Stay-in-Place Formwork
Stay-in-place formwork can consist of concrete or steel components. Partial-depth precast concrete panels are structural components that also act as stay-in-place concrete formwork. The partial-depth precast panels become an integral part of the full-depth deck slab. When concrete formwork is specified by the bridge designer, the designer is typically responsible for the design of the formwork as it is made composite with the bridge system. Article 9.7.4 of the AASHTO LRFD (7th Edition, 2014) specifies the design requirements of partial-depth concrete deck panels. Partial-depth precast concrete panels are discussed earlier in this Deck Design module, and are not elaborated further herein.
Steel stay-in-place forms are not to be considered composite with the concrete slab, per Article 9.7.4.2. Figure 5 shows a photo of stay-in-place formwork installed on a steel I-girder bridge, prior to the placement of the deck slab steel reinforcement. The bridge designer is typically not responsible for the design of the steel stay-in-place formwork, which is instead the responsibility of the contractor or steel stay-in-place formwork provider. However, the designer must consider the additional weight caused by the use of steel stay-in-place formwork in the design of the bridge. A customary allowance of 0.015 ksf is often used by designers, which accounts for the weight of the steel formwork and the additional concrete in the “valleys” of the stay-in-place forms. The weight of formwork is typically provided in the contract documents, and if the allowance is exceeded by the contractor’s choice, the contractor is responsible for showing that the effects on the bridge are acceptable, or providing additional strengthening as needed at no cost to the Owner. Additionally, the steel stay-in-place formwork should not be welded to the top flanges of the girders subject to tensile stresses. Welding is permissible in areas where the top flange is always in compression. The stay-in-place steel deck forms are often attached to an angle, which is attached to the girder top flange by welding or via mechanical attachment, such as a hanger or strap over the top flange of the girder. In regions where the top flange is subject to tension, welding the stay-in-place forms, or their supporting components, directly to the girder can result in a fatigue detail not considered during the design of the steel superstructure. Therefore, in order to facilitate installation of steel stay-in-place forms, the designer should indicate the top flange tension and compression zones on the contract plans (this may be done by showing the CVN testing limits as well.)
