Guidelines for the Installation, Inspection, Maintenance and Repair of Structural Supports for Highway Signs, Luminaries, and Traffic Signals
This document provides guidance for the installation, inspection, maintenance, and repair of structural supports for highway signs, luminaries and traffic signals. The primary purpose is to provide owners with information that can assist them in managing their inventory, identifying potential problem areas, and ensuring safe and satisfactory performance of these types of ancillary highway structures. The primary reason for compiling this guidance is increasing problems with wind induced vibration, fatigue, and even structural collapse of these support systems, as seen in Figure 1. Documented problems with these structures include questionable design and details, poor fabrication practices, and poor installation techniques.
This document is applicable to traffic signal structures, overhead highway signs, highmast luminaries and other large light poles, and also supports used for traffic monitoring equipment of various types, such as cameras. Support structures include cantilever, butterfly, and bridge support (also known as overhead or span type supports). These structures may be installed on foundations in the ground and/or pedestals built into highway side or median barriers or built into parapets or other parts of a bridge. Wood poles and signs, concrete poles, and span wires are not directly addressed, though some of the information contained herein may assist in evaluating those structures.
These Guidelines cover structures fabricated from galvanized or painted structural steel shapes and tubes, aluminum shapes and tubes, and weathering steel. These Guidelines focus on structural aspects of the supports as opposed to sign panels, signal heads, luminaries, or their mechanical and electrical connections. In many cases, due to their location adjacent to or extending over vehicular traffic, a support failure on these structures poses a significant threat to the traveling public.
Design requirements for these structures are contained in the 4th edition of the "AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaries and Traffic Signals" (2001 Specifications) and are not directly discussed herein. In addition, several NCHRP Reports and industry specifications, which contain relevant information are listed in the references.
3.1 Material Types
Most support structures are fabricated from structural steel tubes, angles, and plate or from aluminum tubes, angles and plate. Common material specifications are shown in Table 1. Materials for bolts and anchor rods are discussed in Chapter 6.
|Structural Steel||ASTM A36, A572 or A709 (or AASHTO equivalent)|
|Structural Steel (Weathering)||ASTM A572 W or A588|
|Steel Tubing||ASTM A500|
|Steel Pipe||ASTM A53, Grade B; A595 A, A135, API 52|
Alloy and Temper
|ASTM B209, B210, B211, B221, B241, B247, B308, B429|
Round steel tubes may be either cold formed tubing conforming to ASTM A500 or steel pipe conforming to ASTM A53 for resistance welded pipe, or one of several other ASTM or API (American Petroleum Institute) Specifications. Multi-sided tubes are manufactured by successive bending of a plate and welding the longitudinal seam.
Aluminum members are available in a variety of alloys and tempers as is seen in Table 1. Alloys 6061-T6 and 6063-T5 are the most commonly used. In order to increase strength, aluminum is alloyed with other elements and, for the T series, heat treated. When these tempers are used in welded construction, their allowable stresses are reduced as the heat from welding reduces the beneficial heat treatment. Aluminum is both lightweight and corrosion resistant, which are beneficial when used for ancillary structures. However, the fatigue strength of aluminum is only about 40 percent that of steels with comparable yield strengths, and the modulus of elasticity is one third that of steel, which increases member deflections.
Shop fabrication of aluminum structures generally uses welded construction. Welding electrodes are selected to match the base metal, and welding is performed in accordance with ASW D1.2, "Structural Welding Code Aluminum."
Round Tubes are commonly used in the fabrication of both steel and aluminum structures since they provide:
- Good bending resistance about any axis
- Efficient sections for compression loads
- Good torsional resistance
- Lower drag coefficient and associated wind loading
3.2 Corrosion Protection
To protect steel structures from corroding some type of protective system is required. Though some ancillary steel structures may be painted, protection is most often provided by use of galvanizing or fabrication using weathering steel. Galvanizing is performed using the hot dip process.
In the galvanizing process a zinc coating is metallurgically bonded to the steel surface. The zinc layer provides both a barrier coating for the steel and galvanic protection since the zinc preferentially is consumed before the steel corrodes. The coating process involves three basic steps:
It is essential that the material surface be clean and uncontaminated if a uniform, adherent coating is to result. Surface preparation is usually performed in sequence by caustic (alkaline) cleaning, water rinsing, acid pickling and water rinsing.
The caustic cleaner is used to clean the material of organic contaminants such as dirt, paint markings, grease, and oil, which are not readily removed by acid pickling. Scale and rust are normally removed by pickling in hot sulfuric acid (150°F) or hydrochloric acid at room temperature. Water rinsing usually follows caustic cleaning and acid pickling.
Surface preparation can also be accomplished using abrasive cleaning as an alternative to or in conjunction with chemical cleaning. Abrasive cleaning is a mechanical process by which sand, metallic shot, or grit is propelled against the material by air blasts or rapidly rotating wheels. Abrasive cleaning is often required to remove weld flux from weld areas.
The final cleaning of the steel is performed by a flux. The method of applying the flux to the steel depends upon whether the "wet" or "dry" galvanizing process is used. Dry galvanizing requires that the steel be dipped in an aqueous zinc ammonium chloride solution and then thoroughly dried. This "preflux" prevents oxides from forming on the material surface prior to galvanizing. Wet galvanizing uses a molten flux layer that is floated on top of the bath metal. The final cleaning occurs as the material passes through this flux layer before entering the galvanizing bath.
The material to be coated is immersed in a bath of molten zinc maintained at a temperature of about 850°F. The time of immersion in the galvanizing bath varies, depending upon the dimensions and chemistry of the materials being coated. Materials with thick sections will take a longer time to galvanize than those with thin sections. Materials with a high silicon content will produce thicker coatings for equal immersions in the molten zinc than those with a low silicon content.
In addition to the effects of steel chemistry, the surface appearance and coating thickness are controlled by the galvanizing conditions. These include: a) variations in immersion time and/or bath temperature; b) rate of withdrawal from the galvanizing bath; c) removal of excess zinc by wiping, shaking or centrifuging; and d) control of the cooling rate by water quenching and/or air cooling.
Effective galvanizing requires proper detailing and fabrication practices. Detailing and fabrication must allow both flux and galvanizing zinc to reach all areas of the assembly and to properly drain. The heat of the galvanizing process can cause distortion and induce cracking if not properly addressed. Galvanizing is performed in kettles into which the item to be galvanized must fit. For larger structures, it may be necessary to fabricate them in sections, spliced after galvanizing. In other cases, structures too long for the kettle may be "double dipped"; one half (or more) of the structure is immersed at an angle and galvanized, and the process repeated to galvanize the remaining area. Unless this is done vary carefully, some of the overlap area may be poorly galvanized. Double dipping also creates thermal stresses in a fabricated assembly that may lead to cracking if not properly controlled.
The protective life of a galvanized coating is determined primarily by the thickness of the coating and the severity of exposure. Environments such as exposure to industrial air pollutants or marine environments cause more rapid deterioration of the galvanized coating than, say, clean, dry rural environments.
Hot dip galvanizing of structures is normally specified to conform with ASTM A123 (AASHTO M111-80). This provides for a minimum coating thickness of 3.9 mils on material 0.25 inches and thicker. Based on data from the Hot Dip Galvanizer's Association, service life of 30 to 40 years may be expected in relatively benign environments while only 20 to 30 years may be expected in moderately aggressive atmospheres from a 3.9 mil coating. Many ancillary highway structures have been in service long enough that the galvanizing has deteriorated. In these cases, painting may be considered to extend the structure life.
Weathering steel conforming to ASTM A588, also sometimes called by a trade name Cor-ten steel, uses nickel and chrome as alloying agents to produce a steel that can develop a protective oxide layer on the surface to minimize atmospheric corrosion. The extent of formation of the oxide layer depends on the levels of pollutants in the atmosphere as well as on the length of time the material remains wet. Cycles of wetting followed by complete drying are needed to fully develop the protective surface. Welding electrode types should be specified to be compatible with the weathering steel and high strength weathering steel bolts are also available.
Weathering steel, while used successfully in many projects, has also experienced deterioration in unfavorable environments or due to details that did not allow proper drying. FHWA Technical Advisory T 5140.32, "Uncoated Weathering Steel in Structures", cautions against its use in the following environments:
- Those exposed to highly corrosive air pollution
- Those subject to salt spray or coastal marine salt-laden air
- Those with very high rainfall and humidity or where there is constant wetness
Areas of particular concern for corrosion of weathering steel ancillary structures are the internal corrosion of tubular members, and corrosion in the mating surface of slip joint connections. Tubular members will corrode on the inside if water or an electrolyte is able to continuously contact the surface. Members should contain drain holes and be detailed so that locations are not created where moisture can pool, or debris collect. Figure 2 shows the base of a weathering steel light pole that collapsed due to corrosion at its base. Slip joints draw water into the joint through capillary action. As this area does not dry well, pack rust develops which can generate sufficient force to split the tube vertically, starting at the bottom of the joint. Painting the slip joint surface before assembly, or sealing the bottom edge of the joint are two methods of improving the performance of this area.
Weathering steel bases may also experience premature corrosion when they are buried in soil or covered by vegetation.
3.3 Notch Toughness
Since fatigue cracking is possible in support structures, it is important that the materials used for anchor rods and main structural members have some minimum level of resistance to fatigue, defined by notch toughness (specified with the Charpy-V-notch test, or CVN) in order to avoid brittle fracture from small cracks. Materials that meet minimum CVN requirements are usually also more weldable and less susceptible to weld defects.
Structural members and anchor rods should have CVN at least comparable to the non-fracture-critical bridge requirements; that is, the members and anchor rods should have at least 20 Joules (15 ft-lb) at room temperature and, weld filler should be used which provides CVN of at least 27 Joules (20 ft-lb) at -18°C (0°F). Most present materials used for these support structures are believed to have toughness even greater than these levels of CVN would provide, even though it is not usually specified. Even though most of the grades of steel, filler metal, and anchor rods have adequate notch toughness, there is typically an inverse relationship between notch toughness and strength such that the higher strength materials usually have lower notch toughness. This is another good reason to use only lower-strength grades.
The 2001 AASHTO Specifications require that steel "members" thicker than 13 mm (1/2 inch) meet the bridge steel CVN requirements. This is interpreted to not include anchor rods. It may be expensive or not even possible to get some anchor rods or tubular products with a supplemental CVN requirement. For example, some tubular members will be too thin to get even a sub-size CVN specimen. Since most materials meet the CVN requirement without having to actually specify it or pay for it, supplemental CVN specifications are not recommended for steel members or anchor rods other than for members thicker than 13 mm. Some exceptions may be when:
- There is the possibility of seismic loading;
- There are concerns about weldability of the steel members or anchor rods; or
- A53 Grade B pipe is used, which is not really intended for structural use.
Usually, anchor rods should conform to ASTM F1554-97, Standard Specification for Anchor Bolts, Steel, 36, 55, and 105-ksi Yield Strength, which is essentially the same as AASHTO M314-90, and these anchor rods have good fracture toughness. Alternatively, reinforcing bars conforming to ASTM A706-96, Standard Specification for Low-Alloy Steel Deformed and Plain Bars for Concrete Reinforcement may be threaded and used. Ordinary reinforcing bars conforming to ASTM A615-96, Standard Specification for Deformed and Plain Billet-Steel Bars for Concrete Reinforcement should not be used for these support structures because of possible low toughness.
Using materials with additional notch toughness (above these minimum levels) would not significantly increase the time to failure of the structure by fatigue. This is because cracks grow at an exponential rate, and 95 percent of the life is consumed while the crack size is less than a few millimeters. Even though additional fracture toughness may correspond with a greater crack length at the time of failure, it does not correspond with a significantly longer time to failure.
This is also the reason why, unless there is a suspected crack or there have been cracks in similar details in similar structures, it is usually not cost-effective to perform ultrasonic testing or other non-destructive testing (other than visual) on anchor rods or other details to locate fatigue cracks. There is only a small window of time when the cracks are both large enough to detect and small enough to not yet cause fracture. In some states; however, ultrasonic testing is routine on four bolt bases of cantilever structures and partial testing is carried out on up to eight bolt bases.
4.0 LOAD EFFECTS
Design loads for ancillary structures include dead and live loads, ice loads, and wind loads. For most structures, design will be governed by wind loads.
Dead load includes the weight of the structural support itself, as well as the weights of signs, luminaries, traffic signals, lowering devices and any other appurtenances permanently attached to, and supported by, the structure. Temporary loads that may occur during maintenance should also be considered as dead load. AASHTO also requires that a live load be applied to any walkways and service platforms or ladders.
An ice load, equivalent to a layer of ice 0.60 inches thick, is applied to all surfaces for structures located in regions considered susceptible to ice buildup. This area is shown in Figure 3-1 of the AASHTO Specifications, but generally includes all of the continental United States with the exception of a band running from the California coast and extending across the lower half of the southernmost states, plus all of Florida.
The primary loads applied to sign, signal, and luminaire structures are due to natural winds. The structure must not only have sufficient strength to withstand the maximum expected wind loads, normally a once in 50 year maximum, but also the fatigue effects of fluctuating winds of lower force. There are four wind-loading phenomena that can lead to vibration and fatigue: natural wind gusts, truck-induced gusts, vortex shedding, and galloping. Each type of structure (e.g., signal, sign, or luminaries) is susceptible to either two or three of the four wind loads, as is shown in Table 2. The interaction of the support structure with the wind is dependent on the structure's stiffness and shape. For instance, galloping which can occur in cantilever signal structures, does not occur in bridge support configurations.
|Type of Structure||Galloping||Vortex Shedding||Natural Wind||Truck Gusts|
|Cantilevered Sign (one- or two-chord)||X||X||X|
|Cantilevered Sign (four-chord)||X||X|
|Bridge Support (Sign or Signal)||*||X||X|
Note: X indicates structure is susceptible to this type of loading
* Vortex shedding has occurred in monotube bridge supports and can occur in cantilevered structures if the sign or signal is not attached, such as could occur during installation.
The high flexibility and low damping of cantilevered support structures makes them susceptible to resonant vibration in the wind. The flexible cantilevered structures have low natural frequencies of about 1 Hz (period of vibration of 1 second), which is in the range of typical wind gust frequencies. The closeness of the wind gust frequencies to the natural frequency causes resonance or dynamic amplification of the response. In addition, typical damping ratios in these structures are extremely low (less than one percent of critical damping). The low damping increases the amplification of the wind-induced vibrations.
Usually, the greater the length of the cantilevered mast arm, the more susceptible the support structure will be to wind-induced vibration. In recent years, the span length of the cantilevered mast arms has increased significantly. One example is a 22-m (72-ft) long cantilevered signal support structure near the University of Tampa, which vibrates frequently due to its extreme flexibility.
Personnel involved in installation, inspection, maintenance, and repair of ancillary support structures should understand the wind loading and response of various support structures. This will enable them to recognize these problems in the field and have a better understanding of the important reasons for quality control during the structure erection phase.
4.2 Natural Wind Gusts
The most common type of wind-induced vibration in support structures is from ordinary wind gusts or fluctuations of the wind velocity. In the 2001 Specifications, these gusts are referred to as natural wind gusts to distinguish them from truck-induced wind gusts. Natural wind gusts exert a fluctuating force that is primarily horizontal, and the resulting motion of the mast arm is also primarily horizontal, although there is often a significant vertical motion as well. In the 2001 Specifications, the natural wind gust pressure is applied horizontally to the projected frontal area of all surfaces, including the structural members, as well as the sign and signal attachments.
In cases where fatigue cracking was caused by natural wind gusts, cracking developed over a period of at least several years. In light poles, the connection of the arm supporting the lights to the pole top has cracked. In cantilevered sign and signal support structures, the cracks will usually manifest at the connection of the mast arm to the pole, with cracks forming along the sides of the connection. Natural wind gust loading has also caused cracks at truss connections and at the base of the pole, at the weld joining the pole to the base plate, at the top of the stiffeners, at hand holes, or at the anchor rods.
As shown in Table 2, all structure types are potentially susceptible to natural wind gusts, which act on all members of the structure as well as the attachments. The most flexible and most lightly damped structures are more susceptible, however.
Support structures in certain areas of the country that are very windy are particularly susceptible to vibration and fatigue from natural wind gusts. Typically, these are places where the mean annual wind velocity is greater than 5 m/s (10 mph).
These places where there are frequent constant winds, and support structures are at risk for fatigue cracking due to natural wind gust loading, are not the same places with the maximum peak wind velocities that are identified on wind maps, such as those in ASCE 7-98. These maps show the maximum wind speed that occurs over a 50-year period that is used for strength design. It is noteworthy that there are very few fatigue failures of cantilevered sign and signal supports after hurricanes or other large storms. These once-in-a-lifetime wind gusts have little influence on fatigue.
The fatigue-limit-state natural wind gust loads in the 2001 Specifications are derived for winds that are exceeded about 2 hours each year. Such areas, surprisingly, are not necessarily those with occasional very high winds such as hurricanes or tornadoes. For example, the four places in the United States with the greatest wind velocities that are exceeded at least 2 hours per year are:
- Tatoosh Island, Washington: 28 m/s (62 mph)
- Clayton, New Mexico: 24 m/s (53 mph)
- Point Judith, Rhode Island: 24 m/s (53 mph)
- Cheyenne, Wyoming: 24 m/s (53 mph)
4.3 Truck-Induced Wind Gusts
The passage of trucks beneath support structures induces both horizontal and vertical gust loads on the structure, creating a motion that is primarily vertical but may also include a significant horizontal component as well. The magnitude of the horizontal truck gust is negligible relative to the natural wind gust and is therefore ignored in the specifications. The vertical truck gust load in the specifications is an equivalent static pressure range applied to the underside of the mast arm or truss and any attachments. Due to the large depth of variable-message signs (VMS) in the direction of traffic flow (up to 1.2 m for older signs), the support structures are the most susceptible to truck-induced wind gust fatigue because they present a large area in the horizontal plane, as seen in Figure 3.
Fatigue cracking from truck-induced wind gusts usually develops over a period of several years. However, one case of a failure of a cantilever VMS structure, reported by DeSantis and Haig, was to a structure less than 1 year old. The cracks will usually manifest at the connection of the mast arm to the pole, at truss connections; and at the base of the pole to the weld joining the pole to the base plate, at the top of the stiffeners, at hand holes, or at the anchor rods.
The truck-gust pressure is proportional to the speed of the trucks, so signs located on major highways are more susceptible than those where the trucks are traveling slowly. Also, there is a vertical gradient for the truck-induced gust pressure, so the greater the clearance between the tops of trucks and the bottoms of the signs, the less the susceptibility to truck-induced vibration. The truck-gust loads essentially go to zero at a height of 10 m (32.8 ft) above the roadway.
4.4 Vortex Shedding
Vortex shedding is the shedding of vortices on alternate sides of a symmetric member (i.e., one without any attachments). Vortex shedding can result in resonant oscillations of a pole in a plane normal to the direction of wind flow as shown in Figure 4(A). In the 2001 Specifications, the vortex shedding pressure range for fatigue design is applied horizontally to the projected area of one side of the pole to calculate the fatigue stress ranges in the details.
As shown in Table 2, only luminaries support structures (i.e., light poles) have exhibited problems from vortex shedding. However, other simple cantilever poles, such as camera poles may also be susceptible. Luminaire support structures may occasionally exhibit excessive vibration from natural wind gusts. However, if there is a vibration problem with luminaire supports, it is typically due to vortex shedding.
In some cases, severe vibration from vortex shedding has persisted long enough to cause fatigue cracking. This may not require a long time. For example, in the case of some aluminum light poles that failed after a windstorm in New Jersey, it was found that the fatigue cracks initiated and propagated to failure in just one night. In light poles, cracks are usually observed at the base of the pole or at the weld joining the pole to the base plate or transformer base. If there are stiffeners or gussets reinforcing the pole to base plate connection, then the cracks will typically form at the tops of the stiffeners. In some cast aluminum pole bases, the cracks will occur at various details in the castings. If there are hand holes, cracks may appear around the perimeter of these. Cracked anchor rods have also occurred.
Vortex shedding tends to occur with steady continuous winds at a critical velocity. The velocity need not be very high, but it has been found that significant vibration does not occur unless the velocity is greater than 5 m/s (10 mph). The periodic frequency of the vortex shedding can lock in on the natural frequency of the pole, resulting in very large alternating forces acting transverse to the wind flow direction. There are videotapes of luminaire supports vibrating so severely that the amplitude of the vibration was on the order of the pole diameter. Vortex shedding may result in frequent burnouts of the light filaments. Occasionally, the vibration is so severe that fatigue cracks will appear.
Although vortex shedding can "lock in" and continue as the velocity increases or decreases slightly, if the velocity changes by more than 20 percent, the vortex shedding will stop. Gusty variable winds, such as might occur in a severe storm typically will not cause vortex shedding. In fact, if the wind velocity is greater than 15 m/s (35 mph), the wind is generally too turbulent for vortex shedding to occur. In summary, the winds that are dangerous for vortex shedding are steady winds in the velocity range 5 to 15 m/s (10 to 35 mph).
Recent studies have verified that vortex shedding can occur in tapered as well as prismatic circular poles with almost any diameter. (Note: this is contrary to some statements in the 2001 Specifications) In a few cases, sign and signal support structures have exhibited vortex shedding on the mast arms before the attachments (signs or signal heads) were mounted. After mounting, this is no longer a problem. Consequently, it is recommended that mast arms never be erected before these attachments are mounted. NCHRP Report 469 reports that vortex shedding of long monotube bridge support structures has also occurred.
Galloping is different than vortex shedding but also results in large-amplitude, resonant oscillations perpendicular to the direction of wind flow as shown in Figure 4(B). Unlike vortex shedding, galloping occurs on asymmetric members (i.e., those with signs, signals, or other attachments) rather than circular members. Therefore, it is the mast arms rather than the poles that are susceptible to galloping. Galloping has caused mast arms to move up and down with a range greater than 1 m. According to NCHRP Report 469, a large portion of the vibration and fatigue problems that have been investigated for cantilevered sign and signal support structures were caused by galloping.
Galloping is caused by the attachments (i.e., signs/signals). The number of signs and signal heads, their configuration, area, connection detail, and the direction of wind flow significantly influence the susceptibility for galloping. Signal attachments configured with backplates and subjected to flow from the rear are most susceptible to galloping. However, all types of signal heads and signs have been observed to be affected by galloping, even those with louvered backplates.
Galloping also requires uniform steady winds rather than gusty winds. However, in contrast to vortex shedding, galloping can continue over a large range in wind velocity. The mode of vibration for galloping is swaying of the mast arm in the vertical direction. This mode typically has a frequency closer to 1 Hz (1 second period). Higher modes of vibration have not been observed. Galloping requires twisting of the mast arm as well as vertical motion. Consequently, flexible monotube cantilever support structures are particularly susceptible to galloping. As noted above, monotube bridge support structures may be susceptible to vortex shedding; therefore, one can assume that these structures may also be susceptible to galloping from the sign or signal attachments. However, typical bridge support structures where the sign bridge consists of three-dimensional, three-chord or four-chord trusses are not susceptible to vortex shedding or galloping.
In the 2001 Specifications, the galloping pressure is applied vertically (like a shear stress) to the projected frontal area of all sign and signal attachments. The galloping loads are quite severe, so when they are applicable, they will typically govern the fatigue design. Therefore, mitigation devices will have significant cost benefits in reducing the effects of galloping as opposed to natural wind gusts or truck-induced wind gusts.
In most cases that have been investigated, the fatigue cracking that has occurred from galloping has developed over a long period of a year or more where there may have been many days of winds that caused galloping. In cantilevered sign and signal support structures, the cracks will usually manifest at the connection of the mast arm to the pole or at the base of the pole. In truss mast arms, the cracks may form at the truss connections, usually those closest to the pole. If there is a flanged splice detail in the mast arm close to the pole, this may also be a critical location where cracks may form. At the base of the pole, cracks may form at the weld joining the pole to the base plate. If there are stiffeners or gussets reinforcing the pole to base plate connection, then the cracks will typically form at the tops of the stiffeners. If there are hand holes, cracks may appear around the perimeter of these. Cracked anchor rods have also occurred.
Cantilevered sign support structures with mast arms fabricated in three- or four-chord trusses have also never been reported to vibrate from vortex shedding or galloping. Apparently, these three-dimensional truss mast arms are sufficiently stiff to resist galloping. However, there are many reported instances of cantilevered two-chord trusses with signs and signals galloping. Typically, the longer the cantilever mast arm, the more susceptible to galloping.
Fatigue resistance of particular details is determined by testing full-scale structural members under cyclic loads. The loading is characterized in terms of the nominal stress in the structural member remote from the welds or bolts in the detail. The nominal stress is conveniently obtained from standard design equations using member forces and moments. Testing has indicated that the primary effect of constant amplitude loading can be accounted for in the live-load stress range (i.e., the difference between the maximum and the minimum stress in a loading cycle). Thus, it is the fluctuation of the stress and not the absolute value of the stress that affects fatigue. The constant dead load stress does not affect fatigue.
The strength and type of steel have a negligible effect on the fatigue resistance expected for a particular detail. The welding process and minor deviations in weld quality from AWS D1.1 standards, while important, also do not typically have a significant effect on the fatigue resistance. The independence of the fatigue resistance from the type of steel and weld filler metal greatly simplifies the development of design rules for fatigue since it eliminates the need to generate data for every type of steel. As previously pointed out, the fatigue resistance of aluminum is significantly less than that of steel.
Fatigue failure of a support structure basically occurs because the stress ranges resulting from the wind or truck-induced gusts exceed the fatigue thresholds at critical details. Usually, these failures cannot be blamed on weld defects; rather, they are an indication that the structure is not adequately designed for the fluctuating loads and is experiencing excessively large stress ranges. This is a good reason to believe that other similar structures will soon be having similar cracking problems. Therefore, if a fatigue failure has occurred in a structure, one cannot be complacent about inspecting similar structures. All similar structures should be intensively inspected immediately.
It is a commonly held, but incorrect assumption that cracks may be caused by overloading the structure. In some cases, large variable message signs have been placed on a truss designed for flat green signs, and then fatigue problems have occurred. These problems are likely due to the increased area of the sign, which increases the gust load ranges and the associated stress ranges from wind and trucks, rather than from the increased dead load stress.
In the 2001 Specifications, the fatigue design provisions for support structures are given in Section 11. These fatigue design provisions are similar to those in the bridge specifications. The fatigue design procedures are based on control of the nominal stress range and knowledge of the fatigue threshold of the details. Various details are shown in Figure 11-1 of the 2001 Specifications. Each of these details is assigned a "Stress Category" in Table 11-2, and the fatigue threshold stress range for each category is given in Table 11-3 of this Specification. Support structures should be designed so that the stress ranges due to the fatigue design loads are less than the fatigue thresholds for each detail, thus ensuring that fatigue will not occur even for a large number (hundreds of millions) of applied load cycles. Design calculations should be prepared and stamped by a professional engineer explicitly showing the fatigue design as well as the traditional allowable stress design for strength.
Since the fatigue resistance of various grades of steel and anchor rods is the same, it typically is not cost-effective to use high-strength steel, higher than 420 Mpa (60 ksi) yield strength. If the dimensions of the members or anchor rods are decreased to take advantage of the high-strength steel for the maximum strength design loads, the stress ranges under the fatigue design loads will increase and fatigue will become more of a problem.
5.0 Erection of Sign, Signal, and Light Support Structures
This section recommends procedures for the erection of support structures. Construction of the concrete foundations is not covered. However, guidance on grouting of base plates and the proper installation of anchor rods is presented.
For this discussion, Quality Control (QC) refers to all means and methods utilized by a contractor and/or a subcontractor to assure ancillary structures are properly fabricated, erected and constructed. Quality Assurance (QA) refers to all means and methods utilized by a public owner or an agent acting on their behalf to assure adequate QC is employed by the contractor and that satisfactory quality in the work is achieved.
Numerous defects found during condition assessment inspections have been attributable to poor fabrication or erection practices. Proper erection is the responsibility of the erection contractor. The erection contractor should perform inspection and testing to verify that installation is in accordance with the project specifications. Records should be kept of the dates and results of any inspection, and these records should be available for the Engineer of Record or their representative to review. The Engineer of Record may require that their representative witness the installation and inspection. Inspection by the owner, or its representative, during erection is strongly recommended.
5.2 Shop Inspection
Shop inspection of ancillary structures is encouraged, particularly for welded cantilever sign and overhead bridge structures. The inspections should assure that the structures conform to the design drawings and that details such as drain holes, boltholes, connection details and welds are properly executed. Overall member configuration, alignment, and coating condition should also be checked. In many instances, detailed design of ancillary structures is provided by the vendor, within overall guidance of general configuration plans prepared by the owner. Thus a wide variety of details and fabrication methods are normally encountered in the finished structures. Vendor detail drawings and calculations should be reviewed for conformance with the 2001 Specifications and to identify poor details prior to starting fabrication.
The 2001 Specification requires weld inspection, which should be performed and documented by the fabricator. Section 5.15 of the Specifications provides requirements for steel structures, while Section 6.9 applies to aluminum structures. Weld quality for steel structures should conform to AWS D1.1, Structural Welding Code - Steel, and for aluminum structures to AWS D1.2, Structural Welding Code - Aluminum. For tubular aluminum structures, workmanship requirements for Class I structures are the minimum, while Class II should be considered for critical structures.
Due to the types of joint configurations and welds found in ancillary structures, weld inspection requirements contained in current fabrication specifications are not sufficient to ensure defect free welds during shop fabrication. This is one reason that close field inspection of ancillary structures to find detectable cracks is particularly important. NCHRP Report 469, "Fatigue-Resistant Design of Cantilevered Signal, Sign, and Light Supports," contains a discussion of the Specification requirements for welding inspection and their effectiveness.
5.3 Preparation for Erection
The erection contractor should be responsible for assembling design and fabrication documents and having them available for review by the owner or their representative. The design and fabrication documents should include a set of design drawings; shop drawings; the name and contact information for the designer/consultant of record; design calculations (stamped by a professional engineer explicitly showing the fatigue design as well as the traditional allowable stress design for strength); the name and contact information (including location of plant) of fabricator of record; welding procedure specifications; inspection reports; and copies of material mill certificates for the anchor rods, structural bolts, and the main members (not including stiffeners, base plates, flange plates, and other small plates). As an option, the owner may elect to have some of this information, not directly needed for erection, submitted directly to him/her. The erection contractor, considering the weight and stability of the pieces to be erected, should carefully plan the erection procedure for each job. Light poles, including light supports with relatively short cantilevered arms, may be fully assembled on the ground and erected in one piece. Cantilevered sign and signal supports should be erected in two pieces. It has been found that it is nearly impossible to properly tighten the double-nut anchor rod connection when a completely assembled cantilevered support structure is suspended from the crane. Bridge type structures should have the end frames erected, followed by the bridge.
Prior to erection, the separate parts of the structure, posts, end frames, mast arms, and trusses, should be inspected for bent or damaged members, damaged coatings, distortion, and defective fabrication that would affect proper erection. Structures can be damaged during shipment as well as by improper unloading and storage at the site. Localized defects in the galvanizing coating should be repaired in accordance with the requirements of ASTM A780, "Standard Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings."
In addition to establishing that the structure was properly fabricated, the contractor should visually inspect the structure for cracks. Cracks may occur during fabrication or galvanizing of the structures. In addition, fatigue cracks may occur in structures from the loads they experienced in transport, as has happened with several aluminum trusses with tube to tube welded connections. Fatigue cracks will generally occur between bolt holes or along the toe of the welds parallel to the weld axis. Longitudinal seam welds, any hand holes or other attachments, or any splices or joints should be inspected closely. All defects in the structure should be reported to the owner or their representative so that proper repairs may be authorized.
If it is not clear whether the visual indication is a crack or possibly just a crack in the coating or a scratch, the indication should be further investigated using magnetic particle (MT) testing or dye-penetrant (PT) testing. These testing procedures are explained in detail in AWS D1.1. It is generally not worthwhile to inspect areas using one of these methods unless there is a good reason to suspect there is a crack. Ultrasonic testing may be useful in some circumstances, but generally the cracks will have penetrated through the thickness of the member, and the MT and PT methods can find them more economically. Enclosed tubes, especially the diagonals and verticals in trusses, can rupture from the galvanizing (if adequate vent holes are not used or become clogged). If these tubes fill with water, they should be drained by drilling a 13-mm (0.5-inch) diameter hole near the bottom of the tube, but at least 25 mm (1 inch) from the welded connection. If the water remains undrained, it can accelerate corrosion or rupture the tube if it freezes.
It should be verified prior to beginning erection that there will be no fit-up problems when the parts of the structure are assembled in the field. The slope of the surfaces of parts in contact with bolt heads or nuts, and the faying surfaces, should be equal to or less than 1:20 with respect to a plane that is normal to the bolt axis. However, even if the surfaces are within tolerances, 100-percent mating between the two flanges will usually not be achieved. The structure should be rejected, however, if more than 25 percent of the surface area is visibly not in firm contact after snugging the bolts. This contact surface is discussed further in Section 6.4.
The erector should verify that the foundations are set to the proper elevation and anchor rods are set in the correct pattern and orientation, are of the correct size, and are plumb with the specified extension and thread length above the top of concrete, as seen in Figure 5.
The anchor rods generally should not be subjected to erection or service loads until the concrete reaches its 28-day specified strength. In some cases, with the approval of the owner or owner's Engineer, the post may be attached to the anchor rods after as few as 7 days.
If necessary, camber should be built into the support structure. The base plate should always be designed to be level. The support structure base plate should never be sloped on the anchor rods to achieve a camber, because the misalignment of the base plate will make it nearly impossible to properly tighten the anchor rods.
An ancillary structure inventory database should be established and the inventory number should be clearly placed on the support structure. The inventory number is most often stenciled on with paint, though numbers may also be stamped into the post or even magnetic bar codes can be attached. The design, materials, and fabrication documents discussed previously should be filed in a way that they are accessible and linked to the database. The date of erection should be recorded and the inspection results and any repairs or corrective measures taken should be explained in detail in the database.
5.4 Erection Procedure
Prior to starting actual erection, all traffic control and other safety measures as required by the project specifications must be in place and reviewed for compliance. Erection generally requires at least some lane closures, and may involve short-term total road closures. In areas of high traffic, erection may have to be performed at night, requiring temporary lighting for both a safe and complete installation.
The actual erection process begins by placing and leveling the leveling nuts. Typically, the bottom of the leveling nuts are set approximately 6 mm (0.25 inches) above the top of the concrete. The posts or end frames are then erected on the anchor rods (Figure 6), being careful not to damage the threads on the anchor rods. The base plate should be leveled by bringing up the leveling nuts on the low sides. To minimize shear and bending in the anchor rods, the bottom of the leveling nuts must not be more than one anchor rod diameter above the top of the concrete. Finally, the anchor rod nuts should be tightened. Recommendations for tightening are presented later, in Section 6.5.
After the anchor rod nuts are properly tightened, the post or end frame is released from the crane. For light poles, the erection is now complete, except for the post-erection retightening of the anchor rods, as discussed in Section 6.9.
For cantilevered signal and sign support structures, the erection continues with the mast arm or truss. The cantilevered mast arm or truss is balanced on the crane and is brought up to position and bolted to the post.
For sign bridges, the sections (if there are more than one) are normally connected to one another on the ground and set in one lift. Lifting points must be located so that the bridge is not overstressed during lifting, (Figure 7). With the bridge set on the end supports, alignment should be verified and connections brought up to final tightness. With the cantilever arm, or bridge in position, a check of overhead roadway clearance should be performed and documented.
Signs, signals, and other attachments are normally attached to the cantilever arm or sign bridge on the ground before erection. Even very large heavy variable message sign boxes should be attached to their supporting members on the ground, and this can be done off site. If the attachments are not attached to a cantilever structure when it is erected, they should be attached immediately after erection. This is because the bare mast arms with no attachments are vulnerable to vortex shedding, which can quickly become destructive.