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Concrete tie beam

Concrete tie beam

Concrete tie beam

(OP)
A steel braced frame columns are supported on Spread Footing with tie beam between the columns. I have vertical and horizontal reaction from Frame. So designing tie beam i have to consider that has regular concrete beam with axial load (Horizontal reaction from column) ?
 

RE: Concrete tie beam

That is correct.

BA

RE: Concrete tie beam

(OP)
That axial load acts as compression or tension so for axial compression its like column axial capacity. And for axial tension i will check required Steel area = Pu/(phi*fy)?

 

RE: Concrete tie beam

It is a tension member, not compression.  The only function of the concrete is to protect the steel.  You should check your reinforcement for strength, making sure you have sufficient lap at required splices.  You should also consider the amount of strain in the tie and how it affects the performance of the frame.  You may wish to limit the strain by using a conservative value for tie stress.

BA

RE: Concrete tie beam

That is how I would do it.

 

RE: Concrete tie beam

(OP)
How can i check amount of strain in the tie. I have 2'X 2' beam with 8#6 for axial load of 100 kip.  

RE: Concrete tie beam

Tie members don't have to be wrapped with concrete, but they do have to be protected.  Something like Denso tape is one way.  Whether or not in concrete, I prefer to use couplers rather than lapped bars.

RE: Concrete tie beam

Couplers are a better detail, I agree.

If you are carrying 100 kips with 8-#6 bars, your steel area is 3.52 in^2 and the stress is 28,300 psi.  The total strain is E*L/28,300 where L is the length of the tie.

BA

RE: Concrete tie beam

(OP)
To check footing design i have to check for max load case (Max Vertical load and  max Hori. Load) on footing to check for overturning and sliding both ? And i have to check also for Min. Load case (min load case - uplift and hori. load) on footing for overturning and sliding. ?

RE: Concrete tie beam

Maybe I missed something, but why does everyone think this is a tension tie? The original post said this was a tie beam for a braced frame.

RE: Concrete tie beam

Perhaps the OP can clarify whether the force in the tie beam is tension, compression or either/or.

BA

RE: Concrete tie beam

To me, the word "tie" is synonymous with tension member.  If it is a compression member, it would be called a strut.

RE: Concrete tie beam

(OP)
how about footing design for sliding.
 

RE: Concrete tie beam

hello12345hello,

Quote:

how about footing design for sliding.
Well, how about it?
Is the tie beam a tension member, a compression member or both?  Perhaps you should post a sketch of your structure so that we can get a better understanding of the problem.

BA

RE: Concrete tie beam

(OP)
tie beam acts as tension or compression depends upon loading from wind or seismic load direction. And top of concrete beam is same as top of footing.  

RE: Concrete tie beam

Provide a sketch.

BA

RE: Concrete tie beam

(OP)
If its unclear i will repost it.

RE: Concrete tie beam

Sketch comes through a bit dark.  Anyway, if it is a compression strut sometimes, it would be designed as a column for that load case.  And yes, you have to resist both the horizontal and vertical components of load.  Sliding resistance, passive resistance, resistance by floor slab are all possibilities for the horizontal component.

RE: Concrete tie beam

what is the dimensions of your beam and footing?

RE: Concrete tie beam


hello12345hello

Normally if columns of the steel structure are braced i dont think you really need a Tie beam.Tie beam could be a compressive or tensile member it really depends.
if I were you and I would design a steel structure say 1 floor building and I wouldnt bother to provide tie beam however if my structure is multi story building i would provide Tie beam.

7thsky

RE: Concrete tie beam

Based on the sketch that was sent. If the bracing is tension only, then I don't see the 'tie beam' in tension at any point. Even if it's not tension only bracing, I'm not sure it would ever be in tension.

Outside of a code requirement for the beam/strut, I would only provide the beam/strut if required to resist sliding.

RE: Concrete tie beam

The tie will serve to distribute the horizontal forces. Be conservative in your assumptions about which footing will resist how much sliding.
 

RE: Concrete tie beam

Regardless of if this tie beam is subjected to tension force of no , i provide tie beams in most structure to avoid differential settlement which can be under vertical loads or due to any unexpected factor like leakage of water next to foundation,excavation next to next for any reason

RE: Concrete tie beam

(OP)
100 kip of horizontal load 10X10 Footing won't provide enough passive pressure resistance and Reaction from column is not much when its min. load combination. Can we consider that load transfer into tie beam and provide sliding resistance.?  

RE: Concrete tie beam

The tie/strut member will engage the other footing, but if the two footings combined lack the necessary passive and sliding resistance, additional foundations must be similarly connected until the required resistance is obtained.

BA

RE: Concrete tie beam

Two additional thoughts:

1.  The tie beam will limit lateral movement in the footings due to any seismic activity or subgrade movement.

2.  If the beam is extended to other adjacent footings, it can become a grade beam where it can pick up additional dead load if needed, to resist overturning from the braced frame.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Concrete tie beam

(OP)
BAretired,

if i will consider combine effects of both footings and Min. Load combination, say i have uplift on one footing and downward load on the other footing so Addtional load on other footing will help me to provide sliding resistance. (Sliding coefficient * Vertical Reaction). + (Passive Pressure reaction Times two from both footing). Correct?

 

RE: Concrete tie beam

That is correct.

BA

RE: Concrete tie beam

You might want to look into your code and make sure you don't need to use mechanical or welded splices for this member.

The Canadian code CAN/CSA A23.3 requires mechanical or welded splices for tension tie members, and does not allow lap splices.  The definition in the appendix leaves some leeway for interpretation, but I think this is pretty clearly something that would fall under that clause.  I wouldn't be surprised if there's a similar clause in the ACI code, given that they generally have the same history.

 

RE: Concrete tie beam

AISC has smilar provisions for a member in direct tension (i.e. "tension tie"), but there is some wiggle room provided in the commentary which talks about non-mechanical splices in less-critical tension members.  

RE: Concrete tie beam

(OP)
I have compr. strut/tension tie beam between two footing. To check for overturning at footing base do i have to count overturning moment from Hori. Load.? I will count moment from overturning moment from uplift load.
 

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