Continuous Concrete Beam Intermediate Span Length 1

[iengineerstuff]

Working on a simple residential project, and came across a weird question with concrete beams. I have a wall that is 50'-0" long, with (2) 18' wide garage doors and a 5'-0" wall in between the garage doors. Due to loads and available depth, I have designed concrete beams to go over these openings. Initially, I designed the beam over a single garage door as simply support on either side and got a complete design, however, i figured from the contractors stand point they will probably pour that whole section together creating a continuous beam condition. Upon a redesign, I determined I did not have adequate top steel to sufficiently resist the applied negative moment, so i fixed that issue and moved on. But then it occured to me that, how much bearing length starts to reduce the applied negative moment on the beam. Obviously if I had 8" of intermediate bearing I wouldn't give this a second thought, but I have 5'-0", and I am convinced the negative must be reduced over the long bearing.

To my question, I designed the beam as a 3 span continuous beam, with an 18'-0",5'-0",18'-0" span, so that the negative moment was "distributed" over the intermediate support, and this allowed my original beam design to work. Is this a correct thought process? Or does anyone have a resource I can get into that talks about this specific topic? I googled to no avail. Any help and advice is appreciated.

Thank you!

 

[hetgen]

What is the height of the wall above the garage door?

 

[ProgrammingPE]

Designing the concrete beam as a 3 span continuous beam seems reasonable. The beam bearing stresses are concentrated right at the jambs of the garage doors and the load travels down there, so treating them as independent supports should be close to the actual behavior.

What this effectively does is it changes your simply supported beam into one with a spring that resists moment at one of the ends, so you'll end up with an end moment that is somewhere between wl^2/8 (propped cantilever case) and 0 (simply supported case).

I would adjust your span lengths, though, to account for the required bearing at each end, so maybe something like 19', 4', 19'.

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[Colostruct]

I might take a little bit different approach to this. The center wall acts as a large column. I would consider modeling this as a moment connection since it is my thought that the rigidity would “make” the structure act this way.

In practicality, the approach I learned before we had easy access to frame analysis used wl**2/10. This was supported in the old days by the ACI.

I’ve designed a number of these and it doesn’t seem to add much to throw in a bit extra flexural reinforcing. Shear, I believe, one can be a good approximation treating it as a simple beam.

 

[dik]

How big is the beam? If that 5' is the wall, Depending on the foundations, I might consider two 18' propped cantilevers with a fixed condition at the wall... or something of that ilk.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[iengineerstuff]

hetgen- The wall is 11'-0" tall with the garage doors being 8'-6" tall.

progammingpe - this makes sense to me, running the way I have it is more conservative than running each individually as a propped cantilever, and it works with my current top steel, so I think i am going to stay with my current design. thank you!

dik- the beam is 8x30, and i have sized the foundations to resist the gravity bearing load and any additional overturning load from the loading from the garage, so it is currently a 5'-6"x5'-6"x1'-4" footing. anyways, I have run my beam for both conditions and my flexural unity checks do not get above 75% so i felt comfortable with my current design. appreciate the response.

 

[dik]

Not a problem... generally if a support is greater than 2 or so times the beam depth, I consider it fixed... the stiffness is approx 8x... unless doing a detailed design.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik