PHayzz
Structural
- Jan 26, 2006
- 1
I've been coming to this site for years and have always been able to find what I'm looking for until now, hence this is my first post. I have a project where a bunch of single-story townhomes with walkout basements are being built on a site with a lot of poor soils and fill. The attached 22'x23' garages at these townhomes are at the front with the living spaces and basements behind them. All of the garages were designed with 4" slabs on grade with light WWF. The contractor is finding depths of fill exceeding 7 feet at the garages, which is being removed and replaced with structural fill in lifts. Given this time of year, the excavations are open for a long time and are subject to pooling water, etc. that compromises the compaction effort. The extent of over-excavation and replacement with structural fill is causing enormous change orders and delaying the project.
As an alternative, the contractor has proposed installing structural slabs at the garages. They priced a detail they used from another job (attached; apologies for the poor quality, it's the best I got) that included reinforcing the slab with #4 bottom bars @ 12"o.c. each way throughout, doweled into the foundation walls on all sides. It also showed a column in the middle of the slab. This column extended down to a footing bearing on good soil, and there appeared to be a drop panel at the top, as the top of column was held 8" down from top of slab. Reinforcement in the (assumed) drop panel was (8) - #4 bars 6'-8" long @ 8"o.c. each way. It's hard to tell, but it could be that the slab gradually thickened around the column. Just the plan detail was passed on to me; the details at the foundation wall and column were omitted. Even though the extra concrete and reinforcement is an additional cost, they're claiming that the net result will be far cheaper than proceeding with the over-excavation and backfill at the remaining townhomes.
The contractor says this is a common solution in the area, which is near Kansas City, MO. I have never heard of such a thing and, being the EOR, am tasked with verifying its adequacy and/or designing the solution myself. The unit pricing the contractor has provided does not address thickening the slab, leading me to believe they are figuring on keeping it at 4". My gut tells me I should increase it to 5", possibly more, to gain cover and flexural capacity. However, I'm not sure how to approach designing the slab itself since it doesn't fit into any of the flat plate slab models I've done before - it's supported around the perimeter and has a single column in the middle. I'm tempted to run denser 'column strip' reinforcement in the x- and y- directions, effectively splitting the slab into four quadrants, and space the bars further apart in the remainder of the slab. Although, my preference, as is contractor's I'm sure, is to keep bar spacing uniform throughout. Still, I'm not sure how to figure or distribute the slab moments in this configuration. The punching shear check is easy but the design for flexure is tricky. I'm also tempted to make a FEA model of it to determine the slab thickness and reinforcement layout.
I'd like to know if you've ever seen this type of thing done in a garage, what your thoughts on typical slab and drop panel thicknesses may be, and how you may approach designing the slab for flexure. Any feedback is greatly appreciated, as I've never encountered an animal quite like this one.
As an alternative, the contractor has proposed installing structural slabs at the garages. They priced a detail they used from another job (attached; apologies for the poor quality, it's the best I got) that included reinforcing the slab with #4 bottom bars @ 12"o.c. each way throughout, doweled into the foundation walls on all sides. It also showed a column in the middle of the slab. This column extended down to a footing bearing on good soil, and there appeared to be a drop panel at the top, as the top of column was held 8" down from top of slab. Reinforcement in the (assumed) drop panel was (8) - #4 bars 6'-8" long @ 8"o.c. each way. It's hard to tell, but it could be that the slab gradually thickened around the column. Just the plan detail was passed on to me; the details at the foundation wall and column were omitted. Even though the extra concrete and reinforcement is an additional cost, they're claiming that the net result will be far cheaper than proceeding with the over-excavation and backfill at the remaining townhomes.
The contractor says this is a common solution in the area, which is near Kansas City, MO. I have never heard of such a thing and, being the EOR, am tasked with verifying its adequacy and/or designing the solution myself. The unit pricing the contractor has provided does not address thickening the slab, leading me to believe they are figuring on keeping it at 4". My gut tells me I should increase it to 5", possibly more, to gain cover and flexural capacity. However, I'm not sure how to approach designing the slab itself since it doesn't fit into any of the flat plate slab models I've done before - it's supported around the perimeter and has a single column in the middle. I'm tempted to run denser 'column strip' reinforcement in the x- and y- directions, effectively splitting the slab into four quadrants, and space the bars further apart in the remainder of the slab. Although, my preference, as is contractor's I'm sure, is to keep bar spacing uniform throughout. Still, I'm not sure how to figure or distribute the slab moments in this configuration. The punching shear check is easy but the design for flexure is tricky. I'm also tempted to make a FEA model of it to determine the slab thickness and reinforcement layout.
I'd like to know if you've ever seen this type of thing done in a garage, what your thoughts on typical slab and drop panel thicknesses may be, and how you may approach designing the slab for flexure. Any feedback is greatly appreciated, as I've never encountered an animal quite like this one.
