Transverse reinforcement for columns of special moment frames

[nayr1]

I am designing a structure in SDC D. As per ACI 318-14 18.10.8.1, wall piers must satisfy the special moment frame requirements for columns as per 18.7.4, 18.7.5, and 18.7.6.

[tt]Pier size = 20 in x 14 in
f'c = 6,000 psi
fy = 60,000 psi
Cover = 2.5 in[/tt]

Following 18.7.5.4:

[tt]Ash/s*bc = 0.033
bc = 20 - 2*5 = 15 in
Ash = 0.40 in2 (2 legs no. 4)
s = Ash/(0.033 * bc) = 0.40/(0.033*15) = 0.80 in[/tt]

This results in a required hoop spacing of 3/4", which is not feasible due to the 1" thickness required at the hook overlaps.

Am I correct in my calculations? Is there a clause I am overlooking that allows for a minimum spacing value? I don't believe any spacing closer than approximately 3 in spacing would be beneficial as it would just result in honeycombing due to poor consolidation.

 

[KootK]

I haven't checked your math in detail but, at first glance, I think that you'll want some cross ties parallel with the short side of the column that will contribute to Ash and, therefore, open up the spacing. Your maximum permissible tie spacing is also going to be 14 in / 4 = 3.5". So you'll have dense reinforcing no matter how you slice it.

 

[nayr1]

The longitudinal reinforcing is 6 no. 8. Even if I use no. 5 ties and add a cross tie, the spacing becomes 1 7/8" while the ties are 1 1/4" thick at the overlaps. That leaves 5/8" clear spacing between ties.

3 1/2" spacing would be acceptable, but then I would need 6 legs of no. 5 at that spacing.

 

[HTURKAK]

- Apparently You are correct in your calculations however is not applicable and not good practice..

My approach would be,

- Assuming Pu ≤ 0.3Agfc, the applicable formulas the greater of

i=0.3 *(Ag/Ach - 1 )*(fc/fyt)

ii=0.09 *fc/fyt =0.009

- Regarding first expression , i would choose cover =1.5 in .. in this case Ash/s*bc = 0.0149 ( governs..)

- Choose s = b/4 =3.5 in for ties parallel to long side , bc=11

- Ash/s*bc = 0.0149 Ash= 0.0149*3.5*11=0.58 ( BAR SIZE NO 5 is OK with two legs )

- For ties parallel to short side you will need one cross tie .

Good luck..













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[nayr1]

Unfortunately, I cannot reduce the cover as that is a code requirement and is also mandated by the Engineer of Record. I am the Precast Specialty Engineer and this is a precast concrete structure.

The only alternatives I can see are to:
[ul]
[li]Change the precast panelization to increase the pier size, which might not be accepted by the architect[/li]
[li]Reduce the compressive strength, which might not be accepted by the engineer of record[/li]
[li]Use grade 80 or 100 transverse reinforcement, which might have availability issues[/li]
[/ul]

 

[HTURKAK]

IMO, In this case the only alternative seems to increase the pier size...








If you put garbage in a computer nothing comes out but garbage. But this garbage, having passed through a very expensive machine, is somehow ennobled and none dare criticize it. ( ANONYMOUS )