Raft with Stiffener Beams Design
Raft with Stiffener Beams Design
(OP)
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.
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.






RE: Raft with Stiffener Beams Design
50 kPa soil represents a poor bearing condition. What type soil do you have? What does your geotechnical consultant say about recommended footing types?
RE: Raft with Stiffener Beams Design
Also, I would for my own interest just like to know how such a raft is designed in its most basic form
RE: Raft with Stiffener Beams Design
Sandy silt of low compressibility should have a higher allowable bearing pressure than 50 kPa.
RE: Raft with Stiffener Beams Design
RE: Raft with Stiffener Beams Design
RE: Raft with Stiffener Beams Design
RE: Raft with Stiffener Beams Design
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.
RE: Raft with Stiffener Beams Design
RE: Raft with Stiffener Beams Design
Suffice to say, I have always just used a uniform thickness, so someone with experience with one may have a different opinion.