Installing beam at ground level to reduce pad size/depth

Steel column onto concrete pad.

To reduce the size and depth of a concrete pad, would it be feasible to bolt the column to another length of steel (making an upside down 'T' ) to spread the loading and reduce the chance of punching shear?

Would standard rebar enforcement do the same job?

My thinking is that a long shallow trench with a beam concreted in, would do a similar job as a square (but same surface area) deep concrete pad.

why not dig a standard foundation, or is this not feasible?

You're right in your thinking, but, unless the footing is deep enough to ensure that the load is spread over the entire base area, the footing would need rebar in it. The horizontal beam you suggest would do the same, providing that it's stiff enough to resist bending from the loading; the same would apply to an elongated plate; in fact, any baseplate is designed a) to ensure the pressure underneath it is not sufficient to crush the concrete and b) it's stiff enough to act as an upward cantilever from the face of the post to its extremities.

Nose: The column will be sitting right up against internal garage wall, or as close as I can get it. The other side of the left garage wall is the floor of my shedshop© Other side of the right garage wall is neighbours garage wall/floor. So digging a wide square pad would not be possible unless the column was moved further into the room itself. I had planned to go 1m down and 600mm square.

Shyz: I have a couple of 2 metre long 203x102's gathering dust, I was wondering if these concreted into said 'trench' would be suitable, at the length they are at or even cut down to 1m.

The columns in question are the same size with a 12mm baseplate welded on.

What are you putting on it? An eccentric base will in all probability be ok. How big is the baseplate on the actual column?

A heavyish roof, see the pfc question. (did you delete your original answer or was it the mods?)

Take a 6x5m double garage. Blockwork the 5m door up, with pillars at either end.
Sit a steel beam on either pillar running the 6m length to the back.

Support each beam at the ends and centre with steel columns. The beams then become the wallplate for the new roof.



I was pondering eccentric base, but wouldn't this mean digging deeper to compensate?

220mmx280mm plates, I was thinking this as being too small t.b.h.

If you've got a 280 plate, then using a 600 base out from the wall and the baseplate tight to the wall will mean a difference between centre of loading and centre of base of 300-140 = 160mm . This is greater than L/6, meaning that the whole of the underside of the base out from the wall is not loaded (which is another story, trust me on this!), which in turn means that the pressure at the the leading edge of the base is quite high - and might be higher than the bearing capacity of the soil. However, as there is already a foundation and wall in front of it, the base would have to push those out of the way to rotate and cause a bearing failure.

But...let's assume that the BCO won't accept that, having been educated only in the barest essentials of loading and foundation design. And, anyway, this base would need some designed rebar in it, running at right angles to the wall and secondary rebar running parallel with the wall, which all starts to get a bit silly.

The base you are putting in is 600x600 = 360000mm^2. I'm assuming that this is ok for the load being applied to it - unless you're on sloppy crap, I would think that it's fine. Now, if you were to change this to a rectangular affair, let's say 300 wide out from the existing wall, then the length would need to be 1200 long to get the same area. THe dimension from plate edge to foundation edge would be (1200-280)/2 = 460, so as long as your base is greater than 460mm depth, no reinforcing is required, as the load will spread along the length of this base. Looking across the other way, the centre of the post would still be at 140 off the outer edge of the foundation, ie 10mm off the centreline of the base - in other words, next to no eccentricity and certainly way less than L/6.

If you want to use a 450 wide base out from the wall, the same maffs applies. Base length needed is 800 and the minimum depth is 260mm. Eccentricity across the short dimension will be 55mm, not worth worrying about, as it's still less than L/6.

Therefore, whichever rectangular base you decide to use, there is no need to worry about reinforcing it and you can save those offcuts for something else.

Personal preference? 800x450. Don't forget to go down to the depth of the existing foundation as a minimum, of course, with a min depth of 260mm of footing.

Re the other post, no that was me: the mods (well, a mod) were being particularly stupid the other day, so I wrote delete to save them the hassle of deleting my post, as anything I was posting seemed to arouse their ire. I'll get around to answering it properly .

Most informative Mr Talk! Although I had to read it very slowly at first

Just to clarify a few points.

Probably won't be any BCO involvement. I'm replacing the roof of my detached garage at the rear of the property, the structure will still be the same footprint, the roof will become higher, but within the 4m rule.

The walls are a sectional concrete jobby which sit on the surface, so no foundation to speak of, they might have laid a thicker edge when the base was put down but I won't know until I get digging.

I've done a concrete calc, and it seems that 0.1m³ will do it.
Giving the lack of a proper foundation I'm happy to go deeper, up to about 600mm will be realtively headache free and If I can pour all 4 pads using 1m³ of concrete I'll be a happy DIY'er.

Were I to reinforce for the sake of going the extra mile, what would be suitable? Couple of layers of mesh tied together? (I'd still be happy to be shot of these beams though)

Here is an old pic (pre-shed), one of the 4 columns will be going on the other side of the wall, where the window is.
View media item 2238
Thanks.