Post Load on Slab On Grade

[kam22]

Hello, many great threads here for rack post loading on slabs on grade. However, I can't seem to find answers to my specific questions.
1. I have an existing 7in slab that I am evaluating for post load of 7500 lb often time back to back with another equally load post.

1a. Using the ringo and anderson book, which gives a few methods, I typically use the PCA charts method for new design. However, for this scenario It suggests a slab larger than 7in.
2. I can avoid every saw cut joint. So I might have a post load within a couple feet from an interior (interior to my overall 50ft x 56ft bay surrounded by construction joints) sawcut joint. The joint has a 1 in saw cut and #3@15s continuous through the joint.

2a. When evaluating a post load, should I reduce the thickness by the 1" saw cut so I can count on the slab to transfer the load through the joint?
3. The charts for load back to back were giving a slab thicker than 7in, however, many people online have said a forklift would typically control the slab thickness, so I feel like I am doing something wrong in my efforts because I definitely get the post load to be the controlling factor.

3a. I decided to evaluate as a beam on elastic foundation with free edges and this gets me close, but not when I reduce the thickness by the 1in saw cut. So again, left wondering, do I need to do this reduction in order to transfer load across the saw cut joint?
4. At the construction joints the detailing is a #3 dowel at 24" and a diamond dowel alternating with the #3 dowel at 24" OC. This doesnt match the ACI 360 recommendations. I am used to using Yoder & Witczak, "Principles of Pavement Design" for evaluating load transfer across doweled joints but this layout doesn't fall within the limits of the procedure in this reference.

4a. This is why I decided to treat as a free edge in the beam on elastic foundation model
5. Has any relied on the approach in this article:

https://www.structuremag.org/wp-content/uploads/2014/08/C-StructuralDesign-Azzi-Apr081.pdf

? It produces much higher slab capacities than any other method I have seen. The article assume there is no other post load within the relative stiffness radius but also, if I just say my loads are right on top of each other the capacity is sufficient. If this is the case, why isn't there more information out there using this approach in articles?
6. In the beam on elastic foundation model I am checking the slab moment demand against the cracking moment. Does this seem reasonable? If the slab cracks I assume it no longer has a mechanism for transferring the load across the slab. The positive moment is controlling and if it were to crack wouldnt it crack right below the post on the bottom side of the slab and that be a bad thing?
7. And for my final question; does 7in slab seem reasonable for a back to back 7500 post load? And if so, how do I get the numbers to validate it?

The bearing pressure is not exceeded and the punching shear is good as well.​

 

[phamENG]

7500kip or 7.5kip? Big difference.

 

[Once20036]

Theres lots of posts on here about the structural magazine article. I agree that the article says no other loads within the radius of influence. It seems like many engineers just ignore that limitation. I agree its frustrating that there was no follow up after that article to refine the method.

 

[kam22]

At PhamENG: Thank you, I updated the post. 7500lbf, 15,000lbf back to back in the middle.
Once200036: Thanks for the reassurance. It's hard to rely on one source of truth when I dont find much else using it. Based on the article I can put 4 posts in one spot and be okay. Which wildy exceeds any other approach I have used. Additionally, the off the shelf racking system has a base plate 6inx7in, which seems to be outside the bounds of the article test data. I can apply the equations outside the tables they present, but I worry that im outsie the bounds of what its based on.

 

[bones206]

The Shentu method is useful for companies like Wildeck (one of the article authors works for them), because they need a way to justify installing their mezzanine products on existing thin SOG's to remain competitive. Getting an article published in a reputable magazine helps bolster the perceived legitimacy of an analysis method that just so happens to benefit their business model. I'm not saying the method is invalid, but it is less conservative than traditional working stress methods and it is being promoted in an article written by someone who financially benefits from its use. Just something to consider.

From what I understand, the Shentu FEM analysis method captured the beneficial confining effect of the slab surrounding the loaded zone. This confining effect is probably lessened by the presence of joints, but to what degree is hard to say. The predicted capacity seems to be based on the ultimate failure mode of the slab, which contrasts with the traditional working stress methods. The article recommends a FOS = 3 against this ultimate failure, which seems reasonable, but it is still apparently allowing a higher stress state in the slab than traditional working stress methods. The FEM model used in the Shentu study captured the levels of cracking in the slab in incremental loading stages, so the significant residual capacity beyond the initial cracking could be utilized. It is unclear to me what level of cracking and/or settlement you would expect by setting the FOS = 3 with this method.

To summarize my thoughts, the Shentu method seems legitimate and useful for situations where you need to get the numbers to work and are willing to reduce some conservatism. But there should be an understanding that it probably allows *some* visible cracking and/or noticeable settlement around the post base under full design load (rare). I think most owners would accept this disclaimer as part of the EOR's expectation management in a situation where you are trying to make an existing slab work on paper that has already spent years in service without issues.

PCA's latest has recommendations for reductions at joints, which they refer to as the joint factor (JF).