waytsh
Structural
- Jun 10, 2004
- 373
Hi All,
I have a situation where I am trying to resist huge lateral thrusts from a large PEMB clearspan. For reasons I won't go into we are not able to use cross ties and I am using an independent moment-resisting foundation. Due to the large lateral loads I am utilizing a continuous strip footing down the sidewalls. I supposed you could also call this a mat footing or a two-way combined footing. I am not really sure where one definition ends and another begins. Just to give you an idea of the profile, it is going to be 6' wide, 4' thick, and 172' long with a 2'-6" deep soil shear key under it. Columns will be at 25' on center and the foundation will extend beyond the endwall columns. The size is being dictated by the stability requirements.
The design will need to be in accordance with ACI 318-14 so even though my flexural stresses are very low I will still need to size the rebar on each face to 0.0018Ag. For most of the foundation this is going to be substantial overkill. In order to provide a more economical design I am considering the following:
In the areas where the stresses are low (between the columns), I plan to analyze as a plain concrete per Chapter 14 and just specify minimum T&S (0.0018Ag split between the two faces). The areas under the columns exceeds the flexural stress limits for plain concrete so I will treat as reinforced and use the minimum 0.0018Ag per face. Bars in this area will be fully developed into the plain concrete analysis sections.
My question is whether or not it is permissible to use this type of hybrid approach? Is there a better approach that I am missing? I welcome your thoughts and input.
Thank you!
I have a situation where I am trying to resist huge lateral thrusts from a large PEMB clearspan. For reasons I won't go into we are not able to use cross ties and I am using an independent moment-resisting foundation. Due to the large lateral loads I am utilizing a continuous strip footing down the sidewalls. I supposed you could also call this a mat footing or a two-way combined footing. I am not really sure where one definition ends and another begins. Just to give you an idea of the profile, it is going to be 6' wide, 4' thick, and 172' long with a 2'-6" deep soil shear key under it. Columns will be at 25' on center and the foundation will extend beyond the endwall columns. The size is being dictated by the stability requirements.
The design will need to be in accordance with ACI 318-14 so even though my flexural stresses are very low I will still need to size the rebar on each face to 0.0018Ag. For most of the foundation this is going to be substantial overkill. In order to provide a more economical design I am considering the following:
In the areas where the stresses are low (between the columns), I plan to analyze as a plain concrete per Chapter 14 and just specify minimum T&S (0.0018Ag split between the two faces). The areas under the columns exceeds the flexural stress limits for plain concrete so I will treat as reinforced and use the minimum 0.0018Ag per face. Bars in this area will be fully developed into the plain concrete analysis sections.
My question is whether or not it is permissible to use this type of hybrid approach? Is there a better approach that I am missing? I welcome your thoughts and input.
Thank you!
