Footing for metal buildings 2

[marthab]

In designing foundations for "metal buildings" what is the best way to resist moments on fixed columns? Do hair pins help? It is an independent footing (no monolitic pour).

 

[jheidt2543]

There are basically three ways to resist the horizontal loadings from metal building colums:

1. Use "hair pins", pieces of reinforcing rod that wrap around the column pier and run into the floor slab, the cheapest for smaller forces.

2. Use a "tie rod" between columns, a rod or rebar of a calculated size that runs all the way across the building from one pier to the opposite pier, works for larger forces.

3. Size the footing to resist the overturning force thru concrete mass, the most expensive, but only alternative in some instances.

 

[thaidavid]

marthab,

Be very careful when considering using options 1 and 2, as these could be rendered ineffective by later floor modifications (saw-cutting), which often occur in industrial buildings. The present use, and intended future use, of the building should guide you in this decision. Since it is your design, and your license, on the line, do not let the Owner dissuade you from what you believe to safe and adequate. Predicting future ownership and use of a building 20 years down the road is a tricky proposition.

David

 

[steve1]

Not quite sure why you are trying to "resist moments on fixed columns". Preengineered metal buildings typically are designed as rigid frames with pinned supports, thus resisting the horizontal loads becomes the main issue. This was addressed very well above. Why the "fixed ends"?

 

[PEinc]

The previous comments are all good comments. However, be careful using long tie rods between columns with large tension loads. PL/AE can get relatively large - possibly allowing more structure movement and damage than you anticipated.

 

[jheidt2543]

Here is a follow up question on this topic:

Does anyone have a reference for "hair pin" design for lateral loads in metal building footings? I think it is more than just the pull out strength of the rebar in the slab. The horizontal load must be sufficently distributed to the slab to prevent cracking of the slab too, so the slab reinforcing is also a consideration isn't it?

 

[Focht3]

jheidt2543 is right - slab reinforcement is critical for this foundation system to work properly - within the footing, or overall slab when poured monolithically.

PEinc makes a good point about PL/AE - but metal buildings are flexible and forgiving so this isn't a problem for most metal buildings.

thaidavid makes a valid point about slab penetrations, but the simple fact is that we cannot design for all stupid behaviors - only some. A structural engineer should be consulted whenever a building structure is to be modified - and cutting up a floor slab constitutes a structural modification. The original designer is not responsible unless the planned cutout was communicated to the designer before the design was finished and the structure was completed.

Independently cast (surficial) spread footings present a special problem - uplift can reduce the contact pressure and reduce / eliminate any lateral resistance due to horizontal friction on the bottom of the footing. (This assumes the footing is only about a foot thick.) Passive pressure along the leading edge of the footing - and frame action - are all that would be available to resist the horizontal force. Footing lift-out is a real concern. Since the whole point of most metal buildings is to reduce the framing as much a possible, shallow individual spread footings don't seem to be a hot idea for this building type.

marthab: does this help?

 

[jheidt2543]

One way to address the uplift problem is to check the footing size on the basis of the factored gravity and live loads and also on the basis of uplift. There are quite a few situations in metal building foundation design that the footing size and thickness end up being larger on the basis of the uplift load.

 

[robbe]

Does anyone know a good reference book for foundation design of metal building?

 

[Lakelander]

Robbe
I have used two books as guides to design. Metal Building Systems - Design and Specifications, by Alexander Newman and the Metal Building Systems Manual published by the Metal Building Manufacturers Association. They both provide some very useful information.

http://www.mbma.com/display.cfm?p=6C6636CC-11C3-11D5-9DD60050FC043376

Good Luck.