Steel Baseplate for a Wood Column? 1

[medeek]

I've got a 6x6 DF No. 2 post with about 11,000 lbs vertical load resting on a 6" reinforced slab (assume 3000 psi). According to Tomanovich's GRDSLAB calculator this will not work. Typically I would say cut the concrete and put in a square footing. However, why not just use a steel base plate so as to avoid having to cut the concrete?
Maybe the steel would have to be too thick in order to distribute the load and spread it enough to make it work. Has anyone ever tried this for columns on existing slabs?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[msquared48]

Seldom need too, but I do not see a problem with that.

If it is an existing SOG, I just cut in a new footing and install a Simpson post base. If the slab is elevated, then what you propose makes more sense.

Mike McCann, PE, SE (WA)

 

[KootK]

Maybe the steel would have to be too thick in order to distribute the load and spread it enough to make it work. Has anyone ever tried this for columns on existing slabs?

This is correct and the reason why a base plate probably won't do the trick. I've done similar things for construction loads but that involved a grillage of timber members acting as the base plate. Ultimately, the base plate has to be pretty stiff relative to the SOG.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

The only way this might work is to weld some serious gussets up on the plate to increase its stiffness, but at that point the solution is not very elegant, might as well cut the concrete and pour the new footing.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[CELinOttawa]

I've done this inside a wall, with folded steel. The detail is really very simple and elegant, but only works if you are building a new wall. It can also be labour intensive, as you'll see below. It does, however, have a place in the right jobs...

Think about it this way: Fold a plate, say 24" by 24", so that you have a channel with two equal legs and a depth of 6". That's what happened to work for me the last time I did this, but you can adapt it easily. Quarter inch plate is easy to fabricate and won't be expensive at all. You can also have a welded channel made if quarter inch is too thin for your application.

You need to know a fair amount about the slab to do this right, and should likely consider grout injection as well in order to fill any voids under the slab.

 

[XR250]

for 11k, seems like you would need a pretty large base plate

 

[Triangled]

I've used a 6x oriented as a beam on top of a sill plate and beneath the post to get a little more load distribution. Not sure you can get there for 11k.

 

[manstrom]

I would cut a footing into the slab. You don't know what kind of soil bearing you would get under the slab unless you exposed it and had the soil checked.

Cutting and pouring a new footing is pretty simple. The steel option is a little more complicated.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[CELinOttawa]

Don't forget you still get to consider a widening angle as your force passes through the slab... And it depends also on the soil strength available.

 

[msquared48]

Agree with above. If you have ever seen the base plates of old wood columns in old turn of the last century warehouses, they almost invariably have vertical plate stiffeners to control the bearing plate thickness.

Mike McCann, PE, SE (WA)

 

[precast78]

If you guys calculate this by hand, how do you figure out the bending of the slab? Let's say we assume the allowable bearing pressure is 2000 psf. That means we need about 5.5 sq ft of footing (2'-3" x 2'-3"). Cant we just assume the footing is 2.5'x2.5' and figure out the bending stress? It can't be that much especially if you assume 50:50 distribution each way.

 

[msquared48]

If you have a 16" square baseplate, you should be able to make it work at 2000 psf if the slab is 6" thick.

Mike McCann, PE, SE (WA)

 

[CELinOttawa]

In what I've done, the whole point is to avoid bending in the slab...

 

[precast78]

CELinOttawa, what if the basement is not finish. Does it matter if we bend the slab?

 

[XR250]

I have put a deep LVL in the base of a wall before to distribute out the loads. Strapped it on both sides to the studs for stability. Dunno if I would trust that with 11k, however.

 

[CELinOttawa]

Hi precast78 - Depends on your client and personal tolerance for risk. Personally I'd never do it, and if it leads to cracking, this could be termed an SLS failure.

 

[hawkaz]

What does everyone think of using the Shentu method for post loads on slab on grade? This is the method Enercalc uses.
It gives huge capacities, but I am not totally sold on it yet.

 

[dcarr82775]

Precast78, why risk it? Most likely you know absolutely nothing about the slab, its reinforcing, and most importantly the subgrade prep. Just cut it out and put in a footing.

 

[medeek]

The only post load/slab on grade I've been aware of is the method's used by Tomanovich's GRDSLAB calculator, which I presume come from these references:

1. "Load Testing of Instrumented Pavement Sections - Improved Techniques for Appling the Finite Element
Method to Strain Prediction in PCC Pavement Structures" - by University of Minnesota, Depart. of Civil
Engineering (submitted to MN/DOT, March 24, 2002)
2. "Dowel Bar Optimization: Phases I and II - Final Report" - by Max L. Porter (Iowa State University, 2001)
3. "Guide to Design of Slabs-on-Ground" - ACI 360R-10 - by American Concrete Institute (2010)
4. "Slab Thickness Design for Industrial Concrete Floors on Grade" (IS195.01D) - by Robert G. Packard
(Portland Cement Association, 1976)
5. "Stresses and Strains in Rigid Pavements" (Lecture Notes 3) - by Charles Nunoo, Ph.D., P.E.
(Florida International University, Miami FL - Fall 2002)

How does the Shentu method differ? Do you have any references or sources that I can investigate.

Bottom line though, the trouble of fabricating a special steel base plate coupled with my lack of knowledge of the subgrade, concrete strength and reinforcement have me heavily leaning to just having them bite the bullet and cutting the slab and pouring the new footing. My central post loads at approx. 11,000 lbs are heavy enough that leaving anything to chance would probably not be responsible.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[precast78]

I am just really interested with this subject. If I was to put a column on a slab (new construction), what do I need to do? How do I design it?