Holmenbrua: Analyzing the carrying capacity
HBM’s measurements show: The carrying capacity of the Holmenbrua bridge in Norway is weakened
Increased traffic volume, increased challenges
The load that today’s traffic inflicts on Holmenbrua in Drammen is much higher than what was originally predicted when the bridge was built in the early 1960’s. Due to a significant increase in traffic volume, examinations of the bridge’s construction was initiated to determine if it could withstand the burden it was exposed to or whether improvements were needed.
To analyze the impact the increased traffic volume had on Holmenbrua’s construction, HBM was assigned by Rambøll Norge to conduct the necessary measurements on a given area of the underside of the bridge using strain gauges. The strain gauges were attached to five longitudinal steel rods, or rebars, to log tension and compression in the material by the passing of a truck weighing 50 ton. For full exposure of the rebars and to get the best utilization of the strain gauges possible, a part of the concrete around the rebars was removed.
“It was essential that the surface of material where we attached the strain gauges was treated” says Herman Lingefelt, project engineer at HBM. “We do this to make sure the strain gauges provide the correct measurements. Even wrong coarseness on the surface can result in the glue not transmitting the strain of steel correctly. In this case, we had to grind away the rust and then brush with sandpaper. It may seem simple, but it requires a lot of practical know-how.”
The strain gauges were adjusted to the environment in which they were to operate in, taking into consideration temperature variations in the area. This was so that they would follow the natural expansion and compression in the material caused by temperature. Because Holmenbrua is located in a moist environment, HBM also used a coating over the measuring points to prevent the gauges from losing their grip due to rust or short circuits in the steel.
Weakened carrying capacity
The results of the measurements were consistent with the deflection the bridge experienced and, based on the measurements made by HBM, Rambøll concluded that the bridge’s capacity was so weakened, that reduced weight limits were recommended.
“Measurements like these are very important to uncover security implications that increased load may have on the material in a construction, and shows how important it is to use reliable measurement techniques. Exact knowledge of the bridge's condition is essential for municipal and transport managers,” says Bjarne Hauge of HBM Norway.
Rambøll is satisfied with HBM’s cooperation: ”Using one supplier to conduct the measurements was efficient and saved us time. The work HBM did on Holmenbrua was conducted in a precise and accurate way, and we are very pleased with our cooperation”, says Jon Halden at Rambøll Norge.