Acrow bridge applications range from long term and permanent solutions to detours, temporary and emergency replacement. As with many Acrow applications, the length of time of a temporary installation varies dramatically; the Division Street Bridge is a prime example. Despite rehabilitation projects in the past, the bridge no longer functioned mechanically. As a result, the plan to replace the bridge was sped up by a year and moved to an emergency project which required a temporary replacement solution. While construction windows are typically small, the Division Street project had a timeline that few could realistically achieve. Acrow worked with CDOT and designed and engineered a bridge and installation solution to meet the complex site constraints and camber requirements within the emergency timeframe. The interim Acrow Bridge was designed for two lanes of commuter traffic with two bike lanes and cantilevered 5 foot pedestrian bridges off both sides to provide adequate space for expected foot traffic. It was installed in just 37 days and allowed CDOT to complete the emergency project earlier than anticipated. The general contractor, FH Paschen was awarded Contractor of the Year for 2014 by Chicago Department.
The interim Acrow Bridge is expected to be in place for 4-5 years before the replacement bridge is erected.
The widely recognized engineering formula, T= K x D x P (to be explained later in this article), was used to provide the chart's values, but it must be understood that every bolted joint is unique and the optimum tightening torque should be determined for each application by careful experimentation. A properly tightened bolt is one that is stretched such that it acts like a very ridged spring pulling mating surfaces together. The rotation of a bolt (torque) at some point causes it to stretch (tension). Several factors affect how much tension occurs when a given amount of tightening torque is applied. The first factor is the bolt's diameter. It takes more force to tighten a 3/4-10 bolt than to tighten a 318-16 bolt because it is larger in diameter. The second factor is the bolt's grade. It takes more force to stretch an SAE Grade 8 bolt than it does to stretch an SAE Grade 5 bolt because of the greater material strength. The third factor is the coefficient of friction, frequently referred to as the "nut factor." The value of this factor indicates that harder, smoother, and/or slicker bolting surfaces, such as threads and bearing surfaces, require less rotational force (torque) to stretch (tension) a bolt than do softer, rougher, and stickier surfaces. The basic formula T = K x D x P stated earlier takes these factors into account and provides users with a starting point for establishing an initial target tightening torque.