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Axial tension for reinforced concrete column

Axial tension for reinforced concrete column

(OP)
Situation where I have axial tension on a reinforced concrete column.  I don't due alot of concrete design so please bear with me.  Crack control isn't an issue as in like a water tank design.  My 11th edition of Design of Concrete Structures equation 1.14 says P=fy*As, but the next proceeding line states "to provide adequate safety, the force permitted in a tension member under normal service loads should be of order of 1/2P".  I don't understand the service load portion?  Can't I just use my .9 phi factor and use my ultimate load in equation 1.14?

RE: Axial tension for reinforced concrete column

I've always used the following:


Req'd As =  Pu
          -------
           φfy

where Pu is the factored axial tension force
fy is the yield of the rebar (usually 60 ksi)

This gives you the required strength check for the member as the steel takes everything and concrete takes nothing.

For serviceability I'm not sure what to say here as the natural tendency is for the reinforcing bars to stretch and the resulting cracks in the concrete would vary depending on the uniformity of the load application and the distribution of the reinforcing (i.e. you should have more, smaller bars instead of fewer, large bars).

RE: Axial tension for reinforced concrete column

Req'd As    =     Pu
                      -------
                      ?fy

RE: Axial tension for reinforced concrete column

I messed it up again....just to clarify the equation, here's a third try:


Req'd As    =     Pu
                      -------
                       φfy
 

RE: Axial tension for reinforced concrete column

(OP)
Thanks JAE

I have a fairly large prefab metal building (225'x350').  I'm tying the bases of each column for each frame together to take out the horizontal thrust loads.  My loads are great enough where I need actual grade beam/tension member to take out my tension.  I'm going to use (8) #7.  I'm going to bundle my bars in sets of two.  I think with all my clear distances and so forth I will be able to use a 16"x16" grade beams.  The bundled bars seem to work since I can wrap them at the anchor bolts of the columns.

Is it fair to say that I meet spacing and clear distance requirements for my rebar that I will be able to transfer my load sufficiently (i.e. have enough concrete to transfer the load) through my lap splices?

Another question I have..... This large grade beam can also see compression (not nearly the tensile load).  The grade beam will have soil on 4 sides.  The top side of the beam will unfortunetly have only 8" soil above it.  Can I consider it to be fully braced.  Obviously if I can't I will never get a column that long to work.

Sorry for the long post.  Thanks for your help.



  

RE: Axial tension for reinforced concrete column

I would take the service limit as a limit on cracking.  It may be based on experience or research, but if a text has made it to the 11th edition, then the authors are probably pretty knowledgeable.  I wouldn't just discount their recommendation without a lot of thought.  

RE: Axial tension for reinforced concrete column

I would provide much more reinforcement than required just to limit cracking.

Tension members will almost definately crack if only the minimum required strength steel is added.

csd

RE: Axial tension for reinforced concrete column

". . . to provide adequate safety, the force permitted in a tension member under normal service loads should be of order of 1/2P".

I have the 9th edition, and this sentence is followed by:

"Because the concrete has cracked at loads considerably smaller than this, it does not contribute to the carrying capacity . . .   It does serve, however, as a fire and corrosion proofing . . ."

So it would appear that the intention is to minimize crack sizes.

RE: Axial tension for reinforced concrete column

Something else to keep in mind is the anchor bolts.

With uplift at the column base, consideration will have to be given to the anchor bolt embedment, and how that tension load is transferred to the rebar, verifying adequate edge distance at your piers, and consideration of possible conflicts between anchor bolt locations and vertical rebar and pier ties.

On the last metal building foundation I did, the metal building manufacturer would not specify the anchor bolt length, so you might want to tie down who is responsible for that.

RE: Axial tension for reinforced concrete column

(OP)
Thanks everyone! All your comments are greatly appreciated.  With the reinforcing I will be calling out, my service load will be less than 1/2P.  So I do also meet this limit as the text book suggests.

RE: Axial tension for reinforced concrete column

(OP)
JKW05-Yes, I will be responsible for the anchor bolt length.  I will be verifying embedment, edge distance, etc.

RE: Axial tension for reinforced concrete column

Look at ACI 12.15.5 about lap splices in tension tie members (reference is for 2002 edition).  These need to be mechanical couplers in lieu of a contact lap splice and the splice locations should be staggered.

RE: Axial tension for reinforced concrete column

(OP)
sundale- thanks for the reply.  I guess that answers another one of my questions.  I guess lap splices don't work in my situation.  Mechanical splices will be specified.

RE: Axial tension for reinforced concrete column

(OP)
I think I might be beating this to death but when to I just go to providing a req'd steel plate size and encompass it in concrete to protect it?  I figure I would need like a 3/4" x 8" plate?

RE: Axial tension for reinforced concrete column

In the 8th Edition, the wording indicated that the service axial force should be less than 0.5*fyAs.  With normal load and resistance factors for the design state, it would seem that the service load criteria would govern.

RE: Axial tension for reinforced concrete column

With regard to the mechanical coupler detail: Is it suggested that this if for use in the grade beam tie for the horizontal thrust?  If so, on past projects, I have just detailed a continuous steel rod in a trench with turnbuckle(s). Then extended the rod to the outside faces of the column piers through a bearing plate.  Once the rod is brought up tight, the rod was protected by filling the trench with concrete.

RE: Axial tension for reinforced concrete column

225' clear Span, and a 16"x16" beam in spaning direction ; pardon me but i dont think that this beam would actually act as a tie beam to resist the lateral thrust which you want it to resist, for such large spans not typical beams but "Stressed tendons" or "tie rods" are used thats what i have seen in common practice.

if you are intrested in details! just ask.

regards

RE: Axial tension for reinforced concrete column

(OP)
zaes73-  Please explain more..  I'm guess I'm not seeing what your saying.  What is the difference if I use tie rods or use mechanically fastened rebar?

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