Willett, Hofmann & Associates, Inc. provided engineering services for the contractor constructing the proposed 7-span (1 @ 350’-0”; 1 @ 250’-0”; 1 @ 199’-0”; 1 @ 199’-0”; 3 @ 121’-0”) steel-plate-girder and prestressed-concrete-girder bridge carrying Route 79 over the White River in Clarendon, Arkansas. The first primary task involved design of three 60’ x 45’ cofferdams utilizing AZ 19-700 steel sheet piling and a three-level strut/waler system. The sheet piling penetrated the streambed soil approximately 40’ with an upstand above the soil of 23’ for flood flows. Willett, Hofmann & Associates performed all subsurface/geotechnical analysis and calculations for these cofferdams. The cofferdam required a 10’ seal coat thickness and a double wale system for the lower waler. The cofferdam was designed for buoyancy loads as well as soil and hydraulic loads adding up to 2,800 pounds-per-square-foot to support the ultimate cofferdam void of 60 ft. deep. Analysis was confirmed using PileBuck geotechnical models and various RISA-2D modeling scenarios.

Willett, Hofmann & Associates, Inc. also provided drawings and calculations for a comprehensive erection plan to install and splice-together all of the proposed plate girders spanning the White River. Three temporary “Girder Support System” Towers comprised of steel-wide-flange and steel-HP beams/columns and with HP piles driven into the ground were proposed/designed to support the plate girders in various cantilever and temporary positions. Layout and analysis of these towers were input into a full RISA-3D model to simulate all static, dynamic, and environmental forces. All tower and piling members and all tower connections were designed to resist temporary vertical reactions up to 380,000 pounds and lateral wind/stream reactions up to 60,000 pounds. The stresses and lateral-torsional-buckling forces imposed on plate girders during the erection plan were shown not to their allowable capacities as calculated using the AISC and AASHTO design specifications during the operating rating of the existing structure. The deflections of all splice-connection points were confirmed from the RISA-3D model as all portions and all bolts of all splices properly fit to meet the proposed cambered shape.

In addition to the superstructure and substructure analyses, Willett, Hofmann & Associates, Inc. also calculated all transport stresses of the 10’-deep plate girders before being delivered from the fabricator to the site. To ensure that no parts of the plate girder webs or flanges buckled or yielded during transport, a RISA-3D finite-element-model was developed to confirm all hand-calculations associated with the dead weight and impact forces imposed during delivery. Finally, Willett, Hofmann & Associates, Inc. provided drawings and calculations for all overhang formwork systems required to construct the proposed concrete deck and parapets.