Strap Footing vs. Combined Footing

[vincentpa]

A long time ago when I was designing larger buildings, I had to design a lot of combined footings. One time however, I had to design a strap footing. I can't remember why I had to use a strap instead of a combined footing. Can anyone explain when it is appropriate to use a strap footing over a combined footing? I also forgot the simple design of the strap footing. Can anyone refresh my memory?

 

[DaveAtkins]

Use a strap footing when a combined footing won't work. If a column is very close to the property line, or existing building, and the bearing pressure with a combined footing is too high, a strap footing is the solution.

A strap footing is a beam that is supported by two spread footings, cantilevering over one of them. The column near the property line or existing building sits at the tip of the cantilever.

DaveAtkins

 

[vincentpa]

I could not remember why I used constant shear in design. Thank you for clarifying things or really jogging my memory.

I designed many combined footings where the exterior column was next to the property line but I guess I was always able to get the pressures to work so I never had to ask what happens when they don't. The only time I designed a strap footing was on a project where I was working with a 70 year old engineer. (We started the same day. I was 23. He was 70. He liked to travel around the world.) Anyway, it was a column for a long span truss over an atrium that was part of an addition to a hosptial. He must have known by experience or by running a quick number that a combined footing wouldn't work. I can't remember if he ever explained why or I forgot to ask. It was a VERY fast track project.

 

[hokie66]

The strap footing you refer to is a type of combined footing. "Combined footing" just means that a footing system supports more than one column. The strap (or tie beam, rectifying beam, etc.) combines the two footings in your case.

 

[MarcbSE]

Actually, I think in this case it appears to be two footings supporting only one column. The "strap" beam cantilevers over one footing to pick up the column load. The resulting uplift is resisted by the second footing.

 

[jike]

A strap footing is also refered to as a pumphandle. The footing along the property line is normally eccentric to the column load and the strap (or pumphandle) is used in combination with the load on the other footing to keep the eccentric footing from overturning.

 

[jike]

What Dave Atkins and MarcbSE are describing is what I simply refer to as a grade beam cantilevering over one footing.

 

[DaveAtkins]

jike,

Isn't that what a strap footing is? A grade beam cantilevering over one footing, with the back span going to another footing (preferably with a column on top of it)?

DaveAtkins

 

[vincentpa]

When I am talking about a "combined" footing, I am describing two column footings combined that have the same width generally occuring at a property line or when the columns are close and their footings "naturally" combine. A strap is a type of combined footing, I know. But there are circumstances when to use the "strap" instead of just combining the two footings by filling in the space between them with concrete and rebar (designed of course). I only wanted to know when it was appropriate to use each case.