How to calculate pad footing size and depth

[Ben100]

Hi all,

 

I have several steel columns in my new extension which are taking most of the structural load. The SE has given me the line and point loads, so I now need to calculate the size and depth of the 'pad footings' they sit on. I've looked around, but can't find this information anywhere. Can anyone suggest a method?

 

Cheers,

Ben

[PeterW]

The SE should do it based on the ground conditions. 

[Ben100]

1 minute ago, PeterW said:

The SE should do it based on the ground conditions. 

 

He's too far away, so I'd need to get a local SE to design then if I can't.

[the_r_sole]

.

Edited September 26, 2019 by the_r_sole

[Temp]

Ideally you need to know the load bearing capability of the ground (in Newtons per square meter) and the load (in Newtons). Then it should be easy to divide one by the other to give the required area (in square metres). The issue is what safety factor should be included as well as other precautions (particularly for soft ground or clay).

 

Table 10 in Approved Documents A give some guidance on the width of strip foundations required for different soil types and load. The issues and maths are similar. 

 

I'm not an SE though.

[Ben100]

9 hours ago, Temp said:

Ideally you need to know the load bearing capability of the ground (in Newtons per square meter) and the load (in Newtons). Then it should be easy to divide one by the other to give the required area (in square metres). The issue is what safety factor should be included as well as other precautions (particularly for soft ground or clay).

 

Table 10 in Approved Documents A give some guidance on the width of strip foundations required for different soil types and load. The issues and maths are similar. 

 

I'm not an SE though.

 

This sounds simple enough as the ground is hard compressed sand/stone and I have all the point loads from the SE, which have the safety calculations built in. I’m more concerned about the required thickness of the pads as I’m not clear on how to calculate this.

[LA3222]

1 hour ago, Ben100 said:

 

This sounds simple enough as the ground is hard compressed sand/stone and I have all the point loads from the SE, which have the safety calculations built in. I’m more concerned about the required thickness of the pads as I’m not clear on how to calculate this.

I believe it is the actual ground bearing capacity of the sand/stone that is required.  

 

You have the point loads imposed on the sand/stone - the load that the sand/stone can actually bear will then determine the size of the padstone required?

[Temp]

Section 2E of Approved Doc A also has guidance on the minimum depth (eg how deep the bottom of the foundation should be) in section 2E4 and the thickness in 2E2. 

 

In practice you dig down to the depth recommended in 2E4 and fill it up with concrete to a depth that is a convenient multiple of brick or block courses below the required DPC/FFL level (Checking that the resulting thickness is greater than needed to meet 2E2). That way you don't need to cut blocks to make thinner courses.

 

Some people use more concrete and fewer courses of blocks than needed to meet 2E2. This is sometimes called fully filled foundations but that doesn't mean they are always filled right to the very top just close. It can make sense especially if building in winter/wet weather as your trenches are less likely to fill with water or collapse. You still want to be a convenient multiple below DPC.

 

I think the NHBC also require there to be a cavitiy 225 mm deep below DPC. So that may also limit how high you fill your trench.

Edited September 7, 2019 by Temp