friction or anchor?

[PSF]

I am designing a footing for a heavy transformer (177 kips). My seismic force is approximately 20% of the dead load. Do I need to anchor the transformer to the concrete or will the friction between the transformer and concrete be sufficient to resist lateral force?

 

[steve1]

You must do two checks to determine this.

First check that the resisting moment is at least 1.5 times the overturning moment. This ensures that you have no net tension at the base.

Second check that the sliding resistance is greater than the lateral force acting on the transformer.

 

[haynewp]

You may have to anchor it buy Code anyway. There is certain mechanical equipment that is required to be anchored based on seismic category. For instance in IBC, "Component Force Transfer" 1621.1.1, "Component seismic attachments shall be bolted, welded or otherwise positively fastened without consideration of frictional resistance produced by the effects of gravity".

 

[PSF]

thanks. seems like i have to anchor it anyway by code even though it works for overturning and sliding. There are no holes for attachment on the transformer and the electrical engineers say they never are attached- but the IBC states it pretty clearly.

 

[JAE]

I didn't think that friction could ever be used for seismic...I would anchor it.

 

[UcfSE]

Seismic forces are often vertical as well as lateral. When the machine moves up and down it can lose contact with the concrete and all friction as well. I think you should anchor the thing.

 

[steve1]

This post is a good example of the grey area in the "building" codes when it comes to mechanical equipment installations. Is a stand alone transformer that weights 177k a building? Is it attached to a building? Should it be required to meet "building" code provisions that relate to mechanical/electrical components attached to buildings?

Sometimes, when dealing with mechanical/electrical installations, there are situations where a piece of equipment is so "dense" that it just doesn't need to be anchored to resist normal "building" wind and seismic loads.

However, that being said, if you are going to anchor it anyway then make sure you consider the effects of thermal expansion and contraction on the anchor bolts. Often a combination of oversize and slotted holes in the base plates are used to allow some movement.

 

[samdamon]

PSF-

What seismic performance category are you in? Have you figured out the Ip of this piece of equipment? I ask because IBC (at least NJ version of it) allows you to forego Section 1621 analysis if you are in cat C bldg or less, with Ip=1.0. In other words, mechanical anchors are not required in that situation.

You will need to determine if the transformer is vital to life/safety operations of your building after an earthquake to determine the Ip (See IBC sect 1621.1.7). Would also take into consideration if this transformer is inside or outside the building. If its inside I might design anchors even if they aren't required by the code. Who wants a giant cannonball dancing around loose during a seismic event?

Having said all that, I agree with above posts that one should not rely on friction of any kind to resist seismic loads. In an earthquake, anchors for a building or piece of equipment can get loaded by both vertical and horizontal movements in the subgrade, and it seems quite possible that frictional resistance could be marginal or nil due to elastic movements of the anchors. A "gray back from way back" that I thoroughly respect taught me such, a long time ago.

Let us know what you decide. Regards.