Friction Contribution to Lateral Force Resistance
Friction Contribution to Lateral Force Resistance
(OP)
Hello all. I have a somewhat theoretical question relating to calculating the required tie down force for resisting lateral forces, particularly seismic.
I have attached a sketch to illustrate the scenario I'm thinking of. Let's say I have a rigid box type structure, say 10'x10'x10'. It rests on a bearing pad at each corner. It is not really part of any building structure, just sitting on pavement. We don't want it to slide or tip over. Using ASCE equation 13.3-1, I calculate a seismic lateral force of 1.2*Wp.
The crux of my question is, should I factor in the friction between the bearing pads and surface when calculating the force in any anchor tie down brackets for the base? Let's say I estimate a friction coefficient of 0.5. Then, seismic force = 1.2Wp, and friction force = 0.5Wp. Will the anchors take a load of 1.2Wp or 0.7Wp? There will definitely be friction, I'm just wondering if it is something usually calculated, or just ignored, as a sort of additional safety factor.
Let me know if my reasoning makes sense, and if anybody has thoughts on this type of situation. As always, the help here is much appreciated!
I have attached a sketch to illustrate the scenario I'm thinking of. Let's say I have a rigid box type structure, say 10'x10'x10'. It rests on a bearing pad at each corner. It is not really part of any building structure, just sitting on pavement. We don't want it to slide or tip over. Using ASCE equation 13.3-1, I calculate a seismic lateral force of 1.2*Wp.
The crux of my question is, should I factor in the friction between the bearing pads and surface when calculating the force in any anchor tie down brackets for the base? Let's say I estimate a friction coefficient of 0.5. Then, seismic force = 1.2Wp, and friction force = 0.5Wp. Will the anchors take a load of 1.2Wp or 0.7Wp? There will definitely be friction, I'm just wondering if it is something usually calculated, or just ignored, as a sort of additional safety factor.
Let me know if my reasoning makes sense, and if anybody has thoughts on this type of situation. As always, the help here is much appreciated!






RE: Friction Contribution to Lateral Force Resistance
The only time I've used friction (outside of elements touching soil) is when I was doing some design for temporary stadium seating for an event and they didn't want to use a ton of anchors in their clean slab. The AHJ was making a fuss over an earthquake happening during a 1-day event so they needed to find a way to prevent lateral slip without turning the slab into swiss cheese.
RE: Friction Contribution to Lateral Force Resistance
RE: Friction Contribution to Lateral Force Resistance
As a follow up question, when you're calculating the component seismic force, what do you use for z/h when you have an object like this box, or temporary bleachers, or anything sitting on a ground surface not really connected to a building. I had just been using z/h = 1, but thinking about it, z=0 makes more sense I think. It seems like the z/h = 1 is to account for the amplified motion at the top of a building during shaking. If I can use z=0, it would cut my seismic load down to the minimum required instead of 1.2Wp, which would be easier to design for.
Yes, I agree @atrizzy. The overturning is a matter of the height and balancing the moments between the weight and seismic. No moment arm for friction.
RE: Friction Contribution to Lateral Force Resistance
RE: Friction Contribution to Lateral Force Resistance
RE: Friction Contribution to Lateral Force Resistance
From your description, it seems like you should be using Chapter 15 to design the anchorage, not Chapter 13.
Chapter 13 is for "nonstructural components that are permanently attached to structures".
RE: Friction Contribution to Lateral Force Resistance
RE: Friction Contribution to Lateral Force Resistance
Ah, thanks for pointing that out Deker. That helps clear up the friction question. And it does seem like z=0 is correct also, based on my above paragraph.
RE: Friction Contribution to Lateral Force Resistance
Which is generally the rub (heh) with trying to use friction resistance - it's hard to establish a reasonable coefficient, which forces you to be so conservative to the point where it's not worth including them, even if the code doesn't specifically preclude it.
RE: Friction Contribution to Lateral Force Resistance
Thanks for all the quick advice. Just talking through it was very helpful.
RE: Friction Contribution to Lateral Force Resistance
Effective friction simplified: μ*(0.6-0.7*0.2*SDS)*Wp
You already realized it, but it does little. Sometimes it can help though, especially if seismic is small and you don't want anchors.
Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant
RE: Friction Contribution to Lateral Force Resistance