of bolts in the connection:

Rn,bolt = (C/N)  2.4dtFu                                                                            

The strength of the connection can be determined as the sum of the individual bolt bearing strengths.   For simple shear tabs with one line of bolts, eccentricity can still be ignored in bearing calculations according to the manual.  In fact, letting the bolts deform the material in bearing has the advantage of increasing the flexibility of the connection to reduce moment transfer.  This has bean confirmed by tests for single lines of bolts with small eccentricity.  For extended plates, the C/N method can apply and is simple to perform by hand calculations since C has already been determined for bolt shear.

You may notice that we left out the tearout portion of the bearing equation.  Unfortunately, we do not have a simple method of accounting for tearout in eccentrically loaded bolt groups of shear connections. In typical configurations of a single column of bolts, only one bolt may experience tearout, but it is not clear how to satisfy statics under eccentricity (horizontal components must sum to zero) when for example, the lower bolt experiences tearout and the upper bolt experiences bearing.  We recommend sizing the plate such that the edges distances are large enough so tearout does not control for extended plate shear connections.  There are likely more advanced methods to account for tearout, but any method must satisfy statics and equilibrium,  in addition to having a design with sufficient ductility to redistribute the loads.

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