Have you ever been stopped in your vehicle on a bridge or overpass while a large truck or bus passes by? If so, you probably felt some movement beneath you. That’s because the composite structure of steel, concrete and asphalt that the bridge or overpass was constructed from was flexing! It’s hard to imagine tons of concrete and steel flexing, but nothing is 100% rigid.
When engineers discuss the relative rigidity of a solid material, they call it the modulus of elasticity or Young’s modulus. Different materials have different properties, including their modulus of elasticity. When a force is applied to the material, it will deform. If the resultant stress is within the elastic range of the material, it will elongate or compress, then return to its original state. If the range of elasticity is exceeded, the yield point will be reached and the result is plastic deformation, meaning the material will not return to its original form. If the stress is high enough, ultimate tensile strength can be exceeded and will result in a sudden breakage and release of stored elastic energy.
I told you all this so I can describe an unexpected finding yesterday. I had our coach washed by a local service, David’s RV Wash, that uses deionized water. I wanted the roof thoroughly cleaned. When David climbed onto the roof, he called me over and said I should see something. I climbed up the ladder and was shocked by what he showed me. Our one-piece fiberglass roof was deformed. It had raised ridges in the fiberglass over several of the supporting roof bows. One of the ridges had a fine crack along the top.
Fiberglass roof deformation over roof bow
It looked like the roof support bows had risen up into the fiberglass or the roof itself had sunk into the bows. I’ve been up on the roof of our coach many times and never saw this before. The last time I was on the roof was in July, about five months ago.
I tried to imagine what could have caused this damage since July. After giving the matter much thought, I have a theory. When we were at the Row River over Labor Day weekend, I had a mishap when I was trying to level the coach. This mishap was the result of our leveling jacks working erratically – something I’ve fixed since then.
Here’s what happened. Our site wasn’t level – the coach was in a nose down attitude. And we were on soft ground. I stacked two blocks under each of the front jacks and one block under each of the rear jacks to keep the jacks from sinking into the soft ground. I thought stacking blocks in front would help, allowing the coach to be leveled without extending the front jacks excessively.
Due to ground issue on the HWH hydraulic system control box (that I was unaware of at the time), the auto-level function wasn’t operating properly. When I initiated the auto level, the jacks over-extended abruptly, completely lifting the coach. With the wheels off the ground, we had no parking brake. The coach slid forward, off the blocks.
When the jacks slipped off of the blocks, the coach lurched forward and down into the ground. The jacks dug into the dirt, suddenly stopping the forward and downward motion of the coach. This violent force must have created enough stress for the frame rails to flex. The upward flex of the rails was transferred to the superstructure of the coach, raising the roof bows. At the same time, the sudden stop also flexed the fiberglass roof downward due to inertia.
The contact between the fiberglass roof panel and the roof bows exceeded the elastic range of the fiberglass and it yielded. The resultant plastic deformation created the ridges in the fiberglass roof panel. Meanwhile, the high-strength steel frame and superstructure were within their modulus of elasticity and returned to their original form.
When this mishap occurred, I inspected the jacks and frame under the coach for damage and found minor damage on one jack – which I posted about here. It never occurred to me that the force would result in stress and deformation on the roof. It’s the only explanation I can come up with.
Today, I’ll go to the RV shop and buy EternaBond to repair the crack. I’ll keep an eye on the rest of the roof and hopefully no further damage will be forthcoming.