Powering up after a disaster

07/13/2010


The International Building Code (IBC) is a comprehensive set of building standards that was first proposed in 1997 by the International Code Council (ICC) and adopted in 2000. The IBC sought to harmonize the many national, state, and local codes that govern the design of structures in an effort to eliminate duplicative or conflicting standards and, therefore, make compliance more uniform.

The IBC has been updated on a three-year cycle; the latest version is IBC-2009. Currently, all 50 states and the District of Columbia have adopted version IBC-2000, IBC-2003, IBC-2006, or IBC-2009 as their de facto building code.

While the main focus of the IBC is structural integrity and fire prevention, certain provisions govern the certification and installation of emergency standby power systems used in locations that are seismically active or are subject to high wind loading of up to 150 mph.

Depending on the classification of the structure and type of occupancy, seismically certified emergency standby power systems are required in order to ensure power after a catastrophic event such as an earthquake or wind event.

The primary needs for electrical power after such an event is for the continuing operation of essential facilities to support the community and various life-safety systems that support building egress.

Where power systems are required for continued operation of the facility, the standby generator set and supporting components must be sized to operate all other critical components in the building such as air handling units, air conditioning, cabinet heaters, air distribution boxes, boilers, furnaces, chillers, cooling towers, water heaters, and other similar mechanical, electrical, or plumbing equipment required to keep the building functional.

Where life-safety is of concern, the emergency standby power would be required to operate emergency lighting, elevators, ventilating systems, communication systems, alarms, fire pumps, and other systems involved in protecting life-safety. At a minimum, IBC certification and installation details are required in seismically active locations for the following essential facilities that are classified as Occupancy Category IV in Table 1:

Additionally, an emergency power system that continues to operate following a seismic event plays a positive role in business continuity, allowing the proper shutdown of manufacturing processes or the preservation of computer data—both of which help reduce financial risk.

WHEN TO SPECIFY A SEISMIC POWER SYSTEM

Not every area of the U.S. or type of structure is required to have a seismically certified emergency power system. According to the IBC, a seismically certified emergency power system is required only in locations and structures that meet certain criteria. Figure 1 shows the areas in the country that are seismically active and where seismic design must be considered. The criteria include importance factor (Ip), building occupancy category, site soil class, and spectral response acceleration.

Importance factor: The IBC uses Ip to designate whether an emergency standby power system is a critical or non-critical component. A non-critical component has an Ip of 1.0, but a critical component has an Ip of 1.5 when any of the following conditions apply:

1. The emergency standby power system is required to operate after an earthquake for life-safety purposes such as egress lighting, sprinkler systems, fire protection systems, smoke evacuation, etc.

2. The structure contains hazardous materials

3. The emergency standby power system is located in an Occupancy Category IV structure and is needed for the continued operation of the facility or its failure could impair the continued operation of the facility.

Occupancy category: Table 1 shows the occupancy categories of buildings and other structures as listed in the IBC-2003, 2006, and 2009. Categories I through III do not require a seismically certified emergency power system unless they are located in a seismically active area with short-period response acceleration greater than 0.33 g and the equipment is given an Ip of 1.5 because of number points 1 or 2 above. See Table 2. However, all Category IV structures require such a system when the importance factor is 1.5 (i.e., essential) and SDS is more than 0.167g.

Site classification: In any seismically active zone, the potential for structural damage is influenced by the soil type. The least structural damage can be expected on solid rock (Site Class A), while the most structural damage can be expected on loose, liquefiable soils (Site Class F). See Table 3.

 Short-period response acceleration: This is a number (SDS) derived from the expected ground movement forces (measured in g = acceleration due to gravity) in seismically active locations as defined by the United States Geological Survey (USGS). The value also accounts for the soil type of the location (Figure 1). The higher the SDS value, the more severe are the seismic forces acting upon a structure and its contents. This number is then used in conjunction with an Occupancy Category (I-IV) to determine a seismic design category (A-F). Buildings with seismic design categories C-F have requirements for seismically qualified components when the component Ip = 1.5.

The following three critical parameters are the basis for determining whether a seismically certified emergency power system is required:

  • An SDS of 0.167 g or greater
  • Occupancy category IV with an Ip = 1.5
  • Seismic design category of C, D, E, or F and a component Ip = 1.5.

POWER SYSTEM STRUCTURE MUST ALSO RESIST WIND LOADING

The IBC also addresses wind loading and its effect on the performance of an emergency standby power system. For those states that have adopted the IBC-2003, IBC-2006, or IBC-2009, the building (or enclosure) that houses the emergency standby power system must resist any overturning forces caused by expected high winds, and the generator set must stay mounted to its foundation and be operable after the event in all occupancy category IV structures (essential facilities). The minimum wind speed for design of structures in the U.S. is 85 mph (Figure 2).

According to the provisions in IBC standards, the entire design team is responsible for making sure an emergency standby power system stays online and functional after a seismic or high wind loading event. This group includes emergency standby power system manufacturers, suppliers, installers, design team managers, architects, and consulting engineers. Each has a critical role to play in making sure that structural and nonstructural components perform as designed.

IBC Chapter 17, the “Contractor Responsibility” states:

“Each contractor [i.e., all members of the design team listed above] responsible for the construction of a main wind- or seismic-force-resisting system, designated seismic system, or a wind- or seismic-resisting component…shall submit a written statement of responsibility to the building official and the owner prior to the commencement of work on the system or component. The contractor’s statement of responsibility shall contain acknowledgment of awareness of the special requirements contained in the statement of special inspection.”

SEISMICALLY CERTIFIED EMERGENCY POWER SYSTEMS

It falls to the emergency standby power system manufacturer to provide a product that is certified to withstand the typically expected seismic and high wind loading forces and to continue operating after a seismic event has occurred. The provision in IBC-2009, Section 1708.4 “Seismic Certification of Nonstructural Components” states:

“The registered design professional [usually the architect, consulting engineer, or electrical contractor] shall state the applicable seismic certification requirements for nonstructural components and designated seismic systems on the construction documents.

1. The manufacturer of each designated seismic system component subject to the provisions of ASCE 7 Section 13.2.2 shall test or analyze the component and its mounting system or anchorage and submit a certificate of compliance for review and acceptance by the registered design professional responsible for the design of the designated seismic system and for approval by the building official. Certification shall be based on an actual test on a shake table, by three-dimensional shock tests, by an analytical method using dynamic characteristics and forces, by the use of experience data (i.e., historical data demonstrating acceptable seismic performance) or by more rigorous analysis providing for equivalent safety.

2. Manufacturer’s certification of compliance for the general design requirements of ASCE 7 Section 13.2.1 shall be based on analysis, testing or experience data.”

An emergency standby power system consists of a base, engine, alternator, fuel supply, transfer switches, switchgear, and controls. While the engine generator set is naturally a robust piece of equipment, designing for survival of a seismic event also focuses attention on the generator set mounting to the foundation and external attachments such as fuel lines, exhaust, and electrical connections.

To certify the components of an emergency standby power system, the generator set and its associated systems are subjected to a combination of three-dimensional shake-table testing and mathematical modeling.

The IBC requires that these tests be performed by an independent, approved, third-party supplier that can issue a seismic certificate of compliance when the seismic qualification is successfully completed. Once a particular generator set passes the seismic qualification, it is the responsibility of the manufacturer to label the equipment, indicating the seismic forces to which the equipment was subjected.

CONCLUSIONS

While the IBC addresses all facets of structure design and construction in all 50 U.S. states, it also addresses the performance of a number of non-structural systems such as emergency standby power systems. The IBC’s requirements for emergency standby power systems are intended to ensure that structures within certain occupancy categories will have emergency power after a catastrophic event such as an earthquake or wind event. As such, it has set seismic design and testing standards for the manufacturers of emergency standby power systems.

All members of a structure’s design team—emergency standby power system manufacturers, suppliers, installers, design team managers, architects, and consulting engineers—need to be aware of the seismic and wind loading provisions within IBC for emergency standby power systems. Power system manufacturers have undertaken advanced design and testing programs to comply with the seismic provision within IBC involving three-dimensional shake-table testing, finite element analysis, mathematical modeling, and experience data. Certified power systems are labeled as having passed seismic testing by a qualified, independent testing organization. By working with a power system manufacturer that offers seismically certified products, the structural design team can be assured that it will have an emergency standby power system that will perform as designed after a seismic or high wind loading event.

Table 1

Occupancy          Occupancy category of buildings and other structures category

I               Buildings and other structures that represent a low hazard to human

                life in the event of failure, including but not limited to:

                                • Agricultural facilities

                                • Certain temporary facilities

                                • Minor storage facilities.

II             Buildings and other structures except those listed in occupancy

                categories I, III, and IV.

III            Buildings and other structures that represent a substantial hazard to

                human life in the event of failure, including but not limited to:

                                • Buildings and other structures whose primary occupancy is

                                public assembly with an occupant load greater than 300

                                • Buildings and other structures containing elementary school or

                                day-care facilities with an occupant load greater than 250

                                • Buildings and other structures containing adult education facili-

                                ties such as colleges and universities with an occupant load

                                greater than 500

                                • Group 1-2 occupancies with an occupant load of 50 or more

                                resident patients but not having surgery or emergency treatment

                                facilities

                                • Group 1-3 occupancies

                                • Any other occupancy with an occupant load greater than 5,000

                                • Power-generating stations, water treatment facilities for potable

                                water, wastewater treatment facilities, and other public utility

                                facilities not included in occupancy category IV

                                • Buildings and other structures not included in occupancy

                                category IV containing sufficient quantities of toxic or explosive

                                substances to be dangerous to the public if released.

IV            Buildings and other structures designed as essential facilities,

                including but not limited to:

                                • Group 1-2 occupants having surgery or emergency treatment

                                facilities

                                • Fire, rescue, ambulance, and police stations and emergency

                                vehicle garages

                                • Designated earthquake, hurricane, or other emergency shelters

                                • Designated emergency preparedness, communications and

                                operations centers, and other facilities required for emergency

                                response

                                • Power-generating stations and other public utility facilities

                                required as emergency backup facilities for occupancy category

                                IV structures

                                • Structures containing highly toxic materials as defined by Section

                                307 where quantity of the material exceeds the maximum allowable

                                quantities of Table 307.1 (2)

                                • Aviation control towers, air traffic control centers, and emergency

                                aircraft hangars

                                • Buildings and other structures having critical national defense

                                functions

                                • Water storage facilities and pump structures required to

                                maintain water pressure for fire suppression.

Table 2

Seismic design category based on short-period response accelerations

Value of SDS      Occupancy category

                I or II      III            IV

SDS < 0.167g       A             A             A

0.167g < SDS < 0.33g       B             B             C

0.33g < SDS < 0.50g          C             C             D

0.50g < SDS         D             D             D


Table 3

Site class definitions

Site class              Soil profile name

A             Hard rock

B             Rock

C             Very dense soil and rock

D             Stiff soil profile

E              Soft soil profile

E              Any profile with more than 10 ft. of soil with:

                                • Plasticity index > 20

                                • Moisture content > 40%

                                • Undrained shear strength < 500 psf.

F              Any profile containing soils with:

                                • Liquefiable soils, quick and highly sensitive clays,

                                collapsible weakly cemented soils

                                • Peats and/or highly organic clays

                                • Very high plasticity clays

                                • Very thick soft/medium stiff clays.

About the author

Wells is a product manager at MTU Onsite Energy in Mankato, Minnesota.



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