Seismic Bracing of Non-Structural Components

Introduction

Seismic bracing of nonstructural components represents one of the biggest life safety hazards in our buildings.  Nonstructural components include secondary building components that support the facility’s functions.  Nonstructural damage historically accounts for 20% to 50% of the damage observed in recent earthquake events within the United States.  This should not be too surprising considering the cost of nonstructural components within buildings is often greater than the cost of the primary structure itself.  

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Code requirements to seismically brace nonstructural components are well-documented and have been in the code for many years.  Unfortunately, some requirements are often overlooked because the responsibility to meet the requirements is typically left to the sub-contractors  installing the systems.  This process has the potential to leave building owners at risk for systems that are not adequately braced.

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Nonstructural Systems Required to be Seismically Braced

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The governing code for the seismic bracing of nonstructural components is found in Chapter 13, ASCE/SEI 7-10.   The code offers the following exemptions for seismic bracing:

• Furniture
• Temporary or moveable equipment
• Architectural components in Seismic Design Category (SDC) B if Ip = 1.0
• Mechanical and electrical components in SDC B
• Mechanical and electrical components in SDC C if Ip = 1.0
• Mechanical and electrical components in SDC D, E and F if ALL of the following are met:

• Ip = 1.0
• Component is positively attached to the structure
• Flexible connections are provided between component and duct/pipe/conduit and either

• Component weights 400 lb or less and has center of mass located 4 feet or less above adjacent floor
• Component weights 20 lb or less or in case of a distributed system, 5 lb/ft or less (2” water line)

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Some common architectural components that require seismic restraint are:

• Suspended ceilings

• Seismic restraint not required if ceiling area is less than or equal to 144 sf
• SDC D, E and F require a 2” perimeter supporting closure angle with a seismic separation joint every 2,500 sf with enlarged opening for sprinklers

• Access floors
• Partitions taller than 6 feet with an exception if all of the following are met:

• Height is less than 9 feet, weighs less than 10 psf, horizontal seismic load is less than 5 lb/sf

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Factors Influencing the Bracing Design

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The equation governing the seismic bracing of nonstructural components is as follows:

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                       Fp = (0.4 * ap * Sds * Wp) * (1 + 2 * (z/h))

                                      (Rp / Ip)

Where:

• Fp = seismic design force in the brace
• Sds = spectral acceleration
• ap = component amplification factor (varies from 1 to 2.5)
• Ip = component importance factor
• Wp = component operating weight
• Rp = component response modification factor (varies from 1 to 12)
• z = height in structure of point of attachment of component with respect to the base
• h = average roof height with respect to the base

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As you can see, the primary factors that influence the design force are seismic acceleration, weight of the component, and the elevation of the component relative to the overall building height.  Consequently, the force in the brace increases as seismic motion increase, component weight increases, and the closer the component gets to the structure’s roof.

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Typical Bracing Methods

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Post installed anchors are commonly used into concrete and masonry; however, the anchors must be prequalified for seismic applications.  Power Actuated Fasteners are commonly used in both concrete and steel; however, power actuated fasteners cannot be used for sustained tension loads or for brace applications in SCD D, E, or F unless approved for seismic loading.  It should be noted that power actuated fasteners can be used to support the following:

• Acoustical tile or lay-in panel suspended ceilings
• Distributed systems where service load on any fastener is less than 90 lb
• Fasteners in steel where service load on any fastener is less than 250 lb

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Unistrut, TOLCO and B-Line all have systems specifically design to resist seismic forces and are commonly used.

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Who is Responsible to Design These Systems?

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The building code requires drawings to show supports and attachments of nonstructural components with a quality assurance plan that needs to be prepared by a registered design professional for the use by the owner, authorities having jurisdiction, contractors, and inspectors.  The big question is—who is responsible for this design and drawing preparation?  The engineer of record, or the contractor installing the systems via a delegated design?”

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As you might suspect, the correct answer might be a little of both.  Below is a summary of the pros and cons of each path.

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Engineer of Record

Delegated Design

Pros:

• Engineering complete
• Submitted with permit CDs

Cons:

• A lot of unknown variables
• Solutions will be generic due to unknown equipment requirements
• Assumptions will increase costs
• Type of equipment supplied
• Contractor’s preferences unknown
• Too many different ways to seismic brace systems
• Dictating means/methods/construction sequence
• Service not covered in standard fees

Pros:

• Design specific to supplied equipment
• Contractor dictates means/method/construction sequence
• Most cost effective solution
• Contractor preferences maintained
• Engineering oversight – review by architect/engineer/building official

Cons:

• Engineering per code requirements may fall thru the cracks
• Architect/engineer/building official under obligation to ensure proper engineering is submitted

Non-structural components are the most valuable building assets. The building code provides clear direction on how to seismically brace these elements to protect the owner’s investment and life safety.  The biggest challenges for the design team are to define who is responsible for this design and to ensure all code requirements are met during construction.

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