CMU: Columns vs. Pilasters vs. "In-Wall Piers"

[CN-EIT]

First thank you all in advance. I am a newly minted EIT and pretty excited to be here.

Background:
In referencing ACI 530-11, only definitions for "column" and "pier" are provided. However, in section 1.15 "pilasters" are touched on briefly and defined in the commentary without drawing a distinction between the pilaster and column. In TEK 17-04B, they state "Typically, pilasters are subject to little or no vertical load other than their own weight, and as such serve as flexural members...in many cases it may also be required to support vertical loads...When this occurs, pilasters are designed as columns and function primarily as compression members". Further, "pier" is defined in 1.6 through dimensional constraints which do not conform to what I am referring to as an "In-Wall Pier".

The "in-wall pier" is the portion of CMU which carries a gravity load of 35 kips and an in-plane force. The "pier" is 9'-0" tall and used to transmit the vertical reaction of a lintel to the foundation; it is also located within a shear wall. Both definitions for pier and column in the ACI 530 state they are "isolated". However, I would not consider the in-wall pier as isolated since it is: A)located @ an opening in a wall which will be laid in running bond, & B)any vertical reinforcing would pass through a bond beam which has bars running parallel with the length of the wall.

My questions follows:
What is the difference between a Column, Pilaster, and "In-Wall Pier" both technically and practically (assuming there exists some difference between the theory and reality)?
For the scenario I provided, which design codes would be most applicable?

 

[BridgeSmith]

A column or pier is isolated, i.e. unbraced or discretely braced. If what you have is integral with a wall, it's a pilaster, since it's continuously braced, at least in one direction. However you define it, the capacity is calculated based on the bracing or restraint conditions. I'm a bridge guy, so I can't help you with where to find the applicable provisions in ACI, or the distinction between a pier and a column in ACI. It could be that it's based on shape - columns are round or square, and piers are rectangular, or some other oblong shape, but I'm just guessing there.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

I distinguish column and pier is in general by "size". Column is relatively a slender compression member that more sensitive to flexural loadings/stress; the pier, on the other hand, is a relatively massive compression member that less sensitive to flexural loadings/stress. When you have sizable lateral load, or heavy concentrate load, column and pier are to be used/considered. Pilaster can be used for low-rise building with modest loads.

 

[phamENG]

A column is an isolated, independent member that supports mostly axial loads and resulting eccentric moments (though they can be designed to resist moments from lateral loads, too).

A pilaster is essentially a column, but it's built with the wall. So if you have a 16x16 pilaster and an 8" wall, your pilaster will either be proud of both surfaces 4" or flush with one and proud of the other 8". (All dimensions nominal, of course.) When they say they're used as flexural members, they are referring to a method of design in which you span the walls horizontally. It's a way to get taller walls out of a standard masonry reinforcing pattern. But often times they are also supporting a girder or a truss, so they support axial loads as a column would while also resisting out of plane forces from wind, etc.

A pier is a term used in strength design, and it refers to a section of wall next to an opening (I'm assuming you're not referring to "Foundation Pier" - that's just a little thing in the crawl space of a house). I don't have 530-11 anymore, but 2013 does not say anything about the pier being isolated. If you don't meet the dimensional limits of a pier, you design it as a wall segment.

 

[CN-EIT]

Very good. Thank you for the distinctions and clarifying the general purposes and conditions respective to each.

It would then follow in my case that I have a pilaster (as it is integral with the wall) which is subject to axial load only. I should specify that the shear wall has been designed such that the in-wall pier I am referencing does not have an appreciable in-plane force (i.e. it is being designed for gravity load only).

The last portion of my train of questions is:
1) Having determined the number of cores we need to grout and bear on (based on the allowable bearing stress of CMU = 0.33f'm) we specify longitudinal reinforcement within the cores themselves, is this reinforcement necessary or is it simply "good practice"?
2) Is confinement a concern?
3) If confinement is a concern, are lateral ties necessary for confinement or is the horizontal wire mesh reinforcing embedded in the mortar joints of the wall itself sufficient to provide confinement?

Thank you!

 

[phamENG]

Can you post a picture of what you're proposing? Just a plan view of the detail would be good.

 

[masonrygeek]

Confinement is usually only necessary when dealing with laps/splices of vertical bars. Confinement can be used to shorten the required lap lengths that may be required. Joint reinforcement is not typically used for confinement when used for these purposes. For more information see NCMA TEK 12-06A here:

http://ncma-br.org/pdfs/68/TEK 12-06A1.pdf.