Stacked Beams - Moment Connection

[CANPRO]

Hi All,

I have a situation where I need to support a new steel beam on an existing slab. For a number of reasons, my beam elevation is fixed. And again for a number of reasons, I can't support the beam on the edge of the concrete slab, or the cantilevered portion of the slab(see attached sketch for clarification).

I would like to weld another beam section to the top of my main beam, which I can use to span over to the concrete wall below. This moment connection is not standard.

I've sketched out how I assume the load will get transferred - tension at the end of the W14 and bearing between the two beams which varies linearly. I suppose there would be some shear flow between the two sections that I would need to account for as well. I have some flexibility on the lap length of the two beams - my starting point is minimum 12" (2x the beam depth of the top beam).

Seems fairly simple, but I want to make sure I'm not missing anything. Am I missing anything obvious here, anything I should watch out for?

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1521559936/tips/sketch_dyvqbg.pdf[/url]

 

[TehMightyEngineer]

Back in my EIT days I analyzed a stacked monorail in an old paper mill very similar to your setup. I used a similar analysis of the connection and don't believe it's too far from reality. I suppose my only caution would be to envelope the design to some degree. See what happens if elements you don't intend to take the full load. For example, if the longitudinal welds end up taking some tension make sure they have the capacity for that.

I'd also oversize the stiffeners, they're going to concentrate the load to some degree and may have difficulty uniformly distributing the load without cracking the welds. On these one-off kind of things it's usually easier to just oversize the welds and plates a bit. The extra cost in fabrication will be minimal.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[Tommy385]

I would look at it as "frame corner connection". Depending on a stresses maybe you will have shear field problem, i would add extra diagonal stiffeners in order to take all of stresses from beam webs (similar to strut and tie modeln in RC)

 

[EPCI-Steel]

Looks like a construction aid to me. Are you using the W6 to support the end of the W14 during construction?
If so I would look at using a square HSS as your temporary support, because you have that little W6 loaded up to its capacity and the lateral torsional buckling (LTB) and any incidental side loads that may occure at the free end are going to kill it. A square HSS has equal moment capacity in strong and weak axis and is not susceptible to LTB so is much more robust for construction and the unknowns which may occur.

Also, if the little beam is a construction aid you may not want to do so much welding to the W14. Eventually someone will have to gouge that thing off of there and clean up the weld scars, so making you connection longer and minimizing the amount of weld between the two beams or using clip plates which can easily be cut off will save you time and expense when you have to clean up.

If the little beam is permanent, I would make it larger and make sure you look at possible differential deflections between your steel and concrete structure. Little deflections in one structure can result in very large loads in your connecting element and you have a rapid change in stiffness where your W6 is welded rigidly onto the w14 and you could see fatigue failure of the W6 at that juncture.

 

[CANPRO]

TME, I've never seen this type connection personally but I figured it must have been done before...good to know you've had success with it. Good call with the accounting for uncertain load paths...I should have plenty of capacity in the longitudinal welds, I think I'll work backwards from the final design (assuming load path as I have shown) to see how much cushion I have in the welds for incidental loading. I like those kinds of checks, they help me sleep at night.

Tommy, thanks for the input, I agree that I need to check the additional shear in the beams...I only have a few of these to make so if I need some extra stiffeners its not a big deal, although I may have to increase the depth of the W6 in order to get access to weld the diagonals.

EPCI - good suggestions for the temporary condition, however, this is a permanent connection. I might have missed something, but my W6 is only at about 60% capacity in bending. Thank you for the response.

 

[EPCI-Steel]

Whoops, looked at the wrong W6