Point load added to unknown existing beam with new column under point load 1

[ProLuke]

Hello!
This is the situation:
An existing house with a concrete beam approx 12"x24" spanning 16 feet.
The existing beam holds up approx 2,700 lbs/foot.
We want to add a 5,000 lb concentrated load above the beam.
We want to add a column directly under the 5,000 lb load.

Does this column and footing have to be designed for 5,000 lbs only or 5,000 lbs plus 8 feet of trib width * 2,700 lbs/foot = 26,600 lbs?

Conservatively I wanted to design the column and footing for 26,600 lbs, and the contractor is suggesting it be designed for only the added load of 5,000 lbs.

What do you think? See the attached sketch for clarity.



--Luke
Texas PE
Structural and Civil Engineering
www.framestudio.com

 

[JStructsteel]

Ask your contractor to sign and seal his design, lol.

In seriousness, the 2700lbs is probably not all loaded all the time, so it will be some ratio of that would be collected by the new column. Maybe just the live load?

Thats my take on it.

 

[phamENG]

I'm with JStructsteel. The beam is currently supporting a certain amount of dead load and possibly some live load. I'd ignore the live load as it's really difficult to get an accurate and meaningful figure, and just assume the beam has dead load on it. Unless you jack up the beam during this process, that dead load will stay "in" the beam. Any new live loads that are carried by the beam will go to supports based on stiffness - which means your new support will get its share of the live load.

This brings up another potential issue. You're introducing negative moment into an area that probably isn't reinforced for it. Can the beam take the new support arrangement without some form of augmentation?

 

[rb1957]

I think the contractor is being too simplistic. I think his logic is you have your beam, and all is good. Now you add an extra load, but give a direct loadpath away from the beam, so 5000 lbs load.

but I think the beam is smarter (and lazier) than that. It won't ignore this new loadpath, so it'll attract the tributary load..

another day in paradise, or is paradise one day closer ?

 

[ProLuke]

You are right - the 2700 lbs per foot is both live + dead (worst load combination in this scenario). Live load is only 920 lbs per foot. It does make sense that the dead loads area already supported and we would potentially only be suppporting the new 5,000 lb load plus the live tributary = 12,260 lbs. We were not planning to jack up the beam, just form-in-place a concrete column under it. The negative moment in the existing concrete beam: A good thought, I had not considered that yet. The existing beam is continuous over 4 spans (it is under a bearing wall that goes all the way across a big old house). This is an older house, likely around 1920 it was built. We did hire a scanning company that generally did find there is rebar in the concrete, but how much and where exactly we don't know.

Should I just say no to this idea of adding the column mid-span under the beam?

My original idea was to shift the load to be directly on top of an existing column, then replace the column and footing with one sized for the full existing plus new reaction (42,000 lbs worst case). This would require temporarily supporting the house, removing the existing 10x10 column, saw cutting a large 5'x5' footing, and casting up a new column with known rebar. My fear with this is that when we remove the shoring the existing concrete beam might sink slightly and crack.

--Luke
Texas PE
Structural and Civil Engineering

http://www.framestudio.com

 

[phamENG]

Hmm...I'm always hesitant to alter load paths that I don't fully understand...one reason I don't love working on existing concrete structures without as-built drawings. Before dismissing the idea entirely, I'd compare the new design to the cracking moment of the beam. Assuming it doesn't already have flexural or shear cracks in it, do you anticipate the new arrangment to exceed M[sub]CR[/sub]? Or at least look at the negative moment region as an unreinforced section and see if it passes. If it does, then you could probably roll with it. If it doesn't, then I'd work on shifting the load back over the existing column.

If it goes to the existing column, maybe look at the new vs. original demand/capacity ratios? What does your local existing building code state? In a concrete building, 5,000lbs may fall under the common 10% threshold for re-evaluation/retrofitting.

 

[dold]

In the grand scheme of things, 27k isn't all that much. Is the contractor trying to avoid putting in a footing? 27k / 1.5ksf = 18ft2 ^ 0.5 = ~ 4'x4' footing worst case, right? What's the alternative? Check the slab for 5k point load? I'd tell the contractor to kindly install the footing. Unless it's a lot more involved (piles, etc).

 

[EZBuilding]

If you superimpose the positive bending of the existing dead load with the negative bending due to live load at the location of the column, it should help inform you if you have a concern regarding the addition of the support. My gut feel is that you would be improving the behavior of the concrete beam overall. I would agree with assuming that the load from above transfers directly to the column below.

 

[KootK]

You can create all manner of problems with this if your new column is close to one of the existing supports as you've shown it. You'd potentially tend to put the rightmost support into uplift for one. For all you know, the beam might not have the top steel required to handle the shear reversal. At the same time, your new post and foundation may well see vastly more than 5kip due to the prying action.

You're only increasing the total load on this beam by 12%. I'd be pretty tempted to just let the existing system absorb the new 5k without modification. We normally try to keep overstress down to 5% but, at the same time, one has to keep their wits about them with respect to what is reasonable and what is not. You're contemplating a lot of onerous work here to deal with a load increase that stands a very small chance of being a problem.

As a first step, I'd work the capacity of the existing beam assuming no stirrups, 2500 psi concrete, and minimum flexural steel.

 

[ProLuke]

Thank you all for the thoughts and comments. I did not think of the uplift on the existing column due to being so close - that is something to consider for sure, thank you.
I did consider the idea of not messing with it, but our live-only load increases about 30% - well over the 5% allowable for this case (2015 IEBC for additions to gravity members).

For reference I did check the concrete beam and column assuming #3 rebars in each corner, but that was not enough. I am hesitant to assume any rebar really, but it was a check to see if I'm in the right zone.

As a conclusion I am leaning toward telling them either scrap the project, scrap me as the one signing off on it, or replace a column with a new column & footing.

Thanks again all!

--Luke
Texas PE
Structural and Civil Engineering

http://www.framestudio.com

 

[KootK]

...scrap me as the one signing off on it...

Wha?!? No... We're here to make you a hero, not see you downgraded to AmateurLuke.

What if you deliberately split the beam at the column into two spans? That would simplify the load path substantially.

 

[Brad805]

Part of how I would proceed would depend on how the beam has performed to date? If it is in poor shape that is one thing, but if it is not heavily cracked and appears to be doing its job very well, I might go down the rabbit hole Koot suggests. If the contractor is concerned about costs I would think they could chip out a bit of concrete to expose a few bars. Information would be very helpful. It seems possible to fine tune the new column location to deal with the uplift problem.

 

[kipfoot]

Here's another thought...sometimes it's simpler to move away from existing columns in order to get a new footing in place. If/when the new column induces a crack in the beam, you have two simple spans with low demand. You'll have to show that no shear reinforcement is necessary for the shortened spans at the new column, or chip or use gpr to find stirrups. You can also show that the existing column carries less load than before.

 

[Enable]

Have we explored FRP options here?

Seems to me that FRP maintains the existing load path, and that being the case, the increase in load is relatively small and should be easily handled. It solves KootK's problems with uplift, phamENG's concern about negative moment steel.

 

[ProLuke]

To answer one question about how the existing concrete is performing - it seems in excellent shape; I did not observe any signs of failure.
To answer a question about FRP - that is an idea I didn't think of yet, thanks! One issue about strengthening up the beams and column with FRP is we still have the footing load increased beyond allowable without verifying it. From what I saw the footings are functioning (not sinking) but the sizes we verified on other parts of the house are undersized for default soil bearing... so I don't want to blindly add load to the footings.
I like the idea now of positioning the columns more mid-span and comparing shear and moment diagrams. My gut instinct is that we could add a full tributary-loaded column and footing, and then assume the beam cracks above that column. Adding footings and columns mid-span of this 16' beam (and other similar beams in a line) should make it stronger... similar to "shaker beams" people put under sagging floor joists.

So that is my next step - figure out some mid-span allowable locations, and put the full 5000 lbs plus tributary weight from the adjacent sections of beam.

--Luke
Texas PE
Structural and Civil Engineering

http://www.framestudio.com

 

[Tomfh]

The new column will pick up subsequent load from the original beam (live load, creep loads), not just load from the additional load over. Maybe it works. Maybe it doesn’t. But you can’t just assume the column sees 5000lbs

 

[Enable]

If your footings are undersized to begin with I'd actually lean to the FRP since it'll allow you to get two birds stoned at once.

Here's my thoughts: The FRP could surely handle the additional 5000lbs on the beam. The 2500lbs / column is roughly 5% increase so we're all good on them. It's just the footing where there's an issue due to original undersize, right? Well why cant we do a simple extension with dowels, and if the increase becomes large (seems like it shouldn't given the load?) we can add pilaster things below grade to stiffen up the extension. I'm assuming the footing is some feet below grade for the pilaster thing though.

I'm thinking something like below

 

[Brad805]

If you want to investigate the FRP option you can check with the Sika CarboDur software easily CarboDur Software They have been holding webinars going thru the installation and software recently. It is not overly complicated so long as they read the instructions, but I would want to know the contractor quite well before letting just anyone do it.