Uniform Force Method

[KCPEngr]

I have a question regarding the Uniform Force Method and how it applies. I have the typical gusset plate-Column-Beam connection as shown in Figure 13-2 of the AISC 13th edition manual, but I have a structural metal deck diaphragm attached to the top of the beam. Does the Uniform Force Method really apply since I am dragging all of the horizontal load into my diaphram through my beam and only the vertical load thru the column? Can the horizontal and vertical components be found simply by using the load from the diagonal brace multiplied by the sin/cos of the angle? If the UFM does still apply and I want to move the working point to the Column-Beam interface, do I just apply an additional moment to the column and beam or does the additional moment need to be resisted by the connection as well?

 

[connectegr]

UFM can apply to your condition. The bracing forces a transferred from the gusset plate into the beam and then through the studs into the composite floor. Or through some other positive connection of the deck to the beam flange. However, UFM is only one option for the design of bracing connections. You can use simple vector analysis (KISS method) as you describe. This must consider any connection eccentricities at the gusset to beam flange and gusset to column flange connection. If you move the work points from the member's neutral axes, then this moment must be considered in the member design.

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[Gumpmaster]

The KISS method is totally fine, but you end up with larger moments as connectegr said. I have seen several instances where engineers used the KISS method and didn't account for those moments. Those instances required retrofitting.

Larry Muir's paper "Design of Compact Gusset Plates with the Uniform Force Method" shows several different options to the UFM and provides the free body diagrams for those. It's a good paper and one you should read prior to designing many gussets.

 

[abusementpark]

I don't see why the diaphragm should be a consideration in the connection design.