Gusset plate bolted to single angle bracing 1

[antiwheel]

Hi, I have a single angle knee bracing attached to the column gusset plate with bolts under single shear. Almost all the design examples are dealing with gusset plates with double angles or at least bolts under double shear. My question is, since there is an offset between the gusset plate and the angle leg for the single shear, do I need to consider the offset moment created by the axial force and eccentricity?
M = F*(1/2*Thk_angle+1/2*Thk_gusset) ?
or simply ignore the offset moment? and why?
Thanks

 

[KootK]

or simply ignore the offset moment?

I vote for ignoring it. My reasoning:

1) It's a small eccentricity.
2) Inadvertent moment connection to the angle which is stiffer in weak axis bending kind of resolves it.
3) It's what everybody seems to do.

Obviously, part and parcel of this is designing the single angle for eccentric load delivery.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wannabeSE]

I would use the Steel Construction Manual tables for eccentrically loaded single angles in compression because it is a pain to calculate.

 

[antiwheel]

Thanks Guys. For the single angle I can put an offset in STAAD Pro. to include the eccentricity so my question is also for the gusset plate. When we check the buckling capacity of gusset plate using the whitmore section and compression stubs, do we include the offset moment at all? I found it's very difficult to meet the requirement of AISC 14th equation H1-1 (a and b) since the offset moment is weak axis for the gusset plate.

Thanks

 

[KootK]

The response that I provided was intended for the gusset plates. In STAAD, is your plate fix connected to your angle for weak axis bending?

Also, the eccentricity would be higher than you calculated in your initial post If not a fixed connection. I believe that the eccentricity should be measured to the centroid of the angle.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[antiwheel]

KootK, is there any design example discussing this type of connection? and you are right, the ecc.moment should be from the centroid.

 

[KootK]

Three options come to mind:

1) AISC seismic design manual (great for connections).
2) AISC Vertical Bracing Design guide.
3) AISC 14 Ed design examples (Free)

Sort of figure 3 here: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[humanengr]

The eccentricity on the single angle brace should be considered. If you're using AISC - you can refer to
Table 4-12 of AISC Steel Const. Manual-14th Ed.

The effect of the eccentricity on the gusset plate (out of plane/weak axis bending) is debatable. This is a question probably brought about by the use of computers. This type of design was a "routine" manual calculation in which out of plane bending on the gusset was typically not considered.
Refer to AISC Manual, pg. 9-3 (Connecting Elements Subject to Combined Loading) regarding combined loading on connection elements.

Having said that, in my opinion (and was probably the case with those "routine" manual calculations), if the adequacy of the connection depends on whether or not that eccentricity is considered, then the design is not inherently safe.

I don't know what that brace supports or if there is any redundancy, however, the integrity of the structure should not depend on whether or not there is (say 1/4") eccentricity on a gusset. Connections and gusset plates are too critical to the integrity of a structure to be designed to such an extreme limit. Reference the highway bridge that collapsed in Minnesota. As I recall, gusset plates were "implicated", if not "blamed", for the collapse.
This is analogous to the $15 gasket that brought down a multi-billion dollar space shuttle, resulting in the loss of several astronauts and the school teacher accompanying them.

In conclusion, if you are designing the connection very close to the limits allowed by the code then, YES, consider out of plane bending on the gusset plate and combine that effect with the other load effects acting on the gusset. If, on the other hand, your design is "supple enough", then I would do a quick back of the napkin calculation check on the out of plane bending just to make sure its not an issue.