Homework/Example Problem Structures
© 2007 T. Bartlett Quimby
The PDF drawing set consists of three, 11"x17" drawings. They print well on 8.1/2"x11" paper as well. Additionally, the AutoCAD drawing file and a 3D AutoCAD model are available. The 3D model is a simple centerline model.
Figure Tower-1 is an 3D view of the tower structure that will be designed in the homework problems.
This tower design has been inspired by two free standing communications towers in the Anchorage, Alaska Area. One is a three legged tower that was recently erected on Debar Road near the intersection with Boniface Parkway. The second tower has four legs and overlooks the Eagle River area from the top of Skyline Drive. The tower design presented here is a three legged tower (which is much easier to deal with in many ways) with the possibility of connections from either tower. The drawings show the single angle bracing from the Debar Rd tower with alternates similar to those found on the Eagle River Tower. Various alternatives are explored in the homework problems.
The design tower is three legged tower that is 60 ft tall.
Each leg is fabricated in three segments of approximately 20 ft each. The legs carry the majority of the gravity loads and resist overturning under the effects of lateral loads. The legs are subject to both compression and tension.
The braces provide lateral support of the legs, reducing the effective Euler lengths of the legs, as well as being effective in carrying the lateral forces.
The structure is actually very simplistic. One section can be used for the entire length of each leg or it may change for each of the three segments of each leg. The braces can be designed to be "tension only" and be designed for the worst case loading so that one member fits all. Alternatively, the braces can be designed for both their tension and compression loads, in which case the compression load case will probably control the design of the braces. Again, the braces can be designed for the worst case so that all are the same size, or you can design for several different load ranges.
Note that it is desirable to limit the number of different sizes in a structure. Limiting the number of members can greatly reduce the cost of fabrication and the cost of installation errors in the field. As a result, it is common practice to pick one size of steel member for a group of similarly loaded structural elements within a structure.
The following are images from the two towers. These can be used to understand the drawing details.