Dropped Beam Column Joint Stability

[IE22]

Hi All -

It seems dropped beam column joint stability has always been a topic of debate where I have worked in the past. In a situation like in the attached sketch does the inclusion of a heavy beam saddle provide a stable joint without any sort of intersecting beams or kickers? Would the K factor remain as 1 with the unbraced length from base to top of beam? If considered stable is there any concerns over cross grain bending?

I am interested in hearing other's thoughts!

 

[skeletron]

My interpretation:
Interior free-standing post with floor diaphragm -> K=1, floor-to-floor height for simplicity, I would probably want a CC66 or similar saddle that wraps 2/3 of the beam depth in support but have gone with something nominal like an AC6 B/S depending on the application and location. I don't put too much thought into it because (in my world) the design is heavily driven by the beam. You would probably have some blocking at or near the joint too.

 

[XR250]

I would require a saddle and take K = 1 with L = floor to ceiling height

 

[driftLimiter]

I usually use Skeletron's approach unless but I have also used XR250's. I think its perfectly reasonable to take the actual post height and K=1 but often for simplicity I take the floor to floor height I also don't mind having some additional capacity in my posts.

 

[phamENG]

I agree with skeletron, XR, and drift. Though I do floor to ceiling height. To do height of member, the beam would need to be torsionally fixed by the floor joists. It's not - it'll be torsionally fixed to the post. So the overall buckled shape will be idealized as a parabola from the bearing point (assumed to be a pin) through the the connection of the beam to the floor joists (also assumed to be a pin). (My drawing pad isn't hooked up to my computer, so I'm afraid I won't be sketching that one today...)

Cross grain bending...typically ignored in these circumstances. I would not ignore it if this is a cantilevered post and you're delivering lateral loads to the top edge of the beam that need to get down to the foundation through the connection. If you're not doing that but you still want to think about it, I'd assume an initial out of plumb of the post (1/4" in 8'-0" is a common rough carpentry tolerance for stud walls, not sure about posts) and figure out what kind of cross grain bending develops between the 'pin' between the top edge of the beam and the joists and the top edge of the saddle.

I went down a shallow rabbit hole on tension perpendicular to the grain not long ago (thread507-496245) and one thing that I found was a snippet in the MPC truss design manual that addresses this issue and design of truss connections. You could always look at that and see how you feel about any cross grain bending/tension perpendicular to the grain.

If anyone's interested, check out 7.5.3.2 Design for Tension Perpendicular to Grain Forces [in TPI-1]. It gives equations for how much of the connection needs to be above the center line of the carrying member for various levels of loading and distance from supports.

 

[KootK]

...has always been a topic of debate where I have worked in the past.

#MeToo.

I think that the dirty little secret of this is that it probably works like this:

1) The incoming beam shears effectively prestress the interface between the beams and and post in compression.

2) Because the beams are typically quite wide, the prestress creates a reasonably stiff moment connection between the post and beams for as long as no net tension develops at the interface.

For me, it's primarily the width of beams that makes this setup go 'round. As such, I like my beams nice and wide relative to the posts. I also like my beams not overly tall and either torsionally restrained or loaded in away that minimizes torsion (top loading for example).

I install kickers and/or dropped intersecting beams whenever can but there are plenty of applications where that's not palatable. Ditto for running the post up through the beams.

 

[IE22]

Thanks for all the insights. I think the saddle in addition to the prestress that KootK refers to produces a stable joint. I think ceiling height/bottom of joists is a reasonable height for the column check as the joists should be attached enough to stabilize the top of the beam/col element as a pin (assuming blocking along the beam which I did not draw).