Bridge construction, steel construction: Rollers at east/ launch end acting on bottom chord/ structural element of Mabey steel truss panel temporary bridge over the Merrimack River on Route 113. Most obvious are the four lateral rollers on vertical spindles that keep bottom chord in alignment above the primary rollers. The primary rollers, the tops of which can just be seen to the right of the lateral rollers, on horizontal spindles/axles actually support the truss panels. They rotate when bridge frame is pushed. A bolt, as shown at far left mid-height of photo, is tightened against each primary roller at all other times to lock the roller and prevent the frame from moving. Tyngsborough, Massachusetts 2005.

Bridge construction, steel construction: Rollers at east/ launch end acting on bottom chord/ structural element of Mabey steel truss panel temporary bridge over the Merrimack River on Route 113. Most obvious are the four lateral rollers on vertical spindles that keep bottom chord in alignment above the primary rollers. The primary rollers, the tops of which can just be seen to the right of the lateral rollers, on horizontal spindles/axles actually support the truss panels. They rotate when bridge frame is pushed. A bolt, as shown at far left mid-height of photo, is tightened against each primary roller at all other times to lock the roller and prevent the frame from moving. Tyngsborough, Massachusetts 2005.