Design of concrete foundation for impact load?

[structuresguy]

HI all.

I am looking at the design of a concrete foundation to resist a sudden "impact" load from a cable connected to the concrete. I have the load and decceleration values, so I can calculate the force applied to the concrete.

My question is is it conservative to assume a static load application of the calculated force onto the concrete foundation? the event is dynamic, but not recurring, or reversing. Once the load has peaked, it dies off very quickly to zero. The entire event lasts about 1-2 seconds maximum. Doing an assumed static analysis will obviously be much easier than some form of dynamic analysis. Oh, I forgot, this is not part of a structure, so I have no other interactions other than soil response.


Thanks.

 

[blake989]

no, it is NOT conservative to use a static load. You must calculate an equivalent static loading from the dynamic load. All I remember off the top of my head is the cable is like a spring, with a calculated spring constant omega, just picture your old dynamics class with the free body of a weight attached to the spring that is attached to a solid structure. I will think some more and let you know if I remember some details for you.

 

[structuresguy]

Let me add some more info. The force is applied thru one large cable which is connected to a steel clevis. THe clevis is welded to a steel plate, which I intend to anchor to the concrete foundation. My concern is designing the anchorage so that the clevis plate remains connected to the block of concrete.

The load I would apply would be an equivalent static load calculated from the dynamic loading. I know the mass, velocity and decceleration, so I can calculate the maximum force.

I understand your concern about the sping action of the cable. However, in this case the length of cable is short. I don't feel that spring action would have that much effect in this situation. Though I will get out some of my old dynamics texts, if I can find them, and give myself a quick refresher on the subject.

 

[sgdon]

Also, in stuctural design, engineers always like to use a factor of two to take account of dynamic effect whenever there is an impact load . I wonder if this is conservative here. I would however need to point out that the aplication of the dynamic factors require the consideration of the whole system in place.

 

[blake989]

O.K., I remember some stuff. Peq=((A*E)/L)*Deflection max.

Deflection max= deflection static + deflection dynamic.

Dynamic deflection is obtained from the block with a spring attached to it (from the old dynamics class)and your values.

This is not the only method to do this. Some years back we had to look into a precast panel on a parking garage that an elderly man had rammed with a really big luxury car. I used the method above and my co-worker checked it with some kind of work dissapation method. My value was a load about 5*the weight of the car. With his method it was somewhere about 3.5*weight of car. We used his method.

I am curious, are you doing a bridge retrofit. I have seen cities beginning to strap precast stringers on bridges together over their supports since the Oklahoma barge accident.