ACI 318-19 Shear Provisions vs STM for pile cap

[kjm93]

I'm digging into ACI 318-19 updated shear provisions for the first time by designing a standard 4-pile pile cap. I've found that by using 318-19 I get a pile cap that has nearly 5x(!!) the amount of concrete as a 318-14 beam design method gives me, because I'm down to about 0.5root(f'c) due to the size factor and low flexural reinforcement ratio. So my solution to limit the amount of concrete is to use either a STM approach or introduce vertical shear reinforcement. By doing STM I basically get the same design as I got using ACI 318-14 beam design method - does this seems a bit unconservative now considering all of the 318-19 shear provision updates?

Another question I have regarding pile caps and the new shear provisions is if anyone else thinks the new size factor contradicts the CRSI Pile Cap Design Guide shear strength factor [3.5 - 2.5(Mu/Vu*d)] which increases the shear strength at the column face for deeper members. Should this CRSI factor still be used when checking one-way shear at the column face?

 

[HTURKAK]

I get a pile cap that has nearly 5x(!!) ????

I have copy and pasted the clause 13.2.6.2


13.2.6.2 For one-way shallow foundations, two-way isolated footings, or two-way combined footings and mat foundations it is permissible to neglect the size effect factor specified in 22.5 for one-way shear strength and 22.6 for two-way shear strength.

Probably it will be better to dig more..









Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[kjm93]

I didn't think clause 13.2.6.2 applied to pile caps. Fig. R13.1.1 seems to differentiate pile caps from those types listed in the clause.

 

[HTURKAK]

Pls look ACI 318-19 clause 13.1.1,

13.1.1 This chapter shall apply to the design of nonprestressed and prestressed foundations, including shallow
foundations (a) through (f), deep foundations (g) through (i),
and retaining walls (j) and (k):
(a) Strip footings
(b) Isolated footings
(c) Combined footings
(d) Mat foundations
(e) Grade beams
(f) Pile caps
(g) Piles
(h) Drilled piers
(i) Caissons
(j) Cantilever retaining walls
(k) Counterfort and buttressed cantilever walls

The items (a) through (f) are shallow found.











Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[kjm93]

My third printing has (a) through (e) for shallow foundations. Regardless, 13.2.6.2 only includes one-way shallow foundations which to me means strip footings or grade beams. I believe the size factor is required for pile caps.