Construction of the Bridge Walnut Lane Concrete Arch Bridge, Philadelphia Pennsylvania
Design and construction of the Walnut Lane Bridge consumed three and a half years. The process began in earnest on 13 July 1905, when the Philadelphia City Council appropriated $190,000 for the city's Department of Public Works and Bureau of Surveys to construct a new bridge across the Wissahickon Creek valley. Webster, his assistant Quimby, and their staff went to work on the design. They evidently considered three schemes: a steel viaduct at the estimated cost, a concrete arch for an additional $50,000, and a stone arch of similar design for an additional $100,000. Plans for a concrete arch were presented to the Board of Surveyors and approved on 19 April 1906. The city advertised for bids and opened them on 15 May, awarding the contract to Reilly and Riddle of Roxborough for $234,000. From the notice to proceed, issued on 5 July 1906, the firm had eighteen months to finish the project. The contract was amended in November to include up to $28,000 worth of additional services, with $1,800 allocated for bush hammering the underside of the main arch. The bridge was substantially complete on 14 October 1908, at a cost of $259,440 and more than nine months behind schedule. Even then the bridge carried only pedestrian traffic, with the roadway closed awaiting final grading of the approaches. A formal opening was held on 16 December.
Due to the main arch's immense size, the design of temporary falsework involved intricate calculations by the contractors, which city engineers later approved. For the sake of economy, the contractors used the same falsework twice: once for each rib. Four temporary concrete piers were built in the stream bed to support the falsework and provide a path for moving it from one rib to the other. The piers were excavated to bedrock and equipped with holes for inserting explosives after the falsework was dismantled. The falsework consisted of steel bents 20'-0" high, followed by two tiers ofwooden trusses, 42'-0" and 54'-0" high, containing an estimated 370,000 board feet of timber. Completed on 1 April 1907 under the south rib, the temporary structure weighed an estimated 900 tons.
Timber was an appropriate material for the falsework, not simply because it was inexpensive and easy to erect, but because wood expands when wet. This property was used to great effect in constructing the arch ribs. Using an overhead cableway to deliver materials, the contractors began pouring the south rib by the end of April 1907. Soon thereafter, they began saturating the timber with water. The falsework swelled, widening the gaps or keys between the large blocks of concrete. After concrete was poured into the keys, the contractor allowed the timber to begin drying. The arch settled as the falsework shrank, compressing the keys. This pre-compression was intended to prevent shrinkage or tension cracks from appearing in the completed arch ribs. The concrete was allowed to cure from mid-June until July 22, when crews removed the wedges beneath the falsework. It took thirty men and three days to roll the falsework 34'-0" to its new position and re-insert the wedges. Concrete work on the north rib began on 16 September, again soaking and drying the timber to pre-compress the keys. Workers removed the wedges in early December and began dismantling the falsework. In order to remove the marks left by boards in the formwork, the city agreed to proceed with the proposed bush hammering. Another ten months passed before the bridge's deck was complete.
