smart grid: driven by sustainability - burns &...

TECHBriefs www.burnsmcd.com A quarterly publication by Burns & McDonnell 2008 No. 3

Smart Grid: Driven by SustainabilityUtilities Must Define What It Means, Sort Through Political, Technical Challenges

By Mike Beehler, PEEveryone agrees that retail electric rates and bills are going up in the face of rising worldwide demand for fossil energy resources, expanding fuel transportation costs, climate change, renewable portfolio standards and aging infrastructure. In fact, average electric rates nationwide are up more than 35% in the past six years.

Electric utilities must be prepared to offer solutions to their customers or potentially face a firestorm of protest in years to come. Many believe that the intelligent or Smart Grid is one such solution; however, electric utilities across North America are struggling to answer the question: What is the Smart Grid?

Simply stated, the Smart Grid is the convergence of information and operational technology applied to the electric grid, allowing sustainable options to customers and improved security, reliability and efficiency to utilities.

The Smart Grid can be applied to generation, transmission, distribution, metering and, certainly, beyond the meter on customer facilities (see Figure 1). Distributed generation and the dispatch/storage of renewables, transmission line loading and substation equipment monitoring, distribution power flows and voltage measurement, automated meter reads and turn-on/turn-off service all hold promise. However, if the Smart Grid is to be a solution for offsetting the negative impact of rising rates and bills, it must be deployed in a manner that specifically addresses these rate and bill impacts (see Table 1).

The Political LandscapeCustomers (voters) will soon demand an immediate and tangible response from regulators and lawmakers that will require serious attention to this complex situation.

Lawmakers and regulators may not wait. Congress passed the Energy Independence and Security Act of 2007 (HR6) that discusses but does not fund research and development about the Smart Grid. The California Assembly has passed Senate Bill 1438, which requires investor-owned utilities, the California Energy Commission and the California Independent System Operator to develop a definition of Smart Grid to improve “overall efficiency, reliability and cost-effectiveness of electrical system operations, planning and maintenance” by July 15, 2009, with a plan for implementation by June 30, 2010.

Energy and energy policy is a volatile political issue. According to Public Utilities Fortnightly magazine, presidential candidate Sen. Barack Obama promises to “invest in a digital smart grid … to enable a tremendous increase in renewable generation and accommodate modern energy requirements, such as reliability, smart metering and distributed storage.” Electric utilities would be prudent to start developmental Smart Grid efforts now and, in order to achieve the best political results, consider focusing those efforts on distribution, metering and customer solutions that quickly deliver quantifiable value to the customer.

To that end, the most important part of the simple definition of Smart Grid becomes “allowing sustainable options to customers.”

Figure 1: An intelligent Smart Grid deployment strategy can touch all stages of the electric power life cycle, but it should initially focus on sustainable options for customers at the distribution, meter and customer phases.

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TECHBriefs 2008 No. 3 2 Burns & McDonnell

Taking the Sustainable ViewSustainability is defined as proactive stewardship of the environment, providing for the long-term health and vitality of ecosystems. Applied to obtain the most immediate customer impact on the retail electric power delivery system, sustainability means an improved level of energy efficiency in the transformation, distribution and use of electricity.

This improved level of efficiency translates into such things as lower line and transformer losses for the utility asset owner and conservation and load management opportunities for end-use commercial and residential customers.There are two fundamental approaches to achieving higher efficiency in the

transformation, distribution and use of electricity. One approach is the implementation of substation and distribution automation (DA) systems that improve utility operational efficiency through the application of intelligent equipment devices (IEDs) to remotely monitor, measure, coordinate and operate distribution capacitors, switches, transformers and feeders over a secure, robust telecommunication network. An advanced DA system allows interdevice messaging between substation and distribution IEDs and the supervisory control and data acquisition system using IEC61850 (a standard for equipment interoperability among equipment made by different vendors) or other legacy open protocols such as DNP3 or Modbus. The ability of various new and legacy

Table 1: The execution plan for a Smart Grid deployment must address rate and bill impacts.

Rates and bills are going up. Customers (voters) will soon demand an immediate and tangible response. Utilities need a deployment strategy and a project execution plan. This 12-step process outlines a concept-to-completion deployment strategy that encompasses electric generation, transmission, distribution, metering and the end user with initial Smart Grid efforts clearly focused on the distribution, metering and end-user levels.

1. Program Management: Develop and implement a multifaceted, multiyear contracting strategy and implementation plan to define, promote and deliver a Smart Grid program in the context of high performance, cost control, adherence to schedule, stakeholder relations, revenue protection and prudency review.

2. Business Analysis: Develop strategies, technology assessments and the business case to support regulatory requests and funding for pilots.

3. Distributed Generation: Engineer the connection, dispatch and/or storage of renewable and microscale generation resources to the customer/owner and the electric distribution system.

4. Remote Equipment Monitoring: Design and manage the installation of intelligent equipment devices on major substation equipment and critical transmission spans to remotely monitor asset and environmental condition on a quasi real-time basis.

5. Data Acquisition Technologies: Specify a vendor-neutral advanced meter infrastructure (AMI) system or a substation/DA program that acquires real-time data to support improved security, reliability and operational efficiency of the distribution system.

6. Telecommunications: Study and develop a robust broadband telecommunications system for rural, suburban and urban applications to transfer mission-critical and non-critical data from the customer, distribution feeder or substation to system operations centers.

7. NERC Compliance: Evaluate the physical and cyber security requirements of the distribution system to include substations and system operations centers and develop a plan for compliance with existing mandatory North American Electric Reliability Corp. (NERC) standards and for future cyber security challenges related to AMI.

8. Data Integration Management: Coordinate the integration and long-term management and warehousing of operational and/or customer data from new and legacy systems onto a secure platform that allows data analysis, visualization and reporting by various user groups.

9. Data Analytics and Evaluation: Analyze real-time and archived data to develop a better understanding of load factors, energy usage patterns, equipment conditions, voltage levels, etc., and integrate the data into usable customer programs and/or operation and maintenance algorithms that identify, trend and alert operators to incipient failure.

10. Demand-Side Management: Study the rate impacts of conservation and load management programs, to include demand response programs and the use of dispatchable or stored renewables, using AMI data for various customer classifications. Obtain regulatory approval to test the marketing, performance and acceptance of the programs through pilot projects for customers.

11. Energy Services: Provide design only or turnkey (engineer-procure-construct/EPC) services for commercial and industrial customers that implement energy efficiency or load-shifting projects at their facilities.

12. Home Area Network: Identify, test and analyze the response of new electric household appliances and consumer devices to market price signals from the utility via AMI in the context of existing or pilot rate structures.

Deployment Strategy and Project Execution

Burns & McDonnell 3 TECHBriefs 2008 No. 3

components of the electric distribution system to communicate with one another is expected to lead to better operational efficiency and reliability.

In this case, a utility may improve performance and perhaps reduce operational overheads to help control overall rates charged to customers. Some large and progressive utilities have embraced this advanced DA approach with some early success; however, the customer has no personal control over their electric usage other than an on/off switch. The grid is smart, but it could be smarter.

The second approach is the implementation of advanced meter infrastructure (AMI) that will allow the electric utility to communicate directly with customers and create new opportunities for service. Regulators in many states have embraced and authorized the implementation of AMI, and many utilities are purchasing and installing millions of new electronic, two-way meters and the required broadband communications system to support them. Paul De Martini, director of Edison SmartConnect for Southern California Edison, told Public Utilities Fortnightly, “At the outset, we wanted the message to be that we were going to introduce a new metering program that would benefit our customers. People focus on what the utility gets out of it, but we want to demonstrate that there’s a lot more to be gained by the customer.”

There are many tangible and intangible benefits of an advanced DA system or an AMI-enabled Smart Grid, but the regulatory driver for this effort is sustainability. Regulators will require quantifiable system efficiency improvements or progressive demand response and demand-side management programs that provide load shifting and load reduction leading to more sustainable electric power use. Focusing on Smart Grid benefits outside the sustainability drivers of today’s political environment is imprudent. Utilities expecting full rate recovery on a DA system or an AMI-enabled Smart Grid

must implement improvements that deliver tangible and quantifiable efficiencies to major customer classes.

Project JustificationUtilities that expect to build new transmission, substation and distribution assets may be required during the routing, siting and permitting process to demonstrate how their implementation of a Smart Grid in a region balances the need for the project. Sustainability implies that peaks have been shifted or shaved, loads have been managed and efficiencies have been achieved before new transmission and distribution assets are employed. Regulators, lawmakers and their constituents will demand no less. But there are three major challenges to meeting those demands:

• TheDAorAMIbuild-outrequiresasecure,robust telecommunication network for mission critical and non-mission critical data transport.

• Meterdataintegrationandmanagementforbillions of meter readings turning data into information and, ultimately, action will be culturally disruptive for utilities.

• Demandresponseanddemand-sidemanagement programs allowing for “prices to devices” for residential and small commercial customers must be part of an ultra-simple, readily accepted rate structure.

Thoughtful and prudent attention to these fundamental challenges of implementing the Smart Grid will lead to sustainable options for customers and satisfied regulators that will allow full recovery and return on investment. When that is accomplished, utilities can confidently use the Smart Grid to achieve other security, reliability and efficiency objectives. However, without early success in telecommunications, data integration management and customer programs, the industry will find that it has simply given old ideas a new name.

For more information, please e-mail: [email protected]

Michael E. Beehler, PE, is an associate vice president in the Burns & McDonnell Transmission & Distribution Group. He graduated from the University of Arizona in 1981 with a bachelor’s degree in civil engineering. He received his MBA from the University of Phoenix in 1984. Previously, he held transmission engineering positions at the Tucson Electric Power Co. and the Hawaiian Electric Co. He is a registered professional engineer in eight states and is a fellow in ASCE and a member of IEEE.

Read MoreLook for part two of this Smart Grid series in a future issue of TechBriefs.

For a complete analysis of the major challenges of Smart Grid deployment, read our white paper at:

www.burnsmcd.com/smartgrid


Thinking Long-TermProcuring the Most Economical Aeration Blower System for Your Wastewater Treatment Facility

By Darin Brickman, PEWith wastewater utilities and industries facing aging infrastructure, more stringent regulations, limited resources and increased energy costs, many are examining methods to minimize not only capital budgets but also operations and maintenance (O&M) budgets. Given global energy management issues, entities providing wastewater treatment — municipalities, utilities and some industries — have renewed their focus on the total cost of ownership of energy-intensive mechanical systems.

While the wastewater treatment method used by any entity is dependent upon the type of waste stream, many treatment facilities utilize some form of biological treatment. The most-needed component to ensure proper performance of aerobic biological treatment processes is, simply, oxygen.

Oxygen Demand Equals Power DemandAdequate aerobic biological treatment of industrial and domestic waste streams primarily depends on meeting the oxygen demand exhibited by the treatment process. Meeting this oxygen demand usually represents more than 50% of the total electrical demand of a treatment facility, so minimizing the power consumption from proper treatment has a dramatic impact on the total operating cost.

Oxygen demand can fluctuate due to factors that typically vary with the contributing waste source. For domestic or municipal sources, discharge to the treatment facility typically mirrors water usage in a household. For industrial sources, discharge to the treatment facility typically mirrors the production line.

Oxygen demand is typically expressed as biochemical oxygen demand or chemical oxygen demand. Depending on the waste stream characteristics and the discharge limitations, the total oxygen demand can vary. For example, if oxidation of ammonia nitrogen (i.e. nitrification) by the biological treatment

process is necessary, additional oxygen demand is realized. In the simplest terms, in order to ensure adequate treatment, the amount of oxygen required by the biological treatment process must be provided at any given time. Since providing oxygen requires a power source and, therefore, power consumption, an efficient system is desirable.

This article will not examine nor discuss specific biological treatment processes. Rather, it will focus on the aeration equipment used to supply the required oxygen for a biological treatment process and discuss methods to procure the most cost-effective aeration system.

Aeration SystemsSeveral systems are available to provide oxygen to aerobic biological treatment systems in industrial and municipal wastewater treatment facilities. These systems typically include mechanical aeration or diffused air systems. Mechanical aerators are categorized as surface-type (see Figure 1) or submerged-type and are further separated by device (e.g. turbine, propeller, etc.). Mechanical aerators generally transfer oxygen from the gaseous phase to the liquid phase by exposing the treatment-process liquid to atmospheric oxygen through agitation.

Diffused air systems (see Figure 2) differ from mechanical aeration systems in that oxygen is introduced to the treatment process through a system of aeration blowers, air piping and diffusers to the bottom of the treatment basin. Several types of diffusers are available in today’s market, and each has advantages and disadvantages. The specific application should

Figure 1: A surface-type mechanical aerator exposes the treatment-process liquid to oxygen through agitation.


always consider these factors. In diffused air systems, aeration blowers can have a dramatic effect on system power consumption.

Commonly used aeration blowers include centrifugal and positive displacement. Both types have advantages and disadvantages, but centrifugal blowers generally are used for larger aeration systems. Blower selection should always be completed for the specific application. But for the purposes of this article, the discussion will focus on centrifugal blowers.

Centrifugal Blower SystemsTwo major types of centrifugal blowers are multistage and single-stage. Multistage blowers

consist of a series of impellers fixed to a shaft that rotate within a housing (see Figure 3). The housing is arranged to lead the air from the discharge of one impeller to the inlet of the next impeller. The shaft is directly coupled to the drive motor. Typically, the air output is controlled by an inlet valve where opening or closing the valve controls the air volume through the machine. Due to the pressure drop through the inlet valve, the efficiency of the unit rapidly decreases at partial air flow. The machine also can be susceptible to surge conditions at reduced air flow rates. Usually, a multistage blower that regulates air flow through an inlet throttling valve will have a volumetric turndown from 100% to approximately 65%. Below 65% the machine can become susceptible to surge.

An alternate approach to inlet throttling is a variable frequency drive (VFD). A VFD allows the multistage blower to operate at lower speeds and produce less air flow. Although a VFD provides increased efficiency at lower required air flow rates, potential disadvantages exist. First, the volumetric turndown ability of multistage blowers is not improved with a VFD. When evaluating the use of a VFD with a multistage centrifugal blower, other considerations include the possibility of more

Figure 2: A diffused air system incorporates oxygen through a system of aeration blowers, air piping and diffusers.

Figure 3: Multistage centrifugal blowers use a series of impellers fixed to a shaft that rotates within a housing.


pronounced surge (i.e. if the system is not designed correctly) and the potential need to oversize the system to provide a best efficiency point at the flow conditions most often experienced by the blower system. It should be noted that a VFD has inherent efficiency losses that decrease the overall perceived efficiency advantage if they are not considered in the evaluation.

A second type of centrifugal blower is the single-stage blower (see Figure 4). A single-stage blower is a constant speed machine with three rotating parts: a single impeller, a high-speed pinion and a low-speed gear. Air flow rate is controlled by a series of guide vanes. A dual-vane, single-stage blower uses two sets of control vanes — one on the inlet side and one on the outlet side of the blower. The outlet guide vanes are used to control the aerodynamic shape, or flow pattern, of the air as it is released from the tip of the single impeller. By increasing or restricting this air flow path, the amount of air flow coming off the impeller is controlled.

Along with the outlet guide vanes, the inlet efficiency optimization control vanes, which are controlled by the blower control system and instrumentation, are used to achieve optimal air flow efficiency through the blower, keeping a balanced impeller condition. This condition ensures no more air than is required by the discharge flow demand is moved through the machine. The dual-vane control technology dramatically increases the efficiency of the blower system. This efficiency is maintained throughout the entire volumetric turndown of the blower. Single-stage blowers provide a wide range of available volumetric turndown from 100% to approximately 40% without the risk of surge. Therefore, efficiency (e.g. reduced horsepower consumption) and turndown can be significant advantages of a single-stage blower. The potential disadvantages of single-stage blowers are the increased capital costs

as compared to multistage blowers, and their perceived mechanical complexity.

Evaluating and Procuring the Most Cost-Effective Blower SystemEach centrifugal blower system offers advantages, and both types have manufacturers with proven and reliable blowers being used in biological treatment systems. Because treatment facilities should provide the most cost-effective system, many end users have migrated to an evaluated bid approach. While not a new or revolutionary method of procurement, evaluated bids can be difficult to use effectively when comparing two technologies, such as multistage and single-stage blower systems. At first glance, single-stage blowers can potentially reduce power costs but may result in higher capital costs.

Figure 4: A single-stage centrifugal blower uses a constant speed and controls air flow by a series of guide vanes.


For more information, please e-mail: [email protected]

The opposite may be true for multistage blowers, but on a total system cost over a defined period, a net present worth (NPW) analysis can provide an effective and proven method to compare systems.

NPW analyses can become complex if not conducted logically. Care should be taken to ensure the use of quantifiable evaluation factors. For example, NPW analyses often go to extreme lengths to include cost factors that cannot be verified. Further, the quantifiable and verifiable cost factor should enforce a penalty for not meeting the stated performance or cost.

Burns & McDonnell has used the evaluated bid approach to assist in the pre-purchase procurement of centrifugal blower systems for several clients. In addition to providing the most cost-effective system for the project, properly conducted pre-purchase efforts provide several valuable benefits (see Table 1).

The key technical inputs in evaluated aeration blower bid procurement are:• A detailed air flow rate profile by season and

anticipated oxygen demand by the biological treatment process

• Climactic data, including ambient temperature, relative humidity and barometric pressure

• Operating intervals• Site-specific unit energy costs

It is imperative to evaluate site-specific factors to ensure accurate and fair cost proposals. Based on these inputs, manufacturers provide guaranteed power consumption costs for input into the NPW model. The power costs and any other O&M costs requested in the model combine with the capital costs for the designed system(s) to determine the least NPW cost.

Finally, the key procedural inputs in any evaluated bid procurement should include:

• Evaluation/selection criteria• Compliance submittal process and timing

guarantees• Manufacturer performance guarantee and an

industry standard method of demonstrating performance

• Ownership transfer terms• Delivery options, retainage and

nonperformance penalties

Given these factors, the evaluated bid process requires adequate engineering time to ensure each system is designed to meet the site-specific requirements. For example, the systems must be designed to meet the entire range of oxygen demand and provide the same level of control in order to ensure a fair evaluation.

SummaryWhile the function of aeration blowers is simple (i.e. compress a volume of air over a specific time at a minimum discharge pressure), their sizing, selection and system design are not simple. If the selection criteria include procuring the system that results in the lowest cost of ownership, the task can be even more challenging. However, if the procurement process is properly planned and executed, end users can obtain the least cost and best solution for their facility.

Darin Brickman, PE, is an associate and wastewater department manager in the Burns & McDonnell Denver office. He has more than 16 years of experience in water and wastewater treatment system design and project management. He received his bachelor’s degree in civil engineering and his master’s degree in environmental engineering from South Dakota State University.

Table 1: The evaluated bid approach for pre-purchase decisions provides client benefits.

Evaluated Bid Approach• Provideendusercontroloverequipment

procurement

• Createcompetitivenessbetweenqualitymanufacturers

• Streamlinethedesignprocesssincetheselected equipment is known

• Developapartnershipwiththemanufacturer


Building Information Models (BIM)Envisioning a Future where BIM Brings Value to All Stages of the Design/Construction ProcessBy Mike Fenske, PE, DBIA, and Steve Cline, PEBuilding information modeling (BIM) is moving from dream to reality and changing design and construction processes along the way. Although Burns & McDonnell has been modeling buildings and other facilities in 3D for decades, the term BIM has only recently been accepted by the industry.

Standards such as the National Building Information Modeling Standard have defined BIM as a “representation of the physical and functional characteristics” of a building. Thus, a building information model is more than just an unintelligent 3D model. It includes data compiled from conceptual design through construction and commissioning.

While full implementation of all the possible functionality envisioned is still years away, the industry is moving with accelerating speed.

At Burns & McDonnell, our integrated delivery team is thinking out of the box. What if a design team had modeled the non-critical portions of your facility in advance, allowing the focus of the design to be on the main function of the building rather than the bathrooms? What if an as-built model was integrated with your building control system to allow building operators easy electronic access to information about the fixtures and equipment? What if construction managers could minimize job site personnel and create a cleaner and safer construction site?

Design Data — Adaptable building component models of areas such as offices, control rooms and bathrooms are added to the schematic model.

Architects are able to easily evaluate space constraints while engineers create structural, mechanical and electrical analysis models within the BIM.

Schematic Data — Initiating the building information model (BIM) during schematic design of a facility enables superior space visualization allowing owners to make better-informed decisions.

At the schematic level, the model includes only the most critical components, such as space relationships and building form. The BIM may also be incorporated with GIS data at this early stage to illustrate the facility’s relationship to its surroundings.

Environmental Data — LEED® points are actively tracked as the design progresses, allowing owners and designers to understand the environmental impact of their decisions. The resulting HVAC loads are used to determine if the facility’s central plant can handle new loads.

Energy costs are automatically gathered from local utilities, providing stakeholders an accurate forecast of energy costs.

Bidding and Negotiation Data — Whether the facility is delivered through design-build or the more traditional design-bid-build, subcontractors are able to understand the 2D construction documents better when accompanied by a BIM.

Quantity takeoffs from the model allow for more accurate bids and visualization of the project gives a savvy contractor an understanding of the complexities of the job.

Construction Documents — Extraordinary visualization in the earlier stages of the project has allowed the facility’s stakeholders to make layout and functional changes early in the design process, reducing the amount of time spent in this stage to complete the construction documents.

Two-dimensional construction documents are cut from the model and changes to the BIM result in live changes to the drawings.


Construction Management Data — The facility model is tied to the construction schedule, creating a 4D representation of the building. Construction planners use the 4D model to evaluate construction positions and project sequencing to allow the existing facility to remain operational.

The design is adjusted for an optimal construction sequence fit, saving money and time. Subcontractors coordinate their fabrication models and are able to fabricate more elements off site, creating a safer and cleaner construction site.

Facility Management Data — An as-built BIM tied to actual design and construction documentation allows facility operators easy access to information that is usually only accessible by searching through reams of project records, improving maintenance responsiveness while reducing maintenance cost.

The BIM is tied to facility operation systems so that building engineers can virtually control the building systems while security personnel monitor access.

For more information, please e-mail: [email protected] or [email protected]

Steve Cline, PE, is a structural engineer and BIM services director in the Aviation & Facilities Group. He has five years of structural design experience with institutional, military, transmission and distribution, industrial, environmental and aviation projects. He has a bachelor’s in civil engineering from Purdue University and a master’s in civil engineering with a structural emphasis from the University of Illinois at Champaign-Urbana.

Mike Fenske, PE, DBIA, is an associate vice president and engineering manager in the Burns & McDonnell Aviation & Facilities Group. His experience includes project management for domestic and international aviation and aerospace facility design and construction. He has a bachelor’s in electrical engineering from the University of Nebraska and a master’s in engineering management from the University of Kansas.


for replacement light poles by saying that its standard one-year warranty does not cover naturally occurring harmonic vibration light pole failures. Additional calls to various light pole manufacturers revealed that none of them warrant failure due to harmonic vibration.

Selection ProceduresIt is important to note that the failed light pole met all manufacturer requirements and had been properly selected and installed based on its criteria. Many light pole manufacturers publish wind speed maps and light pole selection criteria for their products. A common light pole selection procedure:

1. Select the light fixture, and obtain its effective projected area (EPA) and weight. The EPA is the area that is loaded by wind. This information is located on the fixture cut sheet.

2. Determine the number of light fixtures and any special mounting methods (arm or bracket) to be installed on the pole. Obtain the EPA and weight for any arms or brackets from the corresponding cut sheets.

3. Total the EPA and the weights of all fixtures, arms and brackets.

4. Select the design wind speed for the project location from the light pole manufacturer’s wind map. Typically, this is a fastest mile wind speed, which is different from the current building code values for a 3-second gust. Conversion tables exist.

5. Select a pole and compare the EPA and fixture weights with the allowable EPA and weights for that pole. If the actual EPA and fixture weights are less than the allowable EPA and maximum weight listed on the cut sheet, the pole meets the requirements.

In this project, both the specified and subcontractor-proposed light poles met these criteria, yet still failed under the destructive effects of vibration under modest wind speeds. The design wind speed was 80 mph ( fastest mile). Generally, when the wind

By Peter Manis, PE, and Wes Jones, PEOn nearly every building project there are non-building structures that require some design and/or engineering. These structures range from trash enclosures and culverts to flagpoles and light poles. In the case of light poles, a common design approach is to either specify a light pole or have the contractor or light pole supplier submit one for approval. In many instances, specifying a light pole is a straightforward process; however, under certain circumstances, additional effort and attention is required to avoid wind-induced resonance.

On a recent project, most of the installed light poles were swaying under recorded wind velocities of 17 to 28 mph with gusts up to 46 mph. The estimated top-of-pole movement was approximately 8 to 12 inches from horizontal repetitive motion, or vibration (see Figure 1).

The next day, one of the light poles was found on the ground with what appeared to be fatigue cracking at the weld between the base plate and the pole. The project was under construction, so subcontractors took down the remaining poles to prevent further failures. Fortunately, there were no injuries since the failure occurred during the night.

Review of the light pole submittal revealed that the subcontractor had proposed a different size and type of pole than what had been originally specified — a 30-foot-tall, 6-inch square aluminum pole. Instead, the subcontractor proposed a 30-foot-tall, 4-inch square steel pole, which was approved because the 4-inch pole more than met the performance specification according to the manufacturer’s literature.

Consultations with the light pole supplier and manufacturer indicated that the failure of the light pole was most likely due to wind-induced harmonic resonance of the light pole and subsequent fatigue cracking of the weld between the base plate and the pole. The light pole manufacturer responded to a request

Wind-Induced Harmonic Resonance:Considerations for Light Pole DesignDesign Factors Reduce Probability of Weld Damage or Failure

Figure 1: Wind velocities of 17 to 28 mph caused side-to-side swaying of 8 to 12 inches. (Graphic courtesy of Structure magazine.)


speed matches the natural frequency of the light pole, resonance will result. In many cases, the resonance is destructive, leading to fatigue cracking of the weld at the base plate to pole interface.

Only in certain circumstances are light poles designed to resist fatigue, according to Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals from the American Association of State Highway and Transportation Officials (AASHTO). These include specific high-level lighting structures, along with overhead cantilevered traffic signal and sign structures. AASHTO indicates that light poles do not normally exhibit fatigue problems, but as this case study indicates, such failures can occur.

Preventive DesignRather than investing time and energy into fatigue analysis and mitigation in common light poles, a cost-effective approach is to minimize the probability of resonance. Two contributing factors to light pole resonance are height and fixture arrangement. One pole manufacturer indicates that light poles with a fixture EPA of less than 2 square feet (very little fixture area) at a height of 25 feet or greater have an increased probability of resonance. While such a slender light pole can withstand the maximum design wind speeds, which generally are above 70 mph, it is susceptible to wind-induced vibration, which typically occurs around 20-40 mph.

As an example, consider the vibration of a flagpole exposed to wind. When there is no flag on the pole, it is quite common to hear cables banging against the pole. This is caused by movement or vibration of the pole. However, when there is a flag at the top of the pole, the wind loading applied to the flag acts to dampen the resonant movement of the pole, eliminating the banging sound. Incidentally, flagpoles have a different foundation anchoring system that typically does not include a base plate or welds. See Guide Specifications for Design of Metal Flagpoles from the National Association of Architectural Metal Manufacturers (NAAMM) for more information on flagpole design.

Consequently, the use of shorter light poles with multiple fixtures will generally reduce the chances of resonance. The shorter length provides a more rigid structure, and having more fixtures at the top equates to greater wind loading. The wind loading and the fixture weight at the top act as dampers to reduce resonant pole movement.

Additionally, although no shape is exempt from wind-induced resonance, it has been noted that round (or octagonal) tapered light poles are less susceptible than square ones. The natural frequency of a tapered light pole varies along its length, which makes it less likely to develop overall resonance from a constant wind.

Further, the geographic location of a light pole may also contribute to the steady-state, low wind speeds that result in light pole resonance. Features such as unobstructed flat land or low-level mountains, where wind can be channeled through an area, may contribute to resonance. Turbulence created by aircraft or vehicular traffic may also be a contributor.

Peter Manis, PE, is a senior structural engineer in the Burns & McDonnell Aviation & Facilities Group. He has bachelor’s and master’s degrees in civil engineering from the Missouri University of Science & Technology and 10 years of experience as a structural engineer on military and industrial projects.

Wes Jones, PE, is a senior electrical engineer in the Aviation & Facilities Group. He has eight years of electrical systems design experience and expertise in medium-voltage power distribution, building electrical systems, central plants and military projects. He received his bachelor’s degree in computer and electrical engineering from the Georgia Institute of Technology.

For more information, please e-mail: [email protected] or [email protected]

Table 1: Study of manufacturer literature yields these recommendations to reduce the likelihood of wind-induced resonance.

Reducing Wind-Induced ResonanceCriteria that should be considered to reduce the probability of wind-induced resonance:

Use round, tapered light poles less than 25 •feet tall, with a minimum 6-inch diameter.Use a minimum of two fixtures per pole to •provide weight at the top for dampening.Include in the pole specifications a •requirement for factory- or field-installed vibration dampers to be provided by the light pole manufacturer.Contact the light pole manufacturer when •there are site-specific concerns that should be considered during light pole design.Provide specific wind loading information •in the documents, and indicate whether wind loading is based on a 3-second gust or fastest mile wind speed.

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2008 No. 3

• Defining Smart Grid• Aeration Blowers

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TECHBriefs

Many light pole manufacturers have attempted to minimize light pole resonance by offering factory- or field-installed dampers. A damper increases the damping coefficient of the light pole, resulting in reduced movement of the light pole. In many cases, these dampers are hanging weights installed either on the surface of the light pole or inside the pole near the top. Dampers are not a cure-all for resonance, because they only change the range of wind speeds that can cause wind-induced resonance.

Best PracticesBased on this information, best practices can be gleaned from manufacturer literature. (See Table 1.) Periodic maintenance and inspection of a light pole can help determine if wind-induced vibration is a concern. Items to be inspected include the weld between the base plate and the light pole shaft and loosening or damage of the light fixture. Frequent lamp replacement is also a sign of pole movement.

The client should be notified of the potential problem — possibly as part of a specifications-required operations and maintenance manual — and a maintenance plan should be implemented. If there is concern during periodic maintenance, the light pole manufacturer and a structural engineer should assist in determining whether wind-induced vibration is the cause.

One Question RemainsIf harmonic resonance is prevalent during or after construction, who picks up the repair bill? In this case, the light pole supplier replaced all 16 poles on the project (with dampers installed), since it was discovered that the light pole manufacturer warranty would not cover this type of failure. The project continued with minimal disruption thanks to that supplier. It is in the best interest of all parties to work together to minimize the probability of wind-induced resonance and to establish measures to monitor future concerns.

References1. AASHTO, Standard

Specification for Structural Supports for Highway Signs, Luminaires and Traffic Signals

2. NAAMM, Guide Specifications for Design of Metal Flagpoles

3. Lithonia Lighting, Light Standards Effects of Vibration Technical Bulletin

4. Valmont Structures, “Pole Owner’s Manual,” Warranty and Maintenance

This article has been reprinted from the March 2008 issue of Structure magazine, published by the National Council of Structural Engineers Associations (NCSEA).