Objectives:
Apply the simulation package, Working Model, to determine the design loads
on an existing truss and then design a modified bridge which can support twice the
original bridge design loads.
Instructional Objectives:
Develop Working Model tools including pin joints, rods, meters, applied forces,
vector display.
Background:
Truss structures are commonly used in a wide variety of applications. A truss
is a rigid structure formed by slender structural members joined at their ends by
bolts, pins, riveted connections or weld joints. A truss structure can carry very
large loads relative to its small structural weight. Examples of truss structures
include bridges, derricks, radio and TV towers, roof structures, highway signs, roller
coasters, building structures and electric power transmission towers.
Analysis of truss structures is a traditional topic in Statics courses. The member forces are determined by the method of joints or the method of sections. While these techniques provide a systematic approach for hand calculations, the design of a complex truss involving the evaluation of alternate configurations requires the use of computer tools. Working Model is one tool which can be used to quickly determine member forces in a variety of truss structures. In this exercise, we will analyze an existing bridge and then will explore modifications required to increase the load capacity of the bridge.
Assignment:
The Edlersburg-Lousville Bridge (Figure 1) in Carroll County, Maryland consists of
two spans of length 216 ft and height 27 ft (Ref. 1). Your assignment consists of a two
part analysis. In part A, you are "reverse engineer" one of these trusses to determine
the maximum uniform distributed load which can safely be applied along the top of the
truss. Assume that each member can support no more than 1,000 kip in tension or
compression and that the original design used a factor of safety of two. In other words,
determine the maximum uniform distributed load (in kip/ft) for which none of the member
loads exceed 500 kip. Recall that in analyzing trusses, distributed loads are converted
to concentrated loads applied at the joints by dividing the net force (distributed load
times member length) applied to a member by two and applying that force to each joint.
In Part B, you are to design a new truss structure of length 216 ft and height no more than 40 ft. Your new truss must be able to support twice the distributed load as the Eldersburg-Louisville bridge with the same factor of safety (2).
To be turned in:
References
1.
Figure 1. Eldersburg-Louisville Bridge (Ref. 1)