Tie beams/Footing girders

[andy1213]

Aside from controlling differential settlements of foundations, structurally speaking, what are the uses of tie beams/footing girders in a structure? I noticed, some engineers model their structure with tie beams while some are not. why?

 

[Lutfi]

This is a loaded question!

1. I use tie beams to carry lateral loads, if needed. They can be supporting masonry walls.

2. If you are relying on roof or floor diaphragm, you will need tie beams to act as chords for the diaphragm. This must be evaluated for tensile and compressive loads.

It really depends on what these elements are being used for. We simply can not add them to the structure without clear purpose.

I hope this helps.


Regards,
Lutfi

 

[SlideRuleEra]

Sometimes, it is the most economical foundation. For certain industrial structures, such as electrostatic precipitators, the only loads are regularly spaced point loads. Put the tie beam intersections directly under each column. Have piling or drilled piers at the intersections. Saves a lot of mass foundation concrete (and dead load on the piling), when compared to a solid slab. For this application a reinforced two-way topping slab (8" thick or so) is normally put on top of the tie beam grid, for maintenance access.

http://www.SlideRuleEra.net

 

[jike]

Engineers use tie beams or grade beams for a variety of reasons. Sometimes I use them to:

carry wall loads
distribute shear to multiple footings
take overturning out of an eccentric footing
transfer vertical load to adjacent footings

I hope this helps answer your question!

 

[haynewp]

Tie beams are also sometimes required to keep the foundation elements from spreading apart during a seismic event.

 

[andy1213]

Thanks for the input guys. By the way, I work with an EPC company specializing in petrochemical projects and I deal with several design subcontractors. Its been the cause of arguments in my office because some engineers insist to put tie beams because they said its a common practice in the petrochemical designs but I am after the economical design and as much as necessary I want to remove those tie beams justified by analysis that they’re not actually needed in the structure, say a steel or concrete piperack structure. Is their reason justifiable or they're just following a set of old uneconomical design practice?

 

[JAE]

What haynewp says - they are required in the UBC & IBC in some areas of specific seismic levels (many times associated with the risk of liquefaction).

I could also see using it to tie in lateral systems and delver the lateral load to the earth in a more competent manner.

I could also see using it if the structure was on a hillside and I was concerned about cut/fill settlements or other instabilities from a geotechnical consideration.

I could also see using it if there were plans (current or future) to excavate beside footings where lateral forces were intended to be resisted by earth pressure.