The Breakroom > The Water Cooler
Building a Bridge
vtsteam:
Well I wouldn't generalize about a generation, for fear of getting generalized as a geriatric curmudgeon myself, but this kid is in line for some life lessons, I'm sure. Not at my hands -- I just want to get this job done. If he can't work, no problem, send someone else.
Anyway, we just made it in time when the concrete truck arrived. Twelve blocks poured in two days. I may work alone stripping the forms over the weekend. since it will rain Monday, and it will be a lot harder then, even with help, slipping in the mud, etc. fun. Woohoo. Not.
awemawson:
What was the issue with the original blocks that you had Steve?
Your picture reminds me of bits of our South Coast when we were preparing to thwart an invasion from across the channel in the early 1940's. Indeed Cuckmere Haven still has the anti tank blocks in place.
vtsteam:
Andrew, they were tapered (pie shaped) -- apparently intended for curved wall landscape use. Front face was 48" wide, rear face was 31" wide. Also they were 18" tall, instead of the plan 24" tall. They weighed 2600 lbs instead of the rectangular block's 4800 lbs.
1.) If laid straight, the front faces abutted, but the embankment faces were spaced apart, the mfr's intention being that 3/4 inch stone would fill the gaps. Fine for static landscape use, but a bridge abutment has moving truck loads over, and tire impact loads, not to mention trees banging into them during a flood. I thought they'd shift too easily.
2.) They were only 42" deep - original design called for 48" deep blocks (4'x4'x2). This difference is particularly critical at the top course, where the steel bridge beams overlap only 1 foot onto the block. Loads could tip the top blocks up a little (or a lot) at each impact in my opinion. Dirt and gravel would enter at the back and further wedge the blocks up
Also affecting tipping resistance: they were less deep, didn't abut each other, thinner (18" vs 24") and weighed about half what he design called for. The centers of gravity of the blocks (and therefore resisting moment) was also further forward toward the wider front face. To me, if you multiply three or four reduction factors by each other, you're left with a very small percentage of the resistance to tipping originally intended.
I was talked into these blocks (when the excavator couldn't find 4x4x2 blocks locally) because they do have a couple of small tenons at the back that fit into mortise grooves, but no one mentioned that they were tapered wedge shaped overall with intended gravel infill. When I saw them going together, and asked where the straight blocks were (I thought the tapered ones were only for the end blocks, laid tight) I was told abut the gravel infill, and a two day "discussion" ensued. :bugeye: Eventually, reason won. :hammer:
vtsteam:
I should add that the system might have been better if the mfr also offered straight blocks. I was confused by the fact that they didn't, but intended tapered blocks for even straight wall sections.
Then I realized the advantage to the mfr. These blocks sell for premium prices (they have a decorative face, and can curve).
But the cost to cast them is reduced because they weigh less -- and cost is directly related to concrete usage. Thus the maker can charge twice as much and use half the material for each. :smart: Not a mystery after all.
awemawson:
Sounds logical, as I would expect from you Steve !
How long before you can pick them up with the cast in loops? Big load and a little bit of green concrete.
Talking of loads - are you casting a pad for the beams to sit on to spread the load?
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