Kevin in Atlanta

Well, my installation hit a bump. While my initial test holes indicated I had the requisite depth, turns out where I did not test was only 4 inches. 5 inches away I have 8". So, now 3 holes hit dirt for on lift. The other all were 8". Contractor way back in '75 did not grade the garage properly so I guess I am looking at have a pad poured. Sadness. How big a pad depth-wise will I need? Luckily, I only need on pad.

MGW-Fla

Pretty sure as long as you have at least 4" of slab that is the minimum suggested depth. GIS told me that I could go with the 3 1/2" even but I don't trust the concrete. I'm awaiting word back from an engineer to get his suggestions for a steel plate instead of cutting out & re-pouring a pad for the posts.

Kevin in Atlanta

Well, I took more precise measurements - looks like 3-3 1/2. Feh! I have a concrete contractor coming out tomorrow - maybe he has some good suggestions.

dr bob

Seems like more than a few builders decide that 2x4 thick is enough to meet the 4" standard. ;( if your 3.5 is good concrete, you might get away with good epoxy anchors. Your concrete guy can tell you best.

MGW-Fla

The 2x4 method has pretty much been the norm here for decades. Problem I have with the epoxy anchors is the length. For the diameter they suggest, the anchors are over 5" long. GIS said only two of the nodules need to be in the concrete, but it just doesn't seem right to me somehow that most of the nodules would lie below the concrete. They say in the instructions with the MaxJax that you have to set the anchors 1/2" below the surface of the concrete then they pull up to level when you set them. That really makes those anchors seem too long to me. The strength rating of the WejIt epoxy anchors, just one is rated to have the strength to equal what all 5 are required to hold, which is 4 times what is actually needed.

Doug_B_928

The epoxy anchors are not set 1/2" below. They are set flush/slightly below the concrete.

FreedP

Let us know the results of your steel plate inquiry. A larger foot would spread out stress on the floor.

dr bob

Spreading the compression loading is not the concern. The lift already distributes that over greater than a square foot under each column. Issue is the tension of the anchors, which depends on the strength of a local 'cone' around each anchor. The effective diameter of that cone area is less when the concrete is too thin, risking fracture and tear-out.

Doug_B_928

^^^Makes sense to me (but I'm not an engineer). Further to this point, if one's floor is not flat the steel plate will be supported by shims and won't be making contact with the floor. Thus, proper installation of the anchors and strong enough concrete around the anchors (the "cone") is crucial.

Kevin in Atlanta

Concrete man stopped by. Suggests I could use a matching steel plate under the maxjax that extends the pad back 12 inches and that would reduce the force by 1/2 or he can cut the concrete and pour a 5000# contract pad for $300.

Rob Edwards

I wouldn't want the columns to have any larger a footprint than they already have, especially some 1/2" to 1" steel plate that you'll be tripping over as you move around the car.

I'd pick door #2, and never worry about your safety margin ever again.

Kevin in Atlanta

Door #3 - cut out a matching recess for the new steel plate.

Rob Edwards

This is where I wish I could put on my Dr. Bob cape and draw out some sort of calculation that would weigh the merits of 2 square feet of 1" steel sitting on now 2 to 2-1/2" of concrete (If I understand correctly you'd be cutting the existing concrete to fit the steel plate flush?) vs. ~1 square foot of lift column base sitting on 3 to 3-1/2" of concrete. But I can't.

So hopefully Dr. Bob will be along to ( my WAG) suggest that less concrete and more steel is not better than more concrete.

dr bob

I'm moonlighting on a power plant project and don't have access to some of my modeling toys. For those that have the tools handy, do the two models and share the tension numbers for the three outside anchors. There is a free Autodesk tool for android that will do simple stuff like this if you want to play. Anyway, look at the tension loads with the bare plate, and again with the 2x2 steel with column under the middle front. Guarding the rear anchor tension will be about 70%, but that's a guess. Jon does this stuff all the time, and I'm sure there are others. Bottom line is that you. An use more anchors smaller and shallower to get the same benefit with the larger footprint.

pbarch1

As Dr. Bob said, it isn't the compression load that required the thickness of the concrete. It is the ability of the anchors to resist the tension load induced by the potential of overturning. With a thin un-reinforced concrete floor the effective cone of concrete that supplies the resistance for the anchors is too small and libel to fracture causing withdrawal of the anchors. Your best bet is to pour a new pad under each post, doweling the new pad into the existing slab.