4.9. SR 10-19-1998 - Laserfiche WebLink

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City of Elk River October 14, 1998 Page 2 of the bridge is placed at the bottom of the beams and the beams themselves form the handrail of the bridge. The fencing is then installed on top of the beams. Through truss: A lighter weight bridge can be constructed if a truss is used. In a through truss, the sides and roof of the bridge are used as structural elements of the truss. Effectively, the structure of the bridge surrounds the walkway. Fencing is attached to the interior of the truss. Pony truss: A pony truss is a combination of a through girder and a through truss bridge. Piers, the middle support columns, can vary widely in appearance, and are an important part of the aesthetics of the bridge. The video shows examples of V, open V, T, Y, column and cap, tapered and simulated stone piers. The Pony Truss and Through Truss designs are able to make use of canned bridge quotations. This means that a bridge supplier would furnish specifications and costs based on the required length and height. The pier and abutments would be designed to match the bridge being supplied. This can slightly reduce the design cost. MAINTENANCE Long term maintenance requirements are another element to consider. Steel bridges generally require more maintenance than concrete or wood types. They require painting every 10 - 15 years. The video does show one type of Iow maintenance steel bridge. It uses CorTen steel that is made to rust evenly on the surface to present the rust color and not need painting. While concrete and wood bridges both generally require Iow maintenance, there is a difference in their life spans. Wood bridges can be expected to last 25 to 30 years while concrete and steel structures will last in excess of 50 years. Due to the shorter life span of wood bridges and higher maintenance of painted steel, we recommend the selection of either a concrete or CorTen steel bridge. OTHER DESIGN ELEMENTS Mn/DOT requires bridges over roads to have a fence to prevent items from being dropped from the bridge. The least expensive and most basic approach is to use standard chain link fence. Chain link can also be coated with PVC, which comes in black, green or brown colors. This is a little more expensive, but generally looks nicer. Another option is to construct a wrought iron fence. This will only work with certain bridge designs and is the most expensive of the three fencing options. Lighting is not required by Mn/DOT, but may be desired as it provides an element of safety. There are three main methods used to light a pedestrian bridge. One way is to string lights along the bridge fencing. This would involve a number of Iow power lights. A Howard R, Green Company O:\PROJ\803450j\0060\450-0902.oct.doc CONSULTING ENGINEERS