Foundation Design/Analysis Questions

[Chosen One]

When considering an isolated spread or wall footing and trying to find the critical depth (d), is the typical ACI Requirement for beam shear of Phi*Vc > 2*Vu waived for stirrups? The examples I have found don't require the normal factor of 2 before stirrups are required. Obviously the goal for simplicity in design is to avoid adding stirrups but I was looking for something in the code stating this.

Also, when designing a column to footing transfer should we use a phi factor of 0.65 for the dowel design in compression if in fact those bars are needed for force transfer? The SERM 6th (page 2-9)uses the factor but a text by McCormac (references 2005 code) on page 367 does not use the factor.

 

[dakota_79]

Correct: footings do not require minimum shear reinf. when 0.5*phi*Vc < Vu < phi*Vc. See ACI 11.4.6 exception (a).

On the second question, I've never designed a pier such that it can't transfer its compression via bearing (don't think that's a good practice), so haven't looked into this too much. However, in general, when summing the strength of steel and concrete (for any situation), you have to use the same phi factor for both.

 

[Phalanx]

Also, when designing a column to footing transfer should we use a phi factor of 0.65 for the dowel design in compression if in fact those bars are needed for force transfer? The SERM 6th (page 2-9)uses the factor but a text by McCormac (references 2005 code) on page 367 does not use the factor.

I don't have either of those books, but here is my take on the issue. If the column has only axial load, then the phi factor is 0.65 (compression failure). If the base of the column is fixed, phi will vary from 0.65 to 0.9 depending on whether the section is tension/compression controlled. In that case, treat the phi factor the same as you would for a beam. Check the strain in the tension steel using your favorite method (i.e. c/d ratio or strain compatibility). With the that info in hand, head to ACI 318-05 Section 9.3.2.2 and use the chart to calculate the correct phi factor.

 

[McEngr]

All good comments guys. Another good thing to consider is the compression development length if in true compression. For a straight bar in compression with a 3" clear, it may not be a thick enough footing. If a fixed base, it's good practice to use a 90 degree hook with the correct bend diameter + 3" clear.

Also, phalanx is spot on. However, the only way that the phi factor is 0.9 is if the section is tension-controlled, which means it would behave like a beam at the joint. This is very unlikely.

My PCA Notes to the ACI 318-08 says it's 0.65, which I think is consistent with most standards. Also, MacGreggor has the same phi factor.