Ties in compression members
Ties in compression members
(OP)
My question is specifically concerning ACI 318 section 7.10.5.3 which says "Ties shall be arranged such that every corner and alternate longitudinal bar shall have lateral support provided by the corner of a tie...and no bar shall be further than 6" clear on each side along the tie from such a laterally supported bar."
Anyone have a good idea why this is required? In the commentary of this section it notes that tests have been performed comparing fully tied columns and columns with no ties at all and there were no appreciable differences in column strength. If this is the case, why have the requirement for all compression members if it is not needed for shear and it does not help in compression?
Anyone have a good idea why this is required? In the commentary of this section it notes that tests have been performed comparing fully tied columns and columns with no ties at all and there were no appreciable differences in column strength. If this is the case, why have the requirement for all compression members if it is not needed for shear and it does not help in compression?






RE: Ties in compression members
RE: Ties in compression members
That's what I picture in my mind when I think about column tie bars.
RE: Ties in compression members
I live in earthquake area (Utah) and have a decent understanding of why lateral support of the longitudinal bars is a good idea for earthquake design (especially in plastic hinge zones), but I didn't think chapter 7 of ACI 318 dealt with earthquake design, which is what confuses me. Chapter 21 has other requirements for ties which deal with earthquakes.
I am not sure I completely buy the idea of the ties being needed for construction, though I suppose it would help somewhat. But if you have a tie that loops around the four sides and connects the four corners, then I would think you would have enough lateral support on the middle bars to keep them from bulging much between the main ties at maximum spacing. But maybe not. That is something to consider anyway.
RE: Ties in compression members
Seismic and construction issues apart, the lateral ties do come in handy for calculating the splice lengths. (in our office we usually ignore that provision)
RE: Ties in compression members
RE: Ties in compression members
BA
RE: Ties in compression members
Shaneelliss - You did mention that provisions of Chapter 21 do not apply but still....
Reference - Instruction material complementing FEMA 451: Confining reinforcing can improve behavior in two ways. First it can enhance strength by restraining lateral strains. Second it can increase the usable concrete compressive strain well beyond the typical value of 0.003. Also published literature on full-scale FRP confined reinforced concrete column point to the same conclusion.
RE: Ties in compression members
RE: Ties in compression members
I dont really deal with concrete columns, but I have always used the tie provisions for piers and pedestals for column footings.
If nothing else, as others have mentioned, the help in construction.
They also act as stirrups in piers with moment, no?>
RE: Ties in compression members
This is what I have done in the past for lightly loaded concrete columns, piers, and pedestals which are not in high seismic zone, and not part of a lateral load resisting system. I use the criterion given in ACI 318 -02, section 21.3.1.1, to distinguish between a flexural member and a compression member: The factored axial compressive force on the member shall not exceed Ag. f'c / 10. (I use this definition given in Chapter 21 for classification purposes only.) If the vertical member qualifies as a flexural member using this definition, then I just follow the provisions for a flexural member. Let us say the vertical member is subjected to shear. If Vu < 0.5( Phi . Vc) then no shear reinforcing is required per section 11.5.5.1. Ties may be provided from construction point of view only.