Structural Picture Frame

[DiademSC]

Hi all,

I'd be grateful for any guidance on the use of structural "picture frames".
I have a knock through from an existing house into a new extension and have specified a classic "goal frame" type arrangement with steel vertical posts supporting the horizontal member. The posts are in turn supported on concrete pad foundations.

The builder has asked if I can specify a "picture frame" type arrangement instead with the lower horizontal member of the frame sitting along the existing footings. He believes that the lower horizontal member of the frame will equally spread the load coming down into it from the two columns at each side and says he has used this arrangement in the past supplied by a structural engineer on a 7.0m wide opening with high loads. My opening size is 6.5m and the vertical loads each side are in the region of 120 kN. Existing footings are 600mm wide x 200mm deep. I feel certain that there will be non linear distribution of the loads to the footings and that its not likely that the footings could take such high load.

Ive searched high and low online for example calculations that demonstrate how to show acceptability of picture frame loads to footings, but without success. Do any of you fine people out there have a calculation that you would be willing to share with me please? Or alternatively do you know of a web page that demonstrates how this is achieved.

Many thanks in advance.

 

[CANPRO]

I believe what you're describing is a classic case of a beam on an elastic foundation. The amount of load distribution will be a function of the stiffness of the bottom beam + footing vs the stiffness of the supporting soil.

You'll need a reasonable approximation of the soil stiffness. From there you can determine how stiff your beam needs to be in order to keep the bearing pressure in an acceptable range and also limit the beam deflection.

 

[KootK]

I feel certain that there will be non linear distribution of the loads to the footings and that its not likely that the footings could take such high load.

Yeah, this sounds pretty unlikely to be feasible. What's the maximum beam depth that you could finagle into this?

 

[XR250]

Can you underpin the foundation at each point load?
I agree with others that this sounds sketchy. You would need a really stiff-ass beam.

 

[rb1957]

I think the builder should stay in his kitchen, and let you work away in yours !

Maybe he'll say "but I've done this (seen it done) in the past".

It doesn't like a good idea to try and distribute concentrated loads over some "soil" base.

Would he rather make 2 small footings or one larger bed (for the beam) ?

Is there a practical reason he doesn't want to make the footings under the posts ?
Or is it "just" hard (uncomfortable) work ?
Or is it something he didn't budget for (so you're eating into his profit) ?

another day in paradise, or is paradise one day closer ?

 

[CANPRO]

Why is it that you need to engage 6.5m of footing for two point loads of 120kN? If you have a fairly modest allowable bearing pressure of 120kPa, you only need 1m² bearing area for each point load, which means you only need to engage a 1.67m length of footing. Put in a short beam between each post and the footing or maybe even an oversized baseplate if you have any faith in the existing footing to distribute load.

If you want to run the numbers on the 6.5m beam solution, you need to look into beams on elastic foundations. If you go through the calculations I think you'll find that a short portion of the beam very near point loads will be effective and majority of the beam will be doing absolutely nothing.