iengineerstuff
Structural
- Aug 4, 2021
- 11
Recently diving back into the bizarre world of the slab on grade(ground) design. I have been pouring over ACI committees, reading papers and reports, and I am trying to gather my thoughts into one post. Hoping some additional discussions/opinions will clear up some uncertainties about the design process and requirements.
My understanding is to design the slab thickness based on design methods provided by COE, PCA, and WRI. From this required thickness, preventing rupture in the concrete due to tensile stresses, I can select my sawcut contraction joint spacing. Sawcut Contraction Joint spacing is based on Fig 6.6, for unreinforced and lightly reinforced slabs. If I increase the ratio to 0.5% or higher, I can eliminate the requirements for sawcut contraction joints.
My understanding is that for a slab to still be considered unreinforced at the contraction joints, I need to have a ratio of 0.1% or less across the joints. Depending on joint spacing/slab thickness (and 100 other factors), increasing the reinforcement ratio at this can cause the joint to lose flexibility and cause random cracking to occur away from the joints.
With these 2 items being said, would it make sense to increase the reinforcement in the slab "panels" to say, 0.3% to reduce crack width, but cut the bars back at each contraction joint. Only Allowing for 0.1% of the reinforcement to act as the load transfer device between panels, and gain enhanced aggregate interlock, while maintaining flexibility.
Also, my understanding is while increasing this reinforcement ratio, I will be able to increase my joint spacing. ACI 360 doesn't seem to explicitly state this numerically, but does mention they are related. Does anyone have a good reference for determining this relationship? I know it used to be the subgrade drag formula, but this has since be removed, it seemed to not consider curling stresses would significantly increase as joints were extended.
For now, my plan for designing slabs going forward is to follow ACI360 for slab thicknesses, and Fig 6.6 for joint spacing. Also making sure reinforcement at saw cut contraction joints doesn't exceed 0.1%. Are these findings sound? How would you increase sawcut joint spacing while accurately designing for increased curling stresses?
Thank you in advance for all responses
My understanding is to design the slab thickness based on design methods provided by COE, PCA, and WRI. From this required thickness, preventing rupture in the concrete due to tensile stresses, I can select my sawcut contraction joint spacing. Sawcut Contraction Joint spacing is based on Fig 6.6, for unreinforced and lightly reinforced slabs. If I increase the ratio to 0.5% or higher, I can eliminate the requirements for sawcut contraction joints.
My understanding is that for a slab to still be considered unreinforced at the contraction joints, I need to have a ratio of 0.1% or less across the joints. Depending on joint spacing/slab thickness (and 100 other factors), increasing the reinforcement ratio at this can cause the joint to lose flexibility and cause random cracking to occur away from the joints.
With these 2 items being said, would it make sense to increase the reinforcement in the slab "panels" to say, 0.3% to reduce crack width, but cut the bars back at each contraction joint. Only Allowing for 0.1% of the reinforcement to act as the load transfer device between panels, and gain enhanced aggregate interlock, while maintaining flexibility.
Also, my understanding is while increasing this reinforcement ratio, I will be able to increase my joint spacing. ACI 360 doesn't seem to explicitly state this numerically, but does mention they are related. Does anyone have a good reference for determining this relationship? I know it used to be the subgrade drag formula, but this has since be removed, it seemed to not consider curling stresses would significantly increase as joints were extended.
For now, my plan for designing slabs going forward is to follow ACI360 for slab thicknesses, and Fig 6.6 for joint spacing. Also making sure reinforcement at saw cut contraction joints doesn't exceed 0.1%. Are these findings sound? How would you increase sawcut joint spacing while accurately designing for increased curling stresses?
Thank you in advance for all responses
