Replacement of wood columns in old building

[mfstructural]

I'm currently working on a project where we are pouring footings below some existing columns in the basement of a 4 story building in Chicago. The issue came about because the building owner hired a contractor to repair the wood columns which were rotted at their bases from years of moisture absorption. The contractor proceeded to cut off the bottom 6 inches or so and wedge a paver brick between the basement slab and bottom of column. The slab was excavated around a couple of the columns and there are no footings per the owner. I have not been to the site. So we will be pouring footings under each column. the question I have is regarding the wood columns though. This is a four story building, with the first story serving as commercial space (furniture store) and rest of the levels as residential. I'd prefer steel in case the basement floods or there is water, the steel will not decay as quickly as a wood column wood (at least that's what I think). But I don't want to do it if it's not necessary. To evaluate the existing columns I'd assume the lowest allowable for a wood column in compression. The column is a 6x6. The city is specifially requesting an engineer to evaluate the columns and their supports, more the supports. I'm not sure I'd be ok leaving them (and building a pedestal on the footing) even if they are ok. I am more comfortable with steel for this application, but it's an expense for the building owner and if I can avoid it and the columns are ok I'd leave them if there was a healthy margin.

 

[jayrod12]

If you're pouring a pad, why not pour a pedestal that keeps the columns 6" above the top of slab. Then they could still be wood, if they work for the loading.

 

[msquared48]

And embed a Sinpson CB66 post base (or something similar) for the 6X6 in the plinth (pedestal).

Mike McCann, PE, SE (WA)

 

[NorthCivil]

Are you experienced in timber? often older timber is much higher quality and grade than newer timber. Especially for larger cuts, like a 6x6.

If you are working on a project like this, you should get familiar with historical local timbers from your area. The fact that the building has stood for X number of years and held fine, even with the bases rotted out, suggests the application might be valid. Not fair to your client to just specify steel replacement at the owners expense because YOU are more comfortable - maybe what that means is you are not competent to take on the project.

Apologize if I come off a little brash, I'm just a huge timber geek and believe we should be using more of it, not less, for sustainability, cost, and workability reasons.

 

[NorthCivil]

tacking onto Jayrod and m2 comments -

the reason the bases are rotted are likely because of either a flooded floor, or direct contact from the timber to the concrete. This is why timber and concrete must never be in direct contact - using a damp proof separator is key. even better is to elevate the timber so the bottom edge of the timber post can breathe.

if the loads allow for it, because you have had rot before I would consider the simpson APB66. its has a 1" gap below the bottom of the post to help limit end-rot.

https://www.strongtie.com/missioncollection_outdooraccents/apb_base/p/apb

just be prudent - there are allowable loads for this post base that are less than the capacity of the post if i recall correctly. around 11 kips

 

[OldBldgGuy]

I am not a big fan of Simpson connectors: too much money for things made for some case other than yours. Any half-assed fabricator can make anything I design for half of Simpson's cost unless we are talking hundreds of units. That aside, cutting off the existing columns and providing a concrete, masonry, steel, or timber pedestal should simply be a matter of looking at what works best. Why is it so difficult? A 6x6 column is carrying the loading so it can't be significant; whatever is easiest to build and ensures future durability is the way to go. Durability means don't have wood resting directly on concrete or masonry. As NorthCivil says (sorta), it's worked up until now, why screw it up just because you're not familiar with it?

 

[XR250]

I am not a big fan of Simpson connectors: too much money for things made for some case other than yours. Any half-assed fabricator can make anything I design for half of Simpson's cost unless we are talking hundreds of units.

OBG,
I'm going have to call BS on that unless your fabricator AND you work for $5.00 per hour. A $20 ABA66 base would be fine. If it does not have the capacity, you can grout under its removable base.

 

[OldBldgGuy]

XR250, you got me on the $20 base but I so often see unreasonably complicated and not that well done hangers that my default position is "make something". For that matter, there are a dozen ways to make a post base with stock steel sections that are quick and cheap, the important part being to separate the wood & the concrete.

 

[NorthCivil]

As far as simpson connectors go, I've had contractors ask if they could fabricate their own replica of the one I had specified, always with the funky hangers that have unique applications. I've heard this is usually due to the supplier having to order it in from the other side of the continent - and the supplier charging appropriately for that - which makes fabbing it locally easier.

the standard hangers, post bases etc are usually cheap if they are common and off the shelf