Raft with Stiffener Beams Design

[sybie99]

I know this shouldnt be difficult, but what are the basics for designing a beam raft, and at what point do you decide that normal strip footings is not the way to go and a raft is better?

Say for a typical 2 storey house, average loads below loadbearing walls are say 75kN/m. If your safe ground bearing pressure is 150kPa, a footing width of only 500mm will suffice. Now, if the safe ground bearing pressure is say 50kPa, a footing of 1.5m a meter wide is required.

Would a beam raft be more economical in case 2?

Now for the design basics, should I ensure the beams are below load bearing walls, and what width should these beams be? I have read that the beams in the raft must be sized so that the load from the walls above does not exceed the safe ground bearing pressure. So in this case, also 1.5m wide. What is the benefit then of a raft in Case 2 if the beams are as wide as a reinforced strip footing would have been? I am definitely missing something, and hence this thread.

 

[hokie66]

In my experience, raft slabs are one way of coping with expansive soils, i.e. clay which expands and contracts with moisture change. The objective is to make the slab work as a unit and stay in plane, as opposed to strip footings which are not joined by a structural slab, therefore tend to move independently.

50 kPa soil represents a poor bearing condition. What type soil do you have? What does your geotechnical consultant say about recommended footing types?

 

[sybie99]

Its sandy silts with low compressibility, he suggested a ground replacement excercise, but I want to see if a raft would be an option.

Also, I would for my own interest just like to know how such a raft is designed in its most basic form

 

[hokie66]

I haven't looked into the design methods of raft slabs for some time, so can't give you much insight. In Australia, the design of footings for residential structures has been codified into prescriptive form, so that once the site parameters are established by investigation, the prescriptive details apply.

Sandy silt of low compressibility should have a higher allowable bearing pressure than 50 kPa.

 

[slickdeals]

In its most basic form, it acts as a two way slab. Imagine a roof slab, upside down with floor loads = soil pressure.

 

[sybie99]

Thanks slickdeals, but still doesnt answer the question on beam width to use, so too beam depth

 

[sybie99]

Come on all you clever people, some guidance on raft design. I know a good way would be to model the raft slab and beams with the loadings from walls and columns and supported by elastic support modelling the soil, using FEA. This would give me an accurate result and I could actually see how mush the raft would deflect as well as what pressure is placed on the soil. But unfortunately I dont have access to such software so need to do a crude hand calc.

 

[structSU10]

So set up a beam model. you can model it as a continuous beam if you have multiple columns or walls, those as the supports, with your soil pressure as a uniform or varying load, as per slickdeals suggestion. You could do this in strips if column geometry is complex, and by ignoring two way action you will be conservative.

From that you can make your shear and bending diagram, and can check capacity. thickness would be limited by punching shear, and normal shear strength of the mat. There are some good simple excel programs for footing analysis too if you need to determine pressure to get started.

 

[sybie99]

Thanks structsu10, appreciate your advice. For irregular walls and columns layout on a uniform thickness slab raft this will work well. If anyone knows where I can find an example of designing a raft with stiffener beams it would please let me know, if this can result in a saving in concrete volume I'd like to follow this route.

 

[structSU10]

I would think constructability of a raft slab with thickened portions in critical zones would be much more time consuming than just a uniform thickness. You would have to do trenched pours for the 'stiffeners' or something, and it may get more complicated with varying rebar patterns and such. With the uniform thickness, in many cases, you can get a uniform rebar pattern (as long as the raft isn't too large where varying rebar would make sense) and the rebar guys could lay it out pretty fast. Generally labor cost is higher than material costs, so I doubt any savings would be realized in most situations.

Suffice to say, I have always just used a uniform thickness, so someone with experience with one may have a different opinion.