Mat Foundation With Perimeter Wall

[Beamman]

Hi all,

I am designing a mat foundation for a building which is 90' x 300' in plan. The building has a full basement therefore there is a concrete basement wall going around the full perimeter.

My question is this: how do I account for the added stiffness the wall gives the mat at the perimeter, or am I splitting hairs here? I am using PCA Mats and my initial mat thickness is 3.6' (7 storey office building). The basement walls are 10' high. I am considering modeling thicker elements around the perimeter or possibly slaving the perimeter nodes.

Thanks in advance for any help/advice on this.

 

[csd72]

I havent designed mat foundations to US codes but I recall that the Australian slabs and footing codes allows you to reduce the footings under concrete/CMU walls to 1'.

I would think that this may be a false economy though as you will have different founding depths for different beams.

 

[civilperson]

Ignore the added stiffness from the perimeter walls. Proper detailing of ties and development of slab bars into the wall make this a tough item to build the slab and then add the walls. To assure connection and development, build the walls first with horizontal bars extending into the slab location and then place the interior slab. Model as a spandrel beam with an added interior slab.

 

[hokie66]

I agree with ignoring the stiffness of the walls, unless your mat is on piles. But I wouldn't cast the walls first. Continuity of the mat foundation is the most important thing. The basement walls could be reinforced concrete masonry.

 

[civilperson]

My previous post was poorly written.
First, ignore the added stiffness of the walls on the mat. (This allows the walls to be placed after the mat).
Second, if the stiffness of the walls is necessary, then cast the walls first with details of reinforcement like a spandrel beam.