In the U.S. there are some tens of thousands of traffic sign structures, not including simple sign supports, and more structures are being added each week. Some states have thousands of these structures. Most of them are on the interstate system and its entrances, and while there are inspection requirements for interstate highway bridges, sign structures do not fall under this federal inspection mandate.
There have been, however, a number of serious failures; at least one of which caused a loss of life. As a result, many states, sometimes with Federal Highway Administration prodding, are investigating their sign structures and establishing maintenance management programs for them.
Many agencies have found that sign structures are orphans. Bridge maintenance sections are busy with normal bridge structures, and traffic sections who maintain the signs on the structures do not have the necessary structural expertise for structural inspections.
As a result, many agencies have had to examine the question of responsibility for the inspection, evaluation and maintenance of sign structures. In most cases, this task has fallen to the agency's bridge and structures section. Even then, additional training and establishment of a sign-structure management program usually has been necessary.
A sign-structure management program should include an inventory, inspection report and maintenance program. With recent improvements in field data gathering methods and databases, there is an opportunity to establish a comprehensive database system that includes inventory, inspection and maintenance data in a readily available format.
Depending upon the particular state or other bridge agency, the amount of information available on sign structures varies greatly from none to fairly detailed records. In most cases, the information is not organized for managing sign-structure inspection and maintenance, but may be available as part of the records for a larger highway project. Even then, design information may be limited, with only typical vender drawings, shop drawings or erection drawings available.
A sign structure inventory system should include, as a minimum, the location of the structure, details of the type of structure and its components, horizontal and vertical clearance measurements, and a record of the size of the signs in place on the structure.
The location of the sign has generally been referenced to roadway mile posts. The growing use of global positioning systems (GPS) and geographic information systems (GIS) by states makes this a very appropriate technology to use, not only for site location, but also as a tool to organize all structure inventory, inspection and maintenance information.
Three main types
There are three main types of sign structures: simple span structures, cantilever structures, and bridge-supported structures. There are many variations in materials, components and overall configuration. The structures may be constructed of steel or aluminum, and some states use a combination of materials, e.g., steel supports with aluminum superstructures. Foundations are generally concrete with steel anchor bolts, but sometimes sign structures are attached to another structure, such as a high-level or long-span bridge.
Superstructures may be fabricated of square or round tubes, angles, I-shapes, or monotubes, and are often galvanized. They may be plane frame structures or space frames and they are frequently of all-welded construction. Many of these structures were designed, fabricated, and erected many years ago, before more stringent fatigue requirements were established.
In some jurisdictions, early sign structures may not have been given the same level of shop inspection and field inspection accorded bridge structures because the sign structures often were purchased as a manufactured product. This can result in the presence of poor workmanship, increasing the chance of cracks at fatigue sensitive details.
Recent studies indicate that higher stresses and a greater number of fatigue cycles than were previously anticipated are caused by wind and upward pressure from large vehicles passing beneath the structures.
The inspection of sign structures involves distinct areas: traffic control, access, inspection techniques and reporting.
Traffic control, or maintenance of traffic (MOT), is a primary concern in the inspection of sign structures. Because sign structures generally are located in congested, heavily traveled areas of controlled-access highways, traffic control must be thoroughly planned before beginning field work.
For shoulder closures or when working in slow-speed, low-volume areas, inspectors may be able to establish their own traffic control, but on high-speed, high-volume controlled-access highways, it may not be economical because of the special training and equipment necessary. The required traffic control effort can be as great as or greater than the actual structure inspection effort. Typically, sign structures are located at entrance and exit ramps making lane closures more complicated. Often, inspectors will have to wait unproductively while lane closures are put in place and moved between locations.
Judicious use of climbing techniques over open traffic lanes can significantly reduce maintenance of traffic requirements, but some agencies have determined that the risk involved in working over open traffic lanes precludes the use of this inspection method.
Generally, access to structures will be accomplished by climbing, by use of a bucket lift or by a combination of the two methods. Ladders could also be used, but such use is generally not cost-efficient.
Climbing is generally limited to truss type structures, supported by climbable support structures. When climbing a structure, the safety of the inspector and the motorists passing below is of paramount importance. Inspectors should be trained in proper climbing techniques, safety and the use of all equipment. All tools should be attached to the inspector with lanyards to prevent dropping them, and these lanyards must be very short so that a dropped tool does not hang dangerously in traffic.
Monotube structures, cantilever structures, bridge-mounted structures and some configurations of truss superstructures may not be climbable. Access to many structures may more easily be obtained using a bucket lift.
Particular care must be used to ensure that bucket lifts are not used over active lanes under any circumstances. Standards for lane closures on controlled-access highways in the Manual for Uniform Traffic Control Devices require that the work area in a closed lane, such as where a bucket lift would be used, be protected by a truck with a vehicle attenuator attached to rear of the truck.
Sign structures should be inspected by trained inspectors who have an understanding of the load path through the structure and the particular types of defects, which have been found or could be expected on these structures.
The structural actions of sign structures are, in many cases, more complicated than those of bridge structures.
The inspector in charge of the inspection team should therefore be a licensed structural engineer or meet the requirements for an NBIS bridge inspection team leader plus have special training in sign structures. For safety, the inspection team should consist of at least two persons.
The inspection should be conducted in an orderly fashion to ensure the entire structure is given a close, hands-on examination of all components.
The principal inspection method is visual examination, usually conducted with the aid of hammers, scrapers and, particularly when climbing, mirrors mounted on extension rods. The prevalence of welded connections in these structures means close, detailed examinations for possible cracks are required. Normally, one inspector climbs the structure or accesses it from a bucket truck and radios notes to another inspector on the ground.
Special inspection techniques may include the use of dye penetrant to locate and define the extent of cracks, magnetic particle or ultrasonic techniques to evaluate welds, ultrasonic thickness measuring devices to measure the remaining thickness of members, drilling of small holes in tubes to detect trapped water, and ultrasonic flaw detectors to examine anchor bolts. Past inspections have indicated that typical and special problems include:
-- Cracked anchor bolts both above and within the concrete,
-- Loose nuts and missing connectors, both on anchor bolts and structural bolts,
-- Cracked and broken welds,
-- Split tubes,
-- Plugged drain holes, debris accumulation and corrosion,
-- Internal corrosion of tubular members,
-- Poor fit-up of flanged connections with cracking and missing bolts, and
-- Structure overload due to installation of signs exceeding design square footage.
The reporting system for the inspections should be a part of the same database management system that includes the inventory, and should be designed for use in the field on laptop computers. It should be designed as a system that accommodates various types of structures and be set up to lead the inspector through a complete inspection. Because of the similarity of components in the various sign types, a standardized system can be used to make note taking more efficient, and help assure a thorough inspection.
A numerical rating system similar to that used for bridges can simplify note taking and make comparison of structures easier. Provisions to include written comments in the database should also be provided. Capabilities of current computers, software, and digital cameras permit gathering design drawings, and photographs into the same files for ready retrieval. The database also can be designed to extract data for inclusion in other department inventories.
The data from the inspection program forms the basis for a maintenance program. The numerical inspection data can aid in identifying and prioritizing individual structure and system-wide maintenance requirements. Inventory information, such as location and traffic volumes, also can be culled from the database as additional elements in repair prioritization.
Sorting of the inspection results by structure type and age provides a means of evaluating the overall performance of sign structures in the inventory. This data is a tool for maintenance planning, and also is valuable feedback for the design and procurement of new structures.
At this time, there is not universal recognition of the need for inspection of sign structures. There have been, however, enough incidents of sign structure failure to indicate the need for inspection by prudent agencies.
A well-developed inspection program cannot only detect defects before they become major problems, but also can form the basis for a planned maintenance program.