Temporary Concrete Block Anchorage

[structeng2]

Hello -

A client has a piece of equipment that is going to be stored on site for about 1 year before installation. They would like it to be temporarily anchored until then. We are exploring using concrete blocks (deadman block, eco block, etc) with weights around 3-4kips per block. The equipment would be fastened to the blocks with a cable at approximately 45deg.

My questions are about the load combinations, stability, and inertial forces of the blocks themselves.

1. Inertial Forces: Considering the block is above grade, it will be excited during a seismic event. This would reduce the blocks capacity to resist loading from the equipment. If the block was embedded, we would ignore inertial forces (just like we do for a regular foundation), however, I am not sure how to get around this when the block is sitting on grade.
2. Load Combos/Stability: For stability, the controlling load combo will be 0.6D + 0.7Eh - 0.7Ev. This reduces the blocks self-weight even further, but again - I don't see a way around it.

What we are seeing is that when considering the inertial forces and stability of the blocks, we are seeing very large block sizes required to resist relatively small cable forces. Curious on your thoughts/input on how you might approach this design.

Thanks!

 

[LittleInch]

Probably not surprising when all the forces are also being experienced by the blocks as well as the item ( large transformer?)

Have you considered screw anchors or burying those blocks?

Or just ignoring seismic as an unlikely / rare event over a one year period?

 

[DanKile]

Could you put the blocks and equipment on a sled? This would keep the blocks in the same location relative to the equipment.

 

[structeng2]

Probably not surprising when all the forces are also being experienced by the blocks as well as the item ( large transformer?)

Have you considered screw anchors or burying those blocks?

Or just ignoring seismic as an unlikely / rare event over a one year period?

Yes, this is a very large transformer. We looked at screw anchors as one option, but the proximity of the transformer to an existing building foundation system makes it difficult to install.

Burying the blocks would be a good option as well if the client is ok with patching and repairing after they move to the final location - even just partial burying would greatly increase the sliding capacity.

We reduced the seismic load in accordance with ASCE37-14 which allows for reducing the site design values based on a temporary period.

Thanks for the ideas!

 

[structeng2]

Could you put the blocks and equipment on a sled? This would keep the blocks in the same location relative to the equipment.

Unfortunately not, they have already set the equipment down on a steel plate and lifting it to put everything on a sled would likely cost more than just getting a few more blocks to hold it in place as it sits. Interesting idea though for sure.

 

[HTURKAK]

Load Combos/Stability: For stability, the controlling load combo will be 0.6D + 0.7Eh - 0.7Ev. This reduces the blocks self-weight even further, but again - I don't see a way around it.

I would analysis for seismic and wind assuming accidental case ( since one year period ) . Probably the equipment will resist to the seismic OT and sliding without any additional measurement.
The picture implies the equipment is primary substation transformer.
If you provide equipment data , seismicity you may get better responds.

 

[structeng2]

I would analysis for seismic and wind assuming accidental case ( since one year period ) . Probably the equipment will resist to the seismic OT and sliding without any additional measurement.
The picture implies the equipment is primary substation transformer.
If you provide equipment data , seismicity you may get better responds.

Thanks, HTURKAK. Could you expand what you mean by 'accidental case'. We are reducing the site design values based on ASCE 37-14, using a 0.2 reduction factor.

My question was more about the concept of including the inertial forces of the blocks themselves in the analysis - which based on this discussion appears is the correct thing to do.

 

[LittleInch]

Is there a gap under the TX?

Could you not just create a tri angular frame and just use the blocking as extra weight?

Or use that to fix the two blocks together?

 

[JStephen]

I assume seismic loading is based on treating this as a fixed rigid object.
One of the ASCE short courses I took discussed seismic loading based on a rigid object rocking. Which increases the period and might reduce seismic loading.
Similarly, if the analysis allows for sliding of the transformer, you might be able to generate lower seismic forces. Maybe greasing that steel plate is the answer!
I don't remember if it's addressed much anywhere, but in some cases, the highest seismic forces are associated with very small displacements. So if the ground shifts by one millimeter, it doesn't really matter how fast it does it.
I had some other ideas, and they're pretty well covered above. One thing that occurs to me, on your diagram, maybe put rigid links where you show the blue lines, rather than just tie-downs, thereby increasing base size.

 

[HTURKAK]

Thanks, HTURKAK. Could you expand what you mean by 'accidental case'. We are reducing the site design values based on ASCE 37-14, using a 0.2 reduction factor.

My question was more about the concept of including the inertial forces of the blocks themselves in the analysis - which based on this discussion appears is the correct thing to do.

-In general seismic codes use the 475 years return period earthquake for general structures , and this corresponds 10% probability of exceedence in 50 years ( 50 years is the design life) . In your case , one can assume temporary situation and use 10 years of earthquake . ( 95 years return period earthquake )
- The proposed deadmen will also experience the same EQ and if there is risk of sliding , the deadmen will also have the same risk.
- If the picture is scaled and the weight is distributed reasonably uniform, overturning is less probable scenario . The equipment will start rocking ( as JSTEPHEN stated )and increase the period. I have visited the Obelisk in Istanbul last year. The obelisk staying there more than 1600 years and apparently alot of strong eq's could not topple .