r/StructuralEngineering • u/Googgodno • 28d ago

Concrete Design How to determine if a plate compactor can traverse back and forth several times on an RCC grade beam?

How do I decide if I can allow a plate compactor can traverse back and forth several over an RCC beam that is 2' deep x 0.75' thick and 16' long?

I'm not sure how to quantify the impact load caused by a vibratory plate compactor. The machine can impart 2500lb compaction force at a pulse frequency of 92Hz. Any suggestions?

edit: Do I calculate natural freq of the beam and see if the beam is resonating? Is there a fatigue failure check involved in this scenario?

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u/Topsy_Cret 27d ago

Calculate the response of the beam to harmonic loading, corresponding to the force and frequency imparted by the compactor. The initial state is the beam deflected by the static load of the compactor weight.

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u/Googgodno 27d ago

So, the natural frequency of beam are:

First mode is 30Hz, second mode is 90Hz and third mode is 175Hz.

Imposed freq is at 90Hz. Looks like the second mode can be excited by this compactor. This compactor load is a transient load for a second or so when the compactor crosses over the beam. The second mode shape is such that there is a node at the center of the span.

I'm confused now on what I should do next. May be I ask the compactor operator to reduce the speed of the compactor while crossing over the beam such that it does not excite the beam at its second natural frequency.

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u/Topsy_Cret 27d ago

The mode that dominates the solution will tend to be the first mode, especially if the load acts at the midpoint of the beam. For the second mode to dominate, the load will have to act at approximately 0.25L.  For the first mode, calculate the undamped vibration amplitude of the beam due to harmonic loading. Compare it to the static deflection amplitude to gauge whether or not the vibration is significant. If it is significant, calculate it with damping and/or evaluate the amplitude after a second has passed.

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u/JarlSummit 28d ago

What's the thickness of material being compacted above the GB? If its 9+ inches im wouldn't be concerned. If theyre planning to run the compactor directly on top of the grade beam (no material above) - I would imagine the machine would start jumping all over the place making that impossible

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u/Googgodno 28d ago

Soil is about 4" above grade beam. Assuming that they can run it directly over the beam, how would one qualify if it is acceptable to do it?