Electrical conduit entry to basement

[EEJaime]

Gentlemen,
I have recieved different answers to this question from our Structural fellows in this company. Thought I would take advantage of the expertise here.

I have a concrete encased electrical conduit bank consisting of 40+ 4" diameter conduit entering a building from below grade on site into a basement level electrical room. The facility is an important facility, in a seismic zone 4 in California. The basement wall is a reinforced concrete wall which at our entry point is 24" thick. I have detailed the conduit to enter through a series of 6" diameter sleeved openings utilizing the "Link-Seal" assembly system with 6" between sleeves vertically and horizontally. The conduit that passes through the wall is a 10' length of rigid galvanized steel conduit. Inside the building it transitions to EMT, outside to PVC.

My question is: At the concrete encasement of the conduit bank approaches the basement wall- can it be poured directly against the wall? Or do I need a gap of a few inches between the basement wall and the beginning of the conduit encasement?

Normally there is no seismic joint gap required below grade, but in this case only half of the installation is below grade. The basement finished floor elevation is +0'-0", the conduit enter about 16' above that.

Any comments would be apprecieated.
Sincerely
EEJaime

 

[hokie66]

I don't know how seismic design affect this, but I prefer the approach of extending the entire conduit encasement through the wall. If the encasement stops and only the conduits extend through, the wall becomes Swiss cheese and not feasible to cast properly. It seems to me that sealing around the encasement is a lot easier than sealing around each conduit penetration.

 

[cvg]

some "flexibility" at the penetration might be advisable assuming that the building and encasement may not move at the same rate and at the same frequency when the shaking occurs. Core drill to get the conduits through the wall, no expensive forming necessary.

What is the purpose of the encasement and is it required to abut the wall for any reason? I don't think it would hurt to stop the encasement short of the wall.

 

[msquared48]

I can see Hokie's concern, but think that providing flexibility at the wall is of more importance considering likely seismic movement both vertically and laterally.

With the rigid element continuing and penetrating the wall, you are inviting compromising the contained conduits in a seismic event and losing whatever service the conduits are providing.

I would provide a junction box outside the wall to dead-end the concrete encasement, and provide flexible connection through the junction box to the embedded sleeves in the wall. The junction box would be able to move at the intersection with the wall plane. A flexible sealant can be used to seal the wall penetrations.

Seems that this would be a standard problem in California and already detailed.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[EEJaime]

msquared,
The problem with your suggestion is that junction boxes must be accessible by electrical code, this will be buried under six levels of building construction. Further half the conduit are left empty for future use and must allow for conductors to be pulled through in the future by use of pull lines we will have in place. It is difficult to pull large conductors through any kind of "flexible" connection without direct access.

This is not the first time this has been done in California I am sure, however we have somewhat of unique condition in that we are going to be under a building that includes the partial basement, very heavy construction, (some of the basement walls where we have building piers tying into the main concourse structure have walls 5-6 feet in thickness). And we have this abnormally large quantity of conduit being distributed through this 24" thick section of retaining wall.

Hokie66, the system of precast sleeves I am using completely seal the penetration. The wall is absolutely swiss cheese and this portion of the wall is not as strong as the rest of the wall, but we plan for that with the structural engineer to reinforce around this section of wall that we are compromising. Since the basement is a high voltage, (well 34.5kV), transformer vault, we wanted to maximize the level of moisture control at the wall and thought this was the best way.

Thank you all for your comments,

EEJaime

 

[ToadJones]

Dumb question,
Can the conduit be routed above grade into building, then routed to the basement?
I always thought it was a bad idea to have conduit penetrate a basement wall below grade.

 

[EEJaime]

Jones,
No we cannot do that. The conduit is routed below a new major airport terminal building concourse. This is the only way. We looked at running it overhead, but the transition and space requirements, not to mention the 40-50 tons of steel that would need to be added to the structure for the length of this 1200 foot long concourse was prohibitive. As were the costs for the large amount of steel conduit, heavy strut conduit racks, seismic bracing and as mentioned a lot of steel structure to support it. We are running a lot of the conduit overhead, but these long, multiple conduit runs need to be below grade.
Thank you,
EEJaime

 

[ToadJones]

...See, I told you it was a dumb question!!!

 

[EEJaime]

Not at all. It was another possible solution that was thoroughly investigated and found to be unworkable, that is all.

Good day,
EEJaime

 

[CTW]

What was the consensus among your structural co-workers?

It seems to me that it's not going to matter whether it's cast against the wall or not. With that many 4" rigid steel conduits, I would expect the conduits to move along with the wall during a seismic event. Although that may depend on the type of backfill material used.

Is the transition from PVC to steel accessible? This may be the point of separation during a seismic event considering how rigid the duct bank will be where the steel conduit enters the building.

 

[EEJaime]

CTW,

The majority thought that a minimal seperation of a few inches would allow the building motion to occur without transferring any stress to the concrete conduit encasement, but the others thought that this would create a point of high stress on the conduit at the point at which the conduit leaves the encasement.

I think that it would be difficult to form the encasement this close to the building wall, so that it would require some kind of semi-rigid spacer that could be shaped to fit against the basement wall with openings for the concuit penetrating through that would stay in place after the pour. I have always just specified the encasement concrete to be poured right up against the building wall.

The backfill is 95% compacted earth, this is near the beach so it is slightly sandy soil naturally, but this will be engineered backfill-the site is an existing taxiway.

Thank you for your input,
EEJaime

 

[cvg]

sandy soil, near the beach in California - sounds like you will have significant motion and perhaps some settlement, possibly liquifaction?. Building foundation settlement will be different than the encased ductbank settlement. Seismic wave amplitude and thus building foundation movement will be much greater than a few inches and will occur in the x, y and z directions. Your idea for a spacer is good. I would provide significant separation and the spacer should be some type of energy absorbing material such as rubber in the gap.

 

[EEJaime]

CVG,
Soils reports indicate no liquefaction zones on the site. I will do some research on what type of spacer material might be availble. I will run this by our Structural guys, they must have some sources for this type of joint material.
Thank you,
EEJaime