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Engineer says speed bumps damaging bridge
David Taylor doesn’t like the speed bumps that flank the island’s three bridges. Further, Taylor – a Boca Grande resident who holds bachelor’s and master’s degrees in engineering and has worked in the field for more than 30 years – says the bumps are doing more harm than good.
In a guest commentary, Taylor contends the “20 mph speed bumps” installed on the approaches to the bridges are, in fact, 5 mph speed bumps that create stress on the bridges as vehicles decelerate and accelerate prior to and after crossing.
These forces, contained in Newton’s Second Law of Motion, combined with the weight of several heavy vehicles on the bridges at the same time, increase static loading, he says.
This translates into more, not less strain on the bridges.
The speed bumps cause further damage when heavy trucks him them at higher rates of speed.
“When the truck hits the speed bump at say 25 mph, it sounds and feels like an earthquake in the surrounding area,” he says.
The vibrations are transmitted through the roadbed onto the bridge.
“GIBA is essentially replicating a localized earthquake multiple times per day. This cannot be good for the bridge.”
He also notes a Swiss study that shows the dynamic impact of heavy vehicles is larger at lower speeds.
Here is the full text of his commentary:
Several months ago GIBA installed speed bumps on the south and middle bridges. Prior to the speed bump installation, the vehicles would simply drive across the bridge with a minimum of acceleration, deceleration and vibration.
Since the installation, one of the two following scenarios develops:
1. The truck comes to a virtual stop at the first speed bump. It slowly crosses the speed bump. It then accelerates to the middle of the bridge, then begins to decelerate to the second speed bump.It then comes to a virtual stop at the second speed bump, crosses it and accelerates on its way.
Of course during this start/stop crossing, other vehicles come right up to its bumper. This means several vehicles are on the bridge at the same time, greatly increasing the static loading.
Additionally, for those of you who recall high school physics, F=MA. (Editor’s note: A body of mass subject to a force undergoes an acceleration that has the same direction as the force and a magnitude that is directly proportional to the force and inversely proportional to the mass. Also known as Newton’s Second Law of Motion).
All of this acceleration and deceleration generates additional forces on the bridge. Of even more concern, concrete is very strong in compression. It is not strong in shear and, unfortunately, that is the direction of these additional forces.
2. In this scenario the truck hits the speed bump at a relatively high rate of speed.
(Does anyone think these are truly 20 mph speed bumps? I have tried them at different speeds and believe they are 5 mph bumps. The signs on the bridges are deceptive. I recommend you try these bumps at the signed 20 mph, but wear a helmet!)When the truck hits the speed bump at say 25 mph, it sounds and feels like an earthquake in the surrounding area. I have been standing more than 100 feet away and felt the ground shake from this loading. Granted, the speed bumps are actually on bridge aprons and not actually on the span.
However, this vibration is being transmitted through the roadbed into the bridge.
GIBA is essentially replicating a localized earthquake multiple times per day. This cannot be good for the bridge.I have done a limited amount of research and found a paper published by the Swiss for testing their bridges. (A link to the paper is online at BocaBeacon.com.) In this paper the authors simulate dynamic bridge loading by placing a plank on the bridge span to induce vibration.
Sound familiar?
Their conclusion: “For a deteriorated pavement, the dynamic impact of heavy vehicles is shown to be larger at lower truck speeds.”
GIBA’s approach has simulated a very deteriorated pavement and reduced the vehicle speed, putting the vibrations closer to the likely natural frequency of the bridge. While the speed bumps are only on the bridge apron, the shock and vibration is certainly entering the bridge structure.
In summary if a truck slows down and causes traffic to build up behind it, it increases the static loading. If it maintains a relatively high rate of speed, it causes a minor earthquake and increases the dynamic loading. In either scenario the speed bumps reduce the lifespan of the bridge.
In managing companies, it has been my experience that attention must be paid to finances large and small. If you worry only about the large expenditures, the smaller ones in aggregate will eat you up.
The amount I recall for the expenditures for the speed bumps was $16,000 or $18,000.
Now, that is not a trivial sum by anyone’s standards. If my analysis is right, they spent this money to actually reduce the lifespan of the bridges.
However, for purposes of argument, assume their approach is correct and that bridge wear is proportional to vehicle speed, something the Swiss paper disputes. Is there a lower cost approach to reducing vehicle speed?
The answer is of course, yes!
How about this approach? Put up speed limit signs and request Charlotte County enforce them?
Since Boca Grande North is obviously a residential area, a 25 mph speed limit is appropriate for safety reasons alone. A Charlotte County sheriff’s car two or three times per week and a few inexpensive signs will reduce the costs substantially.
And, improve the sheriff’s office budget at the same time! All without irritating island residents as much as the speed bumps have.
GIBA has stated the south bridge is in dire need of replacement. I suspect that is true.They have also stated the swing bridge and the middle bridge have life remaining. They have further stated we don’t have the money to replace all three without huge toll increases and without going deeply into debt.
If that is the case, why are they investigating the replacement of all three bridges simultaneously?
It would seem to me that the proper approach is to replace the bridge currently in need of replacement and replace the remaining two bridges as their lifespan comes to an end.Editor’s note: David Taylor has a master of science degree in electrical engineering. He has a number of patents for mechanical and electronic devices, and has been a business owner for more than 30 years. He is a Boca Grande resident.
2 Responses for “Engineer says speed bumps damaging bridge”
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I agree with Mr. Taylor 100 %. The placement of the speed bumps has been a concern of mine from the time they were put in place. I feel my opinion as a daily heavy hauler to Boca for almost 29 years, might pull some weight.
I have witnessed an increase in the number of cars on each bridge at one time.I know for a fact through personal experience, the breaking alone is tremendously stressful when you multiply this formula times the number of vehicles behind my heavy hauler braking simultaneously.
Lets also figure the weight transfer is not only laterally increased but now vertically multiplied by the bounce of the vehicle traversing these Bumps of Destruction. I can hear the peanut gallery now, it is the speed of the vehicles that is our concern.Well, I must say that our drivers have not been written a speeding ticket yet on the Island or causeway in our 29 year history.
It will be just a matter of time that the road will be pounded down into depression on each speed bump as evidence of the concrete that has already replaced the asphalt under the first set at the swing bridge.
How much money does it take to fix Stupidity. Answer none if the voters vote.Time to vote for a change. P.S. I wonder if we would have a new bridge by now, if the county had taken it back before GIBA did. Kevin
I have a master of science degree in mechanical engineering and am a candidate for position 5 in the upcoming GIBA election. I think David Taylor has it exactly right on most of the points in his recent comments on speed bumps. I would like to make a few additional comments along the same lines. They are technical, so please bear with me.
As Mr. Taylor points out, vehicles going over a bridge transmit two general types of forces into the bridge structure; namely, static and dynamic. The static force is simply the weight of the vehicle, while dynamic forces come from motion of the vehicles, and can be of several varieties. Total load on the bridge is the sum of these two forces.
The simplest way to think about dynamic forces is to look at what happens to a vehicles suspension under various conditions. As a car enters the inclined section at the beginning of the bridge, its suspension is compressed and the force required to compress the suspension is transferred into the bridge. As the car reaches the center of the bridge and starts down the other side, the suspesion actually unloads and reduces the load on the bridge. Imagine going over the bridge at 60 MPH – most cars would “bottom out” on the incline, and might “get air” as the car passes over the center and onto the decline. Analytically, what we are experiencing here is vertical acceleration, and the F=MA formula can predict the force which compresses the springs in the car’s suspension, and the loading on the bridge.
Another dynamic force is created by speeding up or braking of the vehicle(lateral acceleration/deceleration), and again, think about the suspension system in your car. During qiuck acceleration, the rear suspension is compressed, while under heavy braking, the front of the car “dives” as the front suspension is compressed. The forces which compress the suspension are transmitted into the roadway, or in this case, the bridge.
Of course there are other dynamic forces created by joints in the bridge span, pot holes, and vibration, including harmonics. Once again, your car’s suspension gives a hint at what forces are going into the bridge.
Now, the key question is how do the current speed bumps affect these static and dynamic forces, and are we helping or hurting ourselves as far as bridge deterioration. The first observation is that the static force of a vehicle is constant. However, as Mr. Taylor points out, the number of vehicles on the bridge is increased substantially by the second(exiting) speed bump, which creates bumber to bumber traffic on the bridge span. So the speed bumps definitely increase the static load on the bridges by several fold. They also create the lateral acceleration (i.e., speeding up/braking) loads mentioned above, which are not present when a vehicle travels over the bridge at a constant speed.
The vertical acceleration loads from entering the bridge incline, as well as span joint and pothole forces, are definitely greater the higher the speed of the vehicle. Consequently, the first(entering) speed bump reduces these forces by reducing the speed at which a vehicle enters the bridge incline.
A proper analysis would have to balance the increased static and lateral acceleration loads against the decreased loads from reduced vertical acceleration. GIBA asserts that their engineering consultants have recommended the speed bumps, but I wonder if they have actually done an analysis of our bridges, including bumber to bumber car spacing on the spans. My suspicion is that their recommendation may have been a general conclusion that lower speeds create less vertical acceleration. My engineering intuition is that the increased static loading creates more bridge load than the decrease we achieve in dynamic loads by lowereing the vehicle speed with speed bumps.
Another observation is that the second(exit) speed bump clearly is hurting, not helping, our objective. The first bump reduces speed entering the bridge, which does reduce the vertical acceleration and loading on the incline section. The second bump creates the bumper to bumper traffic on the bridge span as well as the deceleration loads during braking, both undesireable outcomes. Without the exit bump, vehicles would accelerate, create normal spacing from the trailing vehicle, and exit the bridge smoothly without braking loads. This should be done immediately. A thorough review of our engineering consultants report would also be in order, concerning the balancing of pros and cons for the first bump.
A final observation is that David Taylor should be on the GIBA board. His thoughts on cost control, phased replacement of the bridges, and the desireability of limiting our debt to manageable levels, are all right on.