The Breakroom > The Water Cooler
Building a Bridge
vtsteam:
Dave would pick up a bucket load and swivel around across the bridge. He has a longer than stock dipper arm, and his excavator is heavier than stock for this size, as there is also a counterweight added to offset the longer arm. He also has a thumb and other additions. He says the weight is about 35,000 lbs.
vtsteam:
You can see the thumb here, as he is backfilling behind the lower blocks on the south side.
vtsteam:
Finally it was time to edge out onto the bridge. This went very slowly and carefully. I set a piece of 1/2" rebar into the stream as a vertical guide to check bridge deflection. A piece of orange tape marked the bridge resting position -- against a piece of wood that extended out. We dubbed the Bridge-O-Meter.
vtsteam:
Amazingly (and gratifyingly) the Bridge-O-Meter barely registered his full excavators weight. We had maybe 1/16" deflection with the excavator sitting at mid span. It was hard to see at all. I had earlier calculated that a 40,000 point load, at center span, would deflect .25" on 4 beams of this section of A-992 steel.
The excavator however isn't a point load -- tracks spread it out, and the 9" of solid wood does the same, as well as adding some stiffness, probably. So deflection was minimal. We were really glad to see this, and it meant we had confidence to bring the dump truck over.
Heres the Bridge-O-Meter with Dave mid span. The deflection is not even visible.
vtsteam:
With the lower blocks sufficiently supported with back fill, it was time to place the north side's upper abutment blocks.
Moving these 4800 lb units by driving the tracks, rather than by rotating did cause momentary deflections of up to 1/8" on the meter, but again, surprisingly little compared to our expectations.
The greatest deflection I saw all day was one 1/4" bounce, when Dave was sitting mid span, and tamping down the gravel hard enough to lift the front tracks 6" with a fully extended arm, and then dropping back down.
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