Monday, June 09, 2008
Leave the shaking to the bridge
Tom Angleberger
The New River Valley-based reporter answers your questions Mondays in his column, What's on Your Mind?
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Q: I drive to and from work each day on Elm Avenue. I've noticed that when I'm sitting at a stoplight on the bridge over Interstate 581, the bridge feels like it's shaking. Is this normal? The shaking is pretty noticeable and it feels like it's even making my car shake. After the recent bridge collapse in Minnesota, it's a very disconcerting feeling when I'm waiting for the light to change.
-- Christine Lofgren, Roanoke
A: If you're commuting over the Elm Avenue bridge during rush hour, you're probably in much greater danger of a nervous breakdown than a bridge collapse.
So, relax -- let the calming vibrations soothe you as you wait for the light to change -- think of it as a free massage.
The bridge was inspected as recently as February. And, yes, it did pass that inspection.
The short answer, direct from the Virginia Department of Transportation, is that the vibrations are "completely normal."
The long answer is that the bridge is actually made with such vibrations in mind.
"All bridges have some flexibility, and engineers design for this movement," explained spokesman Jason Bond.
If you've ever used a swinging bridge, you know that it sags in the middle. It turns out that even a concrete and metal bridge will sag, too.
"When traffic pulls onto a bridge, the weight produces more sag and the sag decreases when the traffic pulls off. Bridge engineers ensure that the bridge is designed to accommodate this movement."
They could have built the bridge so that there would be less sagging and shaking, but it would've cost the taxpayers a bundle.
"To eliminate almost all movement on the Elm Avenue bridge over I-581, engineers estimate that the bridge's steel beams would have to double in height from 70 inches to 140 inches."
Instead, they sought a "balance" between the two extremes and the result is a little wiggle.
We probably don't notice this on other bridges because we drive over them without stopping. But not so on Elm Avenue, which draws people from far and wide who just want a chance to stop and rest a little while. And maybe even enjoy a free massage.
Q: What is the difference in a regular rechargeable battery and a solar rechargeable battery? If there is no difference, can they be used interchangeably? I have Malibu brand solar lights that use solar rechargeable batteries.
-- Anne West, Botetourt
A: It appears that these solar lights are really battery-powered lights with solar-powered battery rechargers. I let one charge up for several hours, then removed the AA battery and hooked it up to a little battery-powered fan. It worked.
Most of the rechargeable batteries you see in stores these days are NiMH, or nickel metal hydride. But, the batteries that Malibu uses are NiCd, or nickel cadmium.
I asked Malibu if their lights would work with the NiMH, too.
Spokeswoman Nissy Atassi said they would work "just fine."
However, she noted that a NiMH battery is actually more battery than the lights need.
"NiMH batteries are more expensive than NiCd batteries and have more capacity than what most solar fixtures can recharge during a typical day, so the user would end up paying more for a better battery that cannot be fully utilized by the light."
But, if you've found a good deal on NiMH batteries -- or if it's just more convenient to buy them -- it looks like they've given you a green light. (Get it? It's a "green" light, because it uses solar power.)
If you've got questions, send them in to woym@roanoke.com or leave them on my voice mail at 777-6476 (please be sure to speak clearly and spell your name). I'll need your name, location and phone number or e-mail address.
Look for Tom Angleberger's column on Mondays.
