ACI318 21.13 Members not part of seismic resisting system.

[Prestressed Guy]

I have a question regarding ACI 318 section 21.13 "Members not designated as part of the seismic-force-resisting system"

My question is what is the maximum allowable spacing of stirrups and ties.

I am the precast engineer on a California project which the EOR has designated SDS D in the general notes. It consists of prestressed beams that support the roof of a below grade vault. The beams are 37' long and support hollow core plank with H-20 loading. The gravity part is easy but these beams also support the thrust from the walls of the vault due to retained earth which create an axial compression load of 580 kips static and 1100 kips dynamic. The beams are 40" wide by 36" tall inverted Tee beams with Ag = 1296 in-sq. Ag*f`c/10 = 648 kip.

Given this these beams are more columns than beams. The lateral force system is shear wall with rigid diaphragm so the beams and the columns that support them are not part of the SFRS. The box is extremely rigid so any induced moments will be small relative to the axial and bending moments they are designed for. They will need to be designed per section 21.13.

If I design as an intermediate reinforced column it with the full 1100 kip axial force and all gravity loads I can use So 5.5", Lo 75" with So 11" over the remainder.

 

[Prestressed Guy]

Bump.

Anyone have any thoughts on the issue of ACI 318 section 21.13. I am still trying to resolve is. It is looking to me like I am going to need 6" max spacing for all ties and stirrups but would like to hear others thoughts. This would be significantly more shear steel than what was bid.

 

[KootK]

Given this these beams are more columns than beams

The beams are not columns in one very important respect: floor diaphragm displacements will not produce unintended plastic moments at the ends of the member which would necessitate confinement and tightly spaced stirrups (that's why we do it for real columns).

So, in my opinion, you're off the hook here. I would absolutely run it by the EOR to ensure that everybody's on the same page but, rationally, I see no reason for the tight stirrup spacing here.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Deker]

Were the axial loads specified by the EOR? Under seismic load, you have active plus seismic earth pressure acting on one side of the building and passive earth pressure acting on the opposite side of the building. Since the amount of movement required to develop the full passive pressure typically exceeds the amount of movement experienced by the in-plane basement walls, I think the passive pressure can be ignored. This leaves you with a diaphragm force to be resisted by the grade level topping slab rather than a direct compression between opposite sides of the building. Perhaps the EOR can clarify the seismic load path he or she has designed for.

If you're stuck designing the precast for axial load, I would try to count the topping slab in the axial stress check since the soil pressure will be uniformly distributed along the length of the building. I see no reason why all the axial load should need to be carried by the beams (unless the shoring will be removed prior to pouring the topping slab, but that shouldn't be a concern for the seismic load case). If that gets you below the column compression limit, all you have left is to follow the beam detailing requirements for demands induced by the seismic displacement. As KootK mentioned, if your beam end connections can't generate moment, you're done. If they can, the displacement will need to provided by the EOR. It may be easier to just assume the beams will hinge and detail the transverse reinforcing accordingly.

 

[Prestressed Guy]

Sorry it took so long to respond to this. I forgot to check and did not receive an email notice.

KootK:
As I read the 21.13 provisions they do not specifically address columns as such. They reference "members with factored axial forces" There is no reference to orientation or use. Given that these members are gravity supporting beams with soil / hydrostatic loads of 576 k static / 1100 k dynamic I would say that they definitely have axial forces acting on them.

With precast columns almost always are pin connected at the column / beam joint with or without a 3" cip topping. There really is no way to have end moments other than those that are induced by load eccentricity and P-delta. In spite of that the EOR usually requires conforming to the tie spacing requirements of 21.13.

 

[Prestressed Guy]

Deker:

The loads are prescribed by the EOR. (See below)
The loads are designated as load case H with static = 576 k and dynamic = 1100 k. Initially I assumed that H-static would be factored by 1.6 but H-dynamic was really an E load case so applies a 1.0 factor but I am not sure that is correct.

The full H axial load will be in the beams. The sequence of construction calls for the bracing to be removed prior to the installation of the hollow core plank roof and topping slag. The walls are 36" slurry walls with 24" cip liner inside of that.

One other problem I have run into is that the EOR specified SDS = D but listed Ss = 2.096g and S1 = 0.894g. These values would put it strongly into SDS = E.

 

[Prestressed Guy]

Here are the loads. I tried to upload two files but the perspective kicked off the loads.