Eccentric Bolted shear connection design for Tension forces

[maolson3]

See the attached image for a double-angle bolted connection with a load eccentricity. Do the bolts that connect to the supporting member get design for the tension induced to the upper bolts due to this eccentric-induced moment?

In looking through AISC example problems I have yet to see one that considers combined shear and tension affects on this type of connection.

However, the 13th Edition AISC manual has a section on calculating these tension forces (Pp 7-9, Eccentricity Normal to the Plane of the Faying Surface) and they do not appear to be trivial.

Does anyone have any insight on this or a code reference that allows me to ignore these tensile forces?

 

[nutte]

This connection is assumed to be a simple, pinned, support, so there is no moment on the end of the beam. The load Pu will result in some reaction at this support, say P1, and another reaction at the far end, say P2. This connection is designed for P1, shear only, with no moment. Thus, there is no moment/tension in the bolts in the outstanding leg.

Look at page 10-4 in the 13th edition manual for some discussion on this.

 

[maolson3]

Thanks for the reply. A quick follow-up: let's assume that force Pu gets its P1 dumped into the bolts connecting the beam to the double angle. This P1 is also eccentric to the bolts connecting the double angle to the column flange. This eccentricity also has the potential to induce a moment into the bolt group framing to the column flange, thus inducing tension into the upper bolts and compression into the lower bolts (similar to Figure 7-6 and 7-7 of the 13th edition manual).

Does this need to be considered?

 

[nutte]

This eccentricity is neglected for this connection, for a single vertical line of bolts.

 

[maolson3]

Alright, well what if I had multiple lines of bolts? Would that change anything? I really appreciate the response, I just want to be thorough. Thanks

 

[nutte]

With a double line of bolts, you put the eccentricity on the instanding leg bolts.

Dig through the 9th edition steel manual, and the 2nd and 3rd LRFD manuals. They tended to be more thorough than the current 13th manual is on these issues.

 

[connectegr]

From your detail you are applying a concentrated load relatively close to the beam end connection (a simple beam/ pinned end beam). This force should be considered in you beam design and result in a shear only force at the beam end. There is no eccentricity in the connection material or bolts. If the resultant force is a shear and axial force, then the connection should be design for prying.

I think a better example of your concern is the bolted vertical connection of a column flange brace connection. Using vector analysis or KISS method, the vertical component applied at the column flange is eccentric half the depth of the column. This results is a moment in the bolted connection. I have attached an old 9th Edition calculation for prying. (ref 9th Ed p4-90)

This has changed slightly with the 13th Ed, but I don't have a quick example to show you.

This is also different than the current Uniform Force Method of bracing design.

http://www.FerrellEngineering.com

 

[connectegr]

In reference to the information provided by nutte...

Even with multiple columns of bolts in the beam web, the eccentricity is from the face of the support to the center of the bolt group. The point load still results in shear only at the connection.

Draw the shear diagram

http://www.FerrellEngineering.com

 

[DHKpeWI]

For what it's worth, I agree with nutte's answers.

 

[LSM76]

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 (

http://www.aisc.org).

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:

http://larrymuir.com/Documents/Design of Unstiffened Extended Single Plate Shear Connections.pdf

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.