Glass Design for Sloped Glazing Undergoing Major Renovation

Changes in building code requirements, design practices, available glass products and industry standards complicate the growing use of these large and complex systems to make definitive architectural statements and provide exciting focal points for new commercial buildings.Design criteria to fulfill this mission have been in a state of rapid and continuous creative development for some time, originally prompting AAMA to develop its Glass Design for Sloped Glazing (GDSG) manual back in 1987.Over the intervening years, the performance and selection of glass have become major and critical factors. Emphasis is placed on product requirements for specification performance, energy conservation, daylighting and the safety of building occupants.

In recognition of the accelerating evolution of the products and their application, AAMA is now in the final stages of updating the GDSG document. The pending new edition outlines the design considerations necessary for choosing the proper glass for nonresidential skylight and sloped glazing applications based on the best industry practices and technology, and describes the minimum requirements for sloped glazing as specified in the International Building Code (IBC).

The manual is divided into three major sections.

Section 1: Building Code Requirements for Skylights and Sloped Glazing

This section provides an overview of the scope of the IBC as it relates to skylights and sloped glazing. It references ASCE/SEI 7-10, Minimum Design Loads for Buildings and Other Structures, although noting that this standard may not be useful in defining loads for sloped glazing and skylights or for buildings of unusual shape or geometry. Wind tunnel testing is often the alternative.

 

The new version of GDSG also adds hurricane loads to the general discussion of the types of loads to which sloped glazing may be subjected.

Section 2: Design Considerations

Fundamentally, the responsible design professional should be aware of the potential for a glass fracture and what can happen should this occur. Consequently, the selection and design of the glass for skylights and sloped glazing with the required properties to meet project conditions begins with reference to ASTM E 1300-12a, Standard Practice for Determining Load Resistance of Glass in Buildings. This Standard considers the combination of snow load, wind load, glass slope and glass weight and determines if the proposed glass type will meet the specified load. It covers rectangular annealed, heat-strengthened, fully tempered and laminated glass, supported on any number of edges including glass clamped at one edge, and simply supported on two, three or four edges (insulating glass units are covered for four-edge support only). Guidelines for glass supported at points are not included in ASTM E 1300-12a; however, guidelines relating to point supported glass are addressed in the Design Considerations section of the GDSG manual.

This section features an overview of glass performance factors – strength characteristics, resistance to surface damage, impact from wind-borne materials, load duration, thermal stresses, edge strength, deflection of framing members and control of infiltrating water or condensation. It goes on to point out that designs for sloped glazing systems are often more complex than for vertical glazing because:

• Sloped glass is more susceptible to impact from falling objects, wind borne debris and missiles than vertical glass.




• Sloped glazing in most cases is more likely to fall from the opening when it breaks than vertical glass.




• Snow loads, unlike wind and live loads, may be imposed on the glazing for long periods.




• For most orientations, the temperature sloped glass may reach is usually higher than for vertical glazing due to the sun’s radiation being more normal to the glass surface and the stratification of warm air in the interior’s space. Consequently, the thermal stresses created in the glass most often require heat treated glass (heat-strengthened or fully tempered).

The manual also recognizes that for some projects, good design practices for skylights and sloped glazing may be to meet requirements exceeding those in the codes. There are also some considerations not part of the building code, such as those related to public safety, replacement of glass, energy efficiencies, aesthetics, potential property damage and loss of building use, etc.

Section 3: Design Data

The prediction of the performance of glass in sloped glazing is typically more complex than for vertical glass due to the variety of loads, mixture of glass types and asymmetry of glass shapes. The strength of glass exposed to transient and static loads must be analyzed on a statistical basis. This is typically expressed as: (a) average uniform load resistance for a specified glass type, size and thickness and (b) the coefficient of variation (a measure of the distribution of the glass strength for a large number of lites).

The GDSG document outlines the typical methods for determining overall loads due to a combination of sources, and provides a series of graphs to help in determining equivalent combined load based on glass area and type. Also included are guidelines for designing support frames for sloped glazing.

Next Steps

The Glass Design for Sloped Glazing Task Group (Chair: Paul Bush [PPG]),in conjunction with well-recognized industry consultant Bill Lingnell, has drafted most of the changes judged to be necessary. Open issues still to be resolved as of the 2014 Annual Conference were:

• Addition of a general safety statement




• Addition of criteria for greenhouses, confirming a ridge height of 20ft above grade




• Addition of references to the Florida Building Code HVHZ and /or Miami Dade TAS 201, 202, 203




• Inclusion of an explanation of differences between design wind loads as defined in ASCE/SEI 7-10 vs. the 2005 version




• Acknowledgment of AAMA TIR-A11 for maximum allowable deflection for framing members

Additional technical work remains. The GDSG Task Group has been convening a series of conference calls and meetings at AAMA events to this point and, in order to accelerate substantive progress, is now planning to meet directly with Lingnell in the next few months to consider additional information.

The estimated completion date for the comprehensive manual revision is September 2014, which allows for a new ballot of changes prior to the AAMA Fall Conference.