Lightning Protection

by Design

Lightning protection procurement tends to follow the same procurement strategy that is followed when purchasing life insurance; you buy as much as you feel you need or as much as your budget will allow.

Therefore there are two factors guiding the designer when considering lightning protection: cost and need. Both factors are legitimate design considerations. Since bridges are usually public property, budget is usually beyond the control of the designer and must be programmed by a governmental administrator for projects specific to bridge structures. Need, however, is a factor that requires technical consideration by a qualified lightning protection engineer. This article will focus on the technical considerations of lightning protection engineering specific relative to concrete bridge structures.

The example I will use is the upgrade of the lightning protection system for the Sunshine Skyway Bridge. Lightning protection design is based on the laws of Physics and site specific factors. Therefore, any design must consider location and the structure's risk to lightning damage to either the structure or the electronic and electrical systems that control bridge lightning and traffic control devices. Florida is a high-incident area for lightning. In addition, lightning currents average six times the national average for lightning currents. Design requirements must start with the need assessment and a need assessment must include location, construction and appurtenances.

Figure 2
The original lightning protection system was developed during the architectural design and construction of the structure. This design was based on a design found in the Lightning Protection Institute (LPI) design guide for smoke stacks. Air terminals (lightning rods) were installed in each of the four corners of the two cable stay towers and two separate ground conductors were installed vertically down opposite corners of each of the towers and continued down to the salt water line. The height of the cable stay towers can be seen in the photo in Figure 2 and represents approximately half the distance to the water where the effective ground termination for this system is located.

Bridge Maintenance contacted my office and explained that damage to the structure was found after thunderstorms. This damage varied from damaged surveillance cameras to tripped circuit breakers and pieces of concrete found on the bridge that clearly came from the structure. The pieces of concrete concerned me the most. Some pieces weighed about 50 pounds and if pieces of such mass were to fall from the towers, the risk to life and property quickly was recognized and the Department immediately approved a rigorous research effort to determine the most effective, cost effective and timely design as well as budgeted funds to construct the upgrade to the existing protection system.

A site survey was immediately conducted to determine the existing condition of the lightning protection system and collect data that could be used to develop a computer simulation model (CADD) of the existing system. The model selected was a Pspice mathematical nodal analysis model which offered the engineer many points to insert the lightning attachment forcing function and reiterative "what-if" capabilities that were fast and cost effective. The output of the model was a graphical display of voltage and current over time between any two nodes in the system.

This model revealed that the existing system was very inefficient and could be dangerous to maintenance personnel during a storm. The inefficiency of the system caused large voltages to be developed along the electrical conductors that connected the air terminals to the ground 431 feet below at the water line . The large potentials developed could reach more than a million volts in areas were maintenance personnel regularly worked. In addition, and less predictable but far more ominous was that the large voltages also indicated that the system was acting as a restriction to the lightning discharge path.

The laws of Physics clearly supports the one-time commercial statement that "It's not nice to fool Mother Nature". The delays indicated by the large voltages also indicated that the larger the discharge, the more likely that alternate paths would be used to fulfill the need to complete the discharge of energy between cloud and ground. Lightning protection design is based on knowing where the lightning strikes (the attachment) and if the system designed to control the attachment and conduct attachment currents harmlessly away is insufficient to the task, alternative attachment points and the path or paths taken by attachments to parts of the structure that is not part of the lightning protection system or overload currents become unpredictable and potentially dangerous. Ultimately, a rigorous research effort was approved to determine the most cost effective and capable design for an upgrade of the existing system.

This design took advantage of the geometry of the bridge and the Tampa Bay water salt content as well as the design of the most efficient and protective electrical system to attract, control and manage large lightning discharge currents. The bridge roadway towers 175 feet above the water in the center of the main span and the two cable stay towers exceed 431 feet above the water (MSL). The view from the top of the towers as seen in Figure 3 gives perspective to the task.

Figure 3
There are two towers, each supporting 21 continuous cable stays at their apex. The cable stays are connected to adjustable anchors in a line of 21 anchor arrays radiating away from each tower longitudinally along the centerline of the main span. These cable stays are multi-stranded steel cables wrapped in steel and covered by a round steel case (and qualify as excellent lightning attractors). The upgrade design included grounding the cable stays at the adjustable anchors shown in Figure 4. The cable stays are continuous and travel through a saddle at integral levels in the towers to maintain a constant support angle that maximizes the vertical and horizontal vectors required by the design. Access to the cable stays at the saddles was not possible because the saddles were concrete and surrounded each cable. Therefore the only metallic access to the cable stays that facilitated an adequate mechanical and electrical connection was inside the main span structure at the adjustable anchors.

Figure 4
Figure 4 is a photo of the adjustable anchor connection (called a trumpet) and the horizontal conductor that connects the anchors together. This conductor also continues down to the water line and completes the circuit to ground using the salt water in the bay. The conductors are flat and direct the lightning currents along straight paths and smooth bends. No 90 bends, sharp turns and kinks in the flat strap were allowed.

All connections are exothermic and no mechanical connections are allowed.  This requirement insured reduce future preventive maintenance requirements. The strap design minimizes radiation and the voltage drop per unit length of the conductor. This increases the efficiency of the system and reduces the probability that attachment currents will look for alternate paths to ground as well as reducing the potentials developed along the conductor which addresses the safety consideration highlighted in the original site survey findings.

The design of the air terminals at the top of the two towers was also important to the success of the overall design performance. The air terminals must reliably attract the attachments therefore minimizing the chance that non-protected elements of the bridge structure or its appurtenances would be inadvertently struck by lightning. Considering the height of the towers, the limited access to the top of the towers and the need to provide significant protection to both the concrete structure and the appurtenances (safety railing, obstruction beacons,

Figure 5
communications antennas, etc.), the design needed to be simple, light weight, electrically efficient and very effective. Figure 5 illustrates the small work area, including the access hatch and safety railing and Figure 6 illustrates the red obstruction beacon located in the center. It can't be seen in the photo, but the beacon lense in Figure 6 is deformed from the intense heat generated by the attachment channel.  Both Figures 5 and 6 were taken before the upgrade work to the top of the towers was begun.

The attachment process must overcome the insulating properties of air and ionize the air to facilitate electron flow. The magnitude of the transfer of energy creates very high temperature. In the main channel, currents develop temperatures 5 times that on the surface of the sun; 55,000 degrees F.

Figure 6
Attachment "leader" currents are much lower than main channel currents and the temperatures are proportionally lower, however, it remains apparent that the temperatures are sufficient to cause the obstruction beacon lense glass in Figure 6 to soften without actually making contact with the beacon. This becomes more impressive when you understand that the average attachment and subsequent high temperature exists for less than a second. Temperature over time is required to increase the temperature of a material object. When the time duration becomes very small, greater instantaneous energy is required to do the same work. This leads into the next design consideration; the forcing function.

The forcing function is a statistical distribution based on the normal or natural "bell-curve" distribution. The nominal for Florida is modeled for 60,000 Amperes. The nominal for the US varies from 10,000 to 20,000 Amperes depending on who you talk to . What is really important is the rise time. The rise time used in this model is based on the Institute of Electrical and Electronic Engineers (IEEE) "8x20" standard which is found in the IEEE Standard C-62. The standard waveform rises to maximum current in 8 microseconds and decays to 50% of the maximum in 20 milliseconds, hence the nickname 8x20. This is the one most important consideration when designing the circuits required to handle the enormous currents produced by lightning.

Finally, the location as alluded to above is also a design consideration. The system performance in areas where nominal lightning currents are 10,000 Amperes rather than 60,000 Amperes requires a design of lesser degree to produce acceptable results. This equates to a lesser cost and therefore must be a design consideration. Although steel and wood structures are beyond the scope of this article, material is also a consideration. Expectations of performance such as that of steel being better than wood or concrete is risky and may not be correct if other factors effect the performance of the material to conduct lightning current. Geometry is the key to optimizing performance of any lightning protection system; site geometry, structure geometry, path geometry and conductor geometry.

One design consideration did not come from the upgrade design but came from the testing of the installed system. It was found that physically connecting the flat strap to the steel reinforced concrete did not cause lightning current energy to be dissipated into the concrete. A university research team tested the installation and concluded that the material difference between the concrete and the conductor created a electron barrier similar to that created by a copper strip laminated to a fiber circuit board. The fast rise time of the current simply traveled down the efficient conductor and did not find the material barrier an efficient path to ground.

Advances in lightning research conducted in the last 15 years has produced important information that the lightning protection design engineer can use to better design a lightning protection system.  The capabilities of the desktop computer workstations available to the engineer allow modeling complex models that could only be done on a super computer a few years ago.  It can be now shown that through modeling and competent design that statements such as "attachments is an act of God but damage to your structure or equipment is due to inadequate design of your lightning protection system" is an expected rather than exceptional design goal. Flat strap, Ufer grounds and path geometry are best addressed before construction begins and must be incorporated in the design of the structure and facility resources.

Structure design material should be considered when designing to reduce lightning damage. Although there is no research proof that epoxy coated reinforcing bar has cause lighting induced damage, there is a hypothesis that the use of epoxy coated reinforcing bar in concrete may case concrete "popping" due to induced currents from lightning. The hypothesis is that the epoxy coating will insulate the metal from the concrete prevent contiguous continuity throughout the reinforcing steel. Isolated lengths of metal bars can resonate to the induced currents from a lightning discharge if the length of the bar is integral multiples of the wavelength of a lightning discharge. This hypothesis is consistent with verified induced damage through copper wire which is a major issue with telephone companies and long distance carriers as well as power utilities. The use of fiber has been justified to a large extent because there is no copper in the fiber cable to facilitate the induction of transients from lightning.

Ultimately, the choice will be to protect for lightning damage in the original design or try to install lightning protection later. The second choice is always more expensive. From my experience, I have found that the average lightning protection system will cost anywhere from 2 to 10 times less than the cost to install the equivalent system later. Unfortunately, later will not allow for the addition of Ufer grounds in the concrete foundations and the Ufer ground is a key consideration for tall and vulnerable structures and facilities.

Robert L. Gottschalk is an Electrical Engineer and Florida Registered Professional Engineer. Bob retired from the Florida DOT as an Intelligent Transportation System (ITS) Engineer Administrator and responsible for statewide lightning protection designs for structures and telecommunications facilities, including the statewide microwave radio system infrastructure that supports ITS communications and the statewide Motorist Aid Call Box System (MAS).  Bob is now working as a private consultant and can be reached at (850)656-2582 (voice), (850)656-4799 (fax) or by email at