Foundation Seismic Weight in Base Shear for Sliding? 1

[RDR89]

Hi friends, I have searched this forum and found some good information but nothing definitive from what I've seen, so I'm really asking for opinions and /or best practice at this point. I have an equipment foundation bearing 1'-0" below grade (t/slab = grade). I am wondering if I need to include the weight of the foundation in the calculation of horizontal seismic force for the sliding check. I've seen an engineer at my company include it, but he references ASCE 7-10 15.4-5. I don't think foundations were the intent of Chapter 15. Additionally, per ASCE 7-10, the definition of W per 12.7.2 is everything above the base, which leads me to believe that the foundation weight does not need to be included in the calculation of base shear for the system. If we do include it, I would need to include some sort of soil anchor as sliding will be an issue. Would love to know your thoughts!

 

[1503-44]

If I am not mistaken, ASCE-7 is for Structural steel design. Therefore their shear at the base is caused by the seismic accelerations on the mass of the structure above the base plates, hence they would not consider foundation mass, assuming that the foundation movement is exactly the same as the earth movement all around it and they move together as a unit. If your foundation were to slide in reference to the earth, that would negate the assumption that everything moves as a single solitary system below the structure and additional accelerations on the structure could indeed be generated independently from the seismic accelerations alone. If the foundation slides, you must determine the accelerations and movements of the foundation from the earthquake on the foundation, then in turn apply those to the structure in lieu of the seismic accelerations. Determine the rate of foundation slide including foundation and structure mass, then apply that foundation motion and resulting foundation acceleration pattern to the mass of the structure above instead of the seismic force and acceleration alone.

 

[RDR89]

ASCE 7 is for loads on structures, not steel specific. It also feels like an infinite loop scenario... if I increase the mass to help with stability, I would also increase the seismic weight.

 

[1503-44]

Yes it is a loop. But not necessarily infinite. Increase mass of the foundation until you have enough friction that it does not slide and that will reduce accelerations on the structure above. Increasing mass of the foundation will also reduce movement. Adding mass in the right places, reduces the response spectra. In the wrong places, it may increase it.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[DaveAtkins]

Since you are designing the foundation FOR the seismic force caused by the piece of equipment, the weight of the foundation itself should not be included. Here is how I look at it. The foundation goes along for the ride with the soil, and the inertia of the piece of equipment causes it to stay where it is, which results in a horizontal seismic force on the foundation.

DaveAtkins

 

[1503-44]

No. If you put the equipment on a sliding support, "the ride" is entirely different. The ride felt by the structure is that of the sliding support, no longer that of the soil.

Consider the forces on a high radar mast on a ship, or on an offshore platform in waves. Its forces are not that due to the wave acceleration, but of how the ship responds to the wave, how its mass and buoyancy distribution affects the tower. The tower is connected to the ship, not the waves.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[RDR89]

@DaveAtkins, all the verbiage in ASCE 7 and ACI 318 refers to the structures being supported by the foundations but its not super clear, so I am inclined to think like you that the mass of the foundation itself is not counted in the seismic weight

 

[1503-44]

It is not counted, but if the foundation moves differently than the earth, then the accelerations are not that of the earthquake and you need to figure out what accelerations are actually applied to the structure. It then becomes a problem exactly like a vibrating machine foundation design. The machine foundation mass dampens the response of the machine in a different manner than if the machine was placed directly on soil. Dynamics 101.

If your foundation has wheels" starts sliding and the friction brakes quickly kick in, the stop may be worse than the seismic motion that started it moving.

IMO, you probably do not want sliding to happen at all and are better off pinning it rigidly to the soil, unless it is a very unusual piece of equipment with lots of highly flexible pipe or cables attached to it. Just saying.


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[RDR89]

@1503-44 thanks for the input. Unless I am misunderstanding what you are saying, I don't think anyone is arguing that the behavior/amplification, etc. of the equipment is impacted by the foundation. However, sliding is resisted by the foundation and in smaller part, the surrounding soil. My question isn't about the behavior of the equipment as it related to the foundation, but rather whether the foundation WEIGHT needs to be included in the seismic weight for base shear as it relates to sliding.

 

[phamENG]

For just a 1' thick slab, I would include it in the seismic weight. It will resist movement almost exclusively through friction, so that weight is above the point of lateral resistance.

Turn the edge of the slab down far enough to resist the loads through passive soil pressure - essentially locking the soil and the slab/foundation together - then I would be on board with ignoring the seismic weight of the slab.

Now, I would not lump the concrete mass in with the relatively lightweight machine sitting on top. I'd first check from top of slab up to determine the stiffness of the machine and the anchorage requirements. As long as it's flexible, I'd then do more of a two stage analysis. If the machine is stiff, then maybe lump them but be careful about locating the combined center of mass.

 

[1503-44]

If the foundation does not move differently than the ground, you will have no other accelerations placed on the equipment, other than what is caused by inertial force from the equipment mass itself. But if that force is enough to cause rocking, or sliding of the foundation, than other accelerations are present and probably need to be accounted for.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[phamENG]

If it's enough to cause rocking or sliding, the design engineer failed to do his/her job...

 

[RDR89]

@phamENG, it is a precast slab so not sure a turndown is an option. I've been looking at ASCE 7-22 Eqn 13.3-1 for the horizontal acceleration of non-structural components for the calculation of my equipment seismic loads. I am not sure what to use for the calculation of the horizontal seismic load from the foundation. The period is 0 since there is no height associated with it. The only applicable section I have seen is 15.4 for non building structures but foundations do not seem to meet the intent of that chapter.

 

[phamENG]

The center of mass is of the 12" slab is 6" above the base.

 

[1503-44]

Including foundation weight only "ensures lockage" to the earth, ensuring that all of the system above the earth does not have any additional accelerations applied other than direct seismic load. You are rigidly "attaching the mast to the wave". It does not change the seismic loads on the equipment above. It ensures they are exactly the seismic wave only.

Lump the equipment mass and foundation together to determine if it slides as a unit.
If it does not slide, then you only have seismic forces on the equipment to consider.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[RDR89]

@phamENG the top of slab is at grade. There is no portion of the slab above grade. The foundation is the base and per the definition of seismic weight, it should be everything above the base. What am I missing here?

 

[1503-44]

If the foundation doesn't slide, you're done.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[phamENG]

The base is the point at which lateral loads are imparted to the structure. Do you expect some loose top soil to impart loads to it? I doubt it. They'll be applied on the underside of the slab, where you have friction. So the center of mass of the slab is 6" above the base.

For your machine and it's anchorage, yes - I would consider the base to be the top of the slab. Since the lateral stiffness of the machine is likely much less than a 12" thick slab lying on the ground, a two stage analysis philosophy would apply.

I would take R=1 for checking sliding under the slab.

 

[phamENG]

thread507-385120

Found this from a few years ago. I know KootK's thoughts on seismic loading and ground movements have evolved, so I'm not sure if he'd stand by this or if he has some different ideas, but some good food for thought in there.

 

[RDR89]

@phamENG yep that's the one I kept going back to. I'm still not entirely convinced but erring on the side of conservatism, I will include it.