Elizabeth Calvert

Design of Cantilevered Overhead Sign Supports

The AASHTO 2001 Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals include revised wind load provisions and new criteria for fatigue design. These provisions and criteria differ considerably from those in previous editions of the specifications, and their impact on the design of cantilevered overhead sign supports has not been fully studied. This study assesses the effect of these provisions and criteria on the design of cantilevered overhead sign support structures with the horizontal support composed of a four-chord truss. Wind and fatigue load design calculations of typical structures, located at sites across the United States, were performed with the design provisions of the 2001 supports specifications and compared with design in accordance with the previous edition of the specifications. The induced forces in the primary members of the cantilevered sign support structure were calculated, and corresponding member sizes and weights were estimated. The results of the study demonstrated the effect of the wind and fatigue load provisions on the design of cantilevered overhead sign support structures.

WIND LOAD PROVISIONS IN 2001 AASHTO SUPPORTS SPECIFICATIONS

The AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals (Supports Specifications) has been revised in its entirety through a major research project conducted under the auspices of the National Cooperative Highway Research Program (NCHRP Project 17-10). The new document was approved in 1999 by all state departments of transportation for adoption by AASHTO and was published in 2001. A major part of the revisions included new provisions and criteria for wind loads. These provisions differ considerably from those in previous editions of the specifications. A review of the changes in the wind load provisions of the 2001 Supports Specifications is presented. These changes, which are primarily due to the adoption of a new wind speed map based on a 3-s gust, could result in a significant increase in the magnitude of wind pressure in hurricane areas and a decrease in inland areas depending on location and structure type.

ANALYSIS OF SUBSTATION DEADEND CONCRETE STRUCTURES

Three full-scale concrete pole substation frame structures were tested under specified loading conditions to compare the actual test performance to the theoretical computational predictions based on conventional structural analysis. The objective was to develop refined analytical procedures, conceivably based on improved modeling of the structures, which provide better estimation of the induced forces due to structural loads. Various loading cases were applied to each structural configuration, and all structures were loaded to destruction. All testing was conducted in a vertical configuration, with the poles embedded in crushed granular material to simulate actual field conditions.

A Review of Current Wind Load Provisions for Transmission Pole Structures

Most utility companies in the U.S. use the National Electrical Safety Code (NESC) provisions to determine loads on transmission pole structures. Although several other standards for the calculation of wind loads on non-utility poles do exist, a major national standard that forms the basis of most other standards and is adopted by the building codes is ASCE 7-95 Minimum Design Loads for Buildings and Other Structures . A comparative review is made in this paper of the wind pressure calculated using both NESC and ASCE standards with the main objective of determining differences between those standards and the significant parameters affecting wind load calculations. The adequacy of the current NESC provisions for extreme wind loads will be discussed. The results should provide added insight into the differences between the two standards and may thus be of interest to the committee that is currently revising the ASCE 74 Guidelines for Electrical Transmission Line Structural Loading (under development).

Proposed Revisions to AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals

The 1985 AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals (referred to here as the current Supports Specifications) has been revised in its entirety with significant additions to its content and scope. This work is the result of an extensive research study performed under NCHRP project 17-10. The main objective of the study was to update the previous edition of the Supports Specifications based on the most recent state-of-the-art information for the design and use of structural supports. Considerable effort was made to specify the best practice, realizing the characteristics and limitations of each material. From a design safety standpoint, all materials for structural supports were treated equitably. Safety, aesthetics, and economy were three main guidelines in developing the provisions of the Supports Specifications, but manufacturing practices and experiences were also considered. The proposed specification is presented in a specification/commentary side-by-side format that will enable the engineer to review with ease the basis of Supports Specifications’ provisions. References are updated to provide a resource for the most recent research studies related to the structural behavior and performance of structural supports. An extensive table of contents is provided to allow easy reference and quick access to various provisions of the Supports Specifications. The purpose of this report is to present highlights of the proposed revisions to the specification. The supporting information, scope, and basis for the proposed changes are also discussed.