New design provisions and test methods

by Richard A. Behr, Ph.D., PE, and Scott A.Warner

Reconnaissance reports after major earthquakes such as those in Mexico City, Mexico (1985), and Northridge, California (1994) note architectural glass can be damaged by seismic-induced drifts in building frames. Falling glass fragments during earthquakes represent a life safety hazard to pedestrians and building occupants, yet specific provisions for the seismic design of architectural glass in model building codes have historically been either non-existent or limited to general statements, such as "building drifts shall be accommodated."

In an effort to fill this void, I (Richard Behr) proposed a set of seismic design provisions for architectural glass. They were modified and adopted by consensus for inclusion in the 2000 edition of the National Earthquake Hazard Reduction Program (NEHRP), and published as Federal Emergency Management Agency (FEMA) 368, The 2000 NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures, in 2001.

Both Scott Warner and I participated in the Methods of Test Task Group of the American Architectural Manufacturers Association (AAMA) to develop Recommended Dynamic Test Method for Determining the Seismic Drift Causing Glass Fallout from a Wall System, published as AAMA 501.6-01. {This test method is referenced via AAMA 501.4-00, Recommended Static Test Method for Evaluating Curtain Wall and Storefront Systems Subjected to Seismic and Wind Induced Interstory Drifts, in the 2000 NEHRP provisions for the seismic design of architectural glass components.)

The new NEHRP seismic design provisions for glass and the new AAMA seismic test method for. glass have been adopted (in a slightly modified format) in American Society of Civil Engineers (ASCE) 7-02, Minimum Design Loads for Buildings and Other Structures, which is referenced in the upcoming 2003 International Building Code (IBC) and NFPA 2002 Building Code.

Seismic design provisions
The following sections (9.6.2.4.2 through 9.6.2.10.2) are the seismic design provisions for architectural glass included in ASCE 7-02. Actual section, equation, table, and reference numbers are cited below for accuracy, and ease when referencing the ASCE 7-02 document.

9.6.2.4.2 Glass Glass in glazed curtain walls and storefronts shall be designed and installed in accordance with Sec. 9.6.2.10. [see below]

9.6.2.8.2 Glass Glass in glazed partitions shall be designed and installed in accordance with Sec. 9.6.2.10. [see below]

9.6.2.10 GLASS IN GLAZED CURTAIN WALLS, GLAZED STOREFRONTS AND GLAZED PARTITIONS

9.6.2.10.1 General Glass in glazed curtain walls, glazed storefronts and glazed partitions shall meet the relative displacement requirement of Eq. 9.6.2.10.1-1:

Δ_fallout ≥ 1.25 ID_p   (Eq.9.6.2.101-1)
or 13 mm (0.5 in.), whichever is greater,

where

Exceptions

• Glass with sufficient clearances from its frame such that physical contact between the glass and frame will not occur at the design drift,as demonstrated by Eq. 9.6.2.1 0.1-2, shall be exempted from the provisions of Eq. 9.6.2.1 0.1-1:

Dclear ≥ 1.25 Dp   (Eq. 9.6.2.1 0.1-2)
Dclear = 2c1	(	1 +	hp c2	)
			bp c1

where

hp = the height of the rectangular glass,
bp = the width of the rectangular glass,
c₁ = the clearance (gap) between the vertical glass edges and the frame, and
c₂ = the clearance (gap) between the horizontal glass edges and the frame.
• Fully tempered monolithic glass in Seismic Use Groups I and II located no more than 3 m (10ft) above a walking surface shall be exempted from the provisions of Eq. 9.6.2.10.1-1.


• Annealed or heat-strengthened laminated glass in single thickness with interlayer no less than 0.76 mm (0.030 in.) that is captured mechanically in a wall system glazing pocket,and whose perimeter is secured to the frame by a wet-glazed, gunable curing elastomeric sealant perimeter bead of 13 mm (0.5 in.) minimum glass contact width, or other approved anchorage system, shall be exempted from the provisions of Eq.9.6.2.10.1-1.

9.6.2.10.2 Seismic Drift limits for Glass Components Δfallout, the drift causing glass fallout from the curtain wall, storefront or partition, shall be determined in accordance with Ref. 9.6-20, or by engineering analysis.

Ref 9.6-20 American Architectural Manufacturers Association (AAMA),501.4-00.

Discussion
In essence, Eq. 9.6.2.10.1-1 requires the resistance to glass fallout of an individual glass panel be greater than the relative seismic displacement the component must accommodate as a result of being attached to the primary structural system of the building. In the absence of special drift-accommodating connections between the main building frame and the curtain wall framing members, this relative seismic displacement demand is governed by the calculated seismic inters tory drifts for the specific building being designed for site-specific earthquake conditions.

Two comments apply to the ASCE 7-02 provisions cited above:

• The term D_clear in Eq. 9.6.2.10.1-2 is the relative horizontal displacement between the top and bottom of the glass panel under consideration causing initial glass-to-frame contact. For rectangular glass panels within a rectangular wall frame, an expression for D_clear is given below Eq. 9.6.2.10.1-2. (Note: D_clear is not a glass-to-frame clearance dimension, but the amount of horizontal racking displacement causing initial glass-to-frame contact.)


• Ref. 9.6-20 in the ASCE 7-02 provisions will be revised in future editions of ASCE 7 to read: "AMAA 501.6-2001, Recommended Dynamic Test Method for Determining the Seismic Drift Causing Glass Fallout from a Wall System." This upcoming revision in ASCE 7 is appropriate because AAMA 501.6-2001 is the specific laboratory test method for determining Δ_fallout, the drift causing glass fallout from the curtain wall, storefront, or partition. Reference to AAMA 501.6 is made in AAMA 50 l.4, so the design provisions in ASCE 7-02 are correct in their present form. (For simplicity, future editions of ASCE 7 will make direct reference to AAMA 501.6 rather than the current indirect reference to AAMA 501.6 via AAMA 501.4.)

Seismic test methods
As mentioned above, two seismic test methods (AAMA 501.4 and AAMA 501.6) have been developed by a multi-year consensus process and published recently. AAMA represents all segments of the architectural system and component industry, including wall systems, windows, doors, and skylights. In addition to sponsoring the development and publication of these documents, AAMA also accredits test laboratories to perform these tests.

AAMA 501A-2000, Recommended Static Test Method for Evaluating Curtain Wall and Storefront Systems Subjected to Seismic and Wind Induced Interstory Drifts, is a static racking test method focusing more on the seismic serviceability of curtain and storefront wall systems. AAMA 501.4 contains the following test activity sequence:

• Baseline air leakage (ASTM E 283-91, Standard Test Method for Determining the Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen); static water resistance (ASTM E 331-00, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference); and structural performance at design wind pressure (ASTM E 330-02, Standard Test Method for Structural Performance of Exterior Windows, Doors, Skylights and Curtain Walls by Uniform Static Air Pressure Difference).


• Static racking displacement using a design-specific horizontal racking displacement or a default value of 0.01 x story height.


• Repeat tests in Step A to evaluate changes in wall system serviceability.


• 1.5 x static racking displacement in Step B.


• For Δ_fallout tests, Go to AAMA 501.6.

In contrast, AAMA 501.6-2001, Recommended Dynamic Test Method for Determining the Seismic Drift Causing Glass Fallout from a Wall System Panel, is a dynamic racking test method focusing on the seismic safety of architectural glass components within curtain and storefront wall systems. Essentially, the AAMA 501.6 test involves mounting individual, fully glazed wall panel specimens on a dynamic racking test apparatus, which moves back and forth in sinusoidal motions at gradually and progressively higher racking amplitudes, as in a musical crescendo (Figure 1).

Dynamic racking frequencies are 0.8 Hz at lower racking amplitudes (<±75 mm [3 in.]) and 0.4 Hz at higher racking amplitudes (>±75 mm). The racking amplitude at which glass fallout is first observed for a given specimen is designated as Δ_fallout for that test specimen. The lowest value of racking displacement causing glass fallout for the three specimens tested in accordance with AAMA 501.6 is the reported value of llfallout for that particular wall system glazing configuration. This value of Δ_fallout is the one used in the ASCE 7 -02 seismic design provisions for architectural glass (Eq. 9.6.2.10.1-1 above).

Conclusion
Developed within NEHRP, seismic design provisions for architectural glass, were adopted in ASCE 7 and are now referenced in IBC 2003 and NFPA 5000 Building Code. These new provisions make reference to a customized laboratory test method developed and published by AAMA. Along with the companion laboratory test method, they increase the level of design attention paid to seismic life safety issues associated with architectural glass components in earthquake-prone regions.

References

• AAMA 501.6-01, Recommended Dynamic Test Method for Determining the Seismic Drift Causing Glass Fallout from a Wall System. Schaumburg, IL.
• ASCE 7, Minimum Design Loads for Buildings and Other Structures. Reston, VA.: 2002.
• International Building Code. Falls Church, VA.: 2003.
• FEMA 368, The 2000 NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures. Washington, D.C.: 2001.
• NFPA 5000 Building Code. Quincy, MA.: 2002.

Note
¹ The Occupancy factor assigns a higher level of required earthquake resistance for critical structures, such as hospitals, fire and police stations, emergency operations centers, etc.

Additional Information
Authors Richard A. Behr, Ph.D., PE, is head of the Architectural Engineering Department at Pennsylvania State University. Scott A. Warner is the executive vice president of Architectural Testing, Inc. and chair or AAMA's Methods of Task Group. They can be reached at (814)865-6394.
MasterFormat No. 08050-Basic Door and Window Materials and Methods General Data-Standards UniForat No. B2020-Exterior Windows B2020-Storefronts B2020-Glazed Curtain Walls B2030-Exterior Doors	Key Words Division 8 American Architectural Manufacturers Association American Society of Civil Engineers Architectural glazing ASTM International Building codes	Federal Emergency Management Agency International Code Council Laboratory testing National Earthquake Hazard Reduction Program NFPA
Abstract Significant changes in the way architectural glass components are designed and tested for earthquake resistance are here. Designers, specifiers, and glazing system manufacturers should take note, as new building code provisions and industry-accepted test methods have been published by a number of code bodies, including the ICC, ASCE, and NFPA.