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Volume 41,   Issue 2       February 2006


State Your Case

Three Case Studies about Curtainwall Designs for Wind and Blast

by Charles D. Clift, F.ASCE

Current design for curtainwalls subjected to wind, windborne debris and blast contain relatively new structural requirements that the curtainwall designer must address. This article offers a behind the scenes look at three completed glass cladding projects designed and built in accordance with the new criteria. It also focuses on the structural aspects of these custom curtainwall systems affected by the new requirements. The three projects available as case studies have different site conditions, different combinations of design criteria and distinct curtainwall designs. This project diversity and its impact on the criteria of the resulting designs cannot be compared directly and it is difficult to offer general conclusions. Hence, commentaries on the impact of the new criteria on individual designs are necessarily qualitative in character.

Design Criteria for Wind

The controlling design criteria for curtainwalls often relates to windload and associated mullion deflection limitations. Therefore, it is critical that the curtainwall practitioner be well-versed in wind engineering. Application of wind engineering to building structures is a relatively new field that has matured only in the last half of the 20th century. During the tall building boom days, boundary layer wind tunnel studies were produced frequently and provided precise design criteria for individual projects. More recently, exterior cladding design loads have received a lot of scrutiny due to extensive damage from extreme wind events caused by hurricanes along the Atlantic and Gulf Coasts.

Most currently applicable building codes are based on the International Building Code (IBC) (ICC 2993). The IBC incorporates ASCE 7-02 (ASCE 2003) as a basis for wind design. American Society for Testing and Materials (ASTM) Standards E 1300 (ASTM 2004) and several American Architectural Manufacturers Assoc-iation (AAMA) standards, appropriately referenced in the IBC and local building codes, address design requirements for window glass and glazing systems.

Relevant to the present article, are ASCE 7 method 2 (Buildings of all Heights) and method 3 (Wind Tunnel Procedure) that are prescribed in ASCE 7. Discussion of the case studies will show that the wind tunnel procedure outlined in method 3 may produce smaller design loads for curtainwall systems than the analytical method prescribed in method 2. Glass thickness selections for various types of window glass are determined through the use of ASTM E 1300. Mullion deflection limitations are contained in AAMA (1996).

Design Criteria for Windborne Debris

While there currently are more low-rise projects under construction in the United States compared to the number of tall building projects of past years, attention to wind design criteria has actually increased. This attention is due primarily to debris impact damage that occurs to lower levels of buildings during hurricanes. Building codes in coastal areas have added specific requirements for debris impacts and cyclical pressure testing on glazing systems. The IBC and ASCE 7 incorporate debris impact requirements from ASTM E 1886 and E 1996 (ASTM 2003a; 2003b) that involve large missiles (2 x 4 timbers) and small missiles (representing roof gravel). 

From a materials perspective, the impact standards have caused significant increases in the use of laminated glass and structural silicone glazing. These materials can absorb sufficient energy to provide post-impact containment and stability necessary to protect interior conditions.

Design Criteria for Blast

The most recent focus regarding curtainwall design criteria is not that of natural events, but rather those that are man-made. Blast loading has quickly taken center stage, particularly on government projects, and many changes continue to develop regarding blast loads on glazing systems. One method in current use that contains specific design criteria for curtainwall systems incorporating laminated glass is outlined in ASTM F 2248 (ASTM 200a). Some interesting observations relating to blast effects can be made using high-performance glazing systems designed for extreme wind loads and windborne debris impacts.

Conclusions

Proper application of wind engineering to building cladding requires some specific experience. Projects in hurricane regions require fairly stout design and may be capable of satisfying moderate blast loads. Wind tunnel testing remains a viable option to define design load criteria. Even low-rise building projects in high-velocity wind zones can benefit from wind tunnel modeling due to more precise and often reduced pressure values. As architectural imagination creates innovative cladding designs, the challenge of engineering curtainwall systems will remain a healthy pursuit.

References

AAMA, Aluminum Curtainwall Design Guide Manual, CW-DG-1-96, American Architectural Manufacturers Association, Des Plains, IL, 1996.

ASCE, Minimum Design Loads for Buildings and Other Structures, ASCE 7-02, ASCE, Reston, VA, 2003.

ASTM, Standard Practice for Determining the Load Resistance of Glass in Buildings, ASTM E1300-04, Annual Book of ASTM Standards, ASTM International, W. Conshohocken, PA, 2004.

ASTM, Standard Practice for Specifying an Equivalent 3-Second Duration Design Loading for Blast Resistant Glazing Fabricated with Laminated Glass, ASTM F2248-03, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 2003.

ASTM, Standard Test Method for Performance of Exterior Windows, Glazed Curtainwalls, Doors and Storm Shutters Impacted by Missile(s) and Exposed to Cyclic Pressure Differentials, ASTM E1886-02, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 2003a.

ASTM, Standard Specification for Performance of Exterior Windows, Glazed Curtainwalls, Doors and Storm Shutters Impacted by Windborne Debris is Hurricanes, ASTM E1996-02, ASTM International, West Conshohocken, PA, 2003b.

ICC, International Building Code, International Code Council, Inc., Falls Church, VA, 2003.


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