BEFORE THE AVAILABILITY OF COMPUTERS USING SPECIALIZED ANALYSIS AND DESIGN PROGRAMS, towers were often designed by graphical methods. It was considered prudent to test new designs that would be used repeatedly on a transmission line, thereby confirming the design assumptions with a full-scale test. Today's analysis tools allow engineers to refine designs to an unprecedented degree, and as a result, many utilities feel testing is not warranted. However, while great strides have been made in the analysis and design of latticed steel transmission towers, differences between analysis results and full-scale tests still occur.

CenterPoint Energy's (CNP; Houston, Texas, U.S.) recent experience emphasizes once again the importance of experience and testing in latticed tower design.

CNP performed an economic analysis at the outset of its latest transmission project, which includes a new 49-mile (79-km) two-circuit 345-kV line. Existing latticed steel tower designs were compared with tubular steel poles and new latticed tower designs. The cost of design, detailing and testing was included in the estimate for the new tower designs. The use of Power Line Systems' suite of design and analysis software, specifically PLS-CADD and TOWER, made it possible to quickly estimate the approximate weight of new towers, as well as the weight of the modifications required to make the existing tower designs adequate for the proposed conductor and current design criteria. The material savings from using a more efficient tower design justified the cost of detailing and testing for both the tangent and five-degree structures.

Part of CNP's design philosophy includes a feature to reduce the likelihood of cascading failures. Tower arms are designed for a longitudinal load equal to 50% of the initial tension of the conductor bundle. These loads might be generated by a broken sub-conductor or a hung block during stringing operations. The tower body, however, is designed for the same loads with an increased load factor. CNP design criterion requires the “arm” fail prior to the tower body.

Full-scale testing results compared well with model data in most cases. However, as stated previously, CNP's design philosophy for failure containment requires failure of the arm prior to any failure in the tower under the longitudinal load case. During the test of the tangent tower a limited number of members were strain gaged, including the chord of the crossarm. Two keys discoveries were made during the test. First, the predicted load originally calculated and actual load in the chord varied by about 38%. Second, the displacement at the end of the arm was about 80% greater than predicted by the model.

After review of the results, it was concluded the hanger, which was originally assumed to not support any compression, actually does. This was evident on the angle tower test where strain gages indicate compression loads that are much greater than the calculated compression capacity of the hanger member. These results indicate the load distribution in the crossarm does not follow the analytical model. The crossarm is much stronger longitudinally than expected, in most cases this would be good. However, in CNP's case, the actual failure of the crossarm occurred after the failure of the tower mast. This required modification to the crossarm to limit its longitudinal capacity.

Without full-scale testing these results could not accurately be determined. In addition, the ability to strain gage selected members provided specific load distribution that otherwise could only be estimated. CNP was able to perform these tests at the EPRI Power Delivery Laboratory in Haslet, Texas, prior to the planned closure of the facility later this year.