Curtain Wall Failure Overview

By: Zed Hott PSU AE M.S. Student Fall 2011

Figure 1: A picture of a curtain wall system. (Hodan, 2011).

Curtain walls, as defined by the Whole Building Design Guide (WBDG), is a wall which does not carry floor or roof loads. An image of a typical curtain wall section can be seen in Figure 1. These walls transfer wind loads and its own gravity load back to the structural support of the building. Curtain walls are typically thin, aluminum framed, with glass panels or thin stone, which hang on the structural element of a building like a "curtain," hence the name. They first became popular in construction around the end of World War II, because after the war aluminum was no longer scarce and tall building designs became more popular. Since the design became so predominant in construction, the most common types of failures associated with these systems are water damage, glass damage, poor thermal and visual performance, construction, and design errors. This article will discuss the most common failures, which are water damage, glass damage, and design and installation errors, as well as show examples of where these failure occurred, and provide information on how to prevent future failures.

Moisture Damage

The most common type failure in curtain wall sections is moisture damage. Damage from water infiltration includes premature deterioration of the wall structure, finish damage, mold and mildew, and decreased interior air-quality (Schwartz, 2001). Repairs from poor water protection are often expensive to fix, but can be easily prevented in initial design and construction of the curtain wall. Moisture damage can be classified into two distinct parts. The first being water infiltration while the second is condensation.

Water infiltration creates the largest potential for moisture damage in a building. Sealants are the primary use of preventing water penetration into a curtain wall; however, sealants can break apart. Poor adhesion can cause the sealant to break away. The thermal expansion coefficient of aluminum is 2.5 times that of glass, the large relative displacements often cause sealants to break. A perfectly water-tight curtain wall cannot be maintained by sealants alone. The general rule of thumb when concerning water penetration in curtain walls is not to rely entirely on sealant. Sealants are not perfect. Sealants break, pull away, or can not be installed correctly. The best way to avoid water damage is to have redundancy in water protection, by incorporating an in-wall drainage system along with careful consideration of a proper sealant. Please see Figure 2 to see proper waterproofing techniques.

In water repair situations, an engineer should consider adding a second line of defense, if possible. In order to properly ensure an effective curtain wall design, laboratory and field mock-ups are necessary based on American Architectural Manufacturers Association (AAMA) tests 501.1, AAMA 501.2, ASTM E331, ASTM E547, ASTM E1105 (Viegener and Brown, 2010). Performance in these tests allows the engineer to see problems which they may not have thought of, and allow them to make a simple, significantly cheaper fix; when compared to fixing the problem once the curtain wall had been installed. Building owners are also responsible for preventing some forms of water penetration damage. Curtain wall sealants are only designed to last 10 to 15 years; therefore, regular maintenance and upkeep of sealants is a must to prevent damage.

Figure 2: Proper waterproofing of curtain walls. (Based on Kosis, 2011).

Condensation occurs when the temperature of the glass or aluminum frame in a curtain wall reaches the dew point temperature of the interior space conditions. Water forms on the surface of the glass or aluminum, and can cause damage to the unit. Basic design against condensation in ensuring the Condensation Resistance Factor (CRF) of a given curtain wall section meets the requirement of the space, which is based off of the expected temperature and humidity of the space. Designers should be aware the CRF is an average, and can not account for cold spaces in the facility which can cause localized condensation. The WBDG states, when designing a curtain wall glass unit in areas where high humidity is required within the space (such as hospitals) or where configurations are abnormal, software modeling is a must, to ensure condensation does not occur. The WBDG also states laboratory tests simulating indoor and outdoor air temperatures and humidity of the space is good practice to see how a glass panel will perform. Specified tests are AAMA 1503.1 and National Fenestration Rating Council (NFRC) 500 (Viegener and Brown, 2010). A great way to prevent condensation in frames of curtain walls is to use thermally broken aluminum. Thermal breaking is where a piece of plastic is incorporated in the frame, which significantly decreases the heat flow in (or out) of a curtain wall. This reduction of heat flow raises the surface temperature of the aluminum, and decreases the possibility of condensation on the aluminum. Another prevention which can be incorporated in design is limiting the amount of non-thermally-broken aluminum exposed to exterior conditions (Viegener and Brown, 2010).

Many mistakes have been made in curtain wall waterproofing design in the past. Being informed of proper waterproofing techniques and learning from past mistakes can help prevent future waterproofing failures. The following case studies provide examples of past water damage failures in curtain wall sections.

Case Study: The Johnson Wax Research Tower, WI

A prime example of a curtain wall demonstrating poor waterproofing is The Johnson Wax Research Tower in Wisconsin. This tower was designed by Frank Lloyd Wright, and constructed in 1939. The facade features large amounts of Pyrex tubing. Upon initial construction a sealant made from lead oxide was incorporated. The sealant did not adhere properly to the Pyrex, which caused the building to leak drastically. For the next 20 years, many different sealants were applied, all ending with the same results, a leaky building. In 1952 a specially designed sealant pulled away from the structure exposing large amounts of glass joints. About 40 years after initial construction, in 1958, a silicone rubber sealant was put in place in the joints of the building, and the building stopped leaking (Feld and Carper, 1997). From this failure we can learn the importance of properly selecting sealants for curtain wall sections, because improper selection can lead to multiple failures.

Case Study: St. Claire's Hospital, WI

St. Claire's hospital, located in Weston, Wisconsin, finished construction in winter of 2005. Shortly after initial occupation, the windows, which were insulated glass units, began to show condensation. The interior of the hospital had to be humidified in the winter, for patient care and equipment safety. An investigation by WJE found failed sealant in the window units, which allowed the warm moist air to come in contact with the cold glass, which caused condensation. The repair for this involved a quick replacement of windows, with proper sealant and considerations for the higher humidity in the interior of the building (WJE, 2005). From this failure we can learn the importance not only of proper sealant, but proper consideration of interior room temperatures and humidity in design of curtain wall sections. Figure 3 shows a similar failure in a window unit in a building on the Penn State campus.

Figure 3: Condensation inside a window on Penn State's campus. (Taken by Hott, 2011).

Glass Failures

Glass failures in curtain walls can be split up into several different categories. Nickel Sulfide (NiS) inclusions, thermal cracking, and damage from impact are the most common types of glass damage. NiS inclusions, also known as "glass cancer", are imperfections incorporated in the glass when it is manufactured. NiS remains at high temperatures, after the rest of the glass has cooled. After the NiS cools, the inclusions exapand in volume and crack the glass. This effect is most commonly seen in tempered glass. In order to stop NiS inclusions from cracking in a curtain wall, the engineer should consider not using tempered glass, or perform a heat soak test (Gromowski, 2010). To read more on NiS inclusions, please see the WikiFailure report specifically on NiS inclusions.

Thermal cracking of glass is another concern which the engineer should consider when designing a curtain wall. Thermal cracks occur in the glass when large temperature differences in the glass cause high stresses within the pane, forcing the glass to crack (Chowdhurt and Cortie, 2007). Thermal cracks are easy to detect because they perpendicular to the frame and usually expand the whole window section (McCowan and Kivela, 2011). Please see Figure 4, an image of a thermal crack found in a investigation by engineers at SGH. Failures are more likely to occur when an absorptive coating is placed on the glass. These coatings are put in place to reduce the cooling load of the building, but can come at a cost to the glass integrity because they absorb solar radiation and keep it stored in the glass. The stored energy increases the temperature of the glass, and can cause it to expanded unevenly, creating a crack. The more effective (ie more sun absorbed) the more likely the glass is to crack. If the glass support allows some movement, the likeliness of a thermal crack occurring decreases slightly. In order to properly design for absorptive coatings, an engineer should consider the stresses which will be induced in the glass by the sun and the coating and see if the stresses will likelycause the glass to crack. Also, an engineer can specify the use of heat strengthened glass, a stronger form of glass, which can take higher thermal stresses. Other alternatives such as a reflective, not absorptive coating should be considered, if applicable.

Figure 4: A thermal crack in a curtain wall section. (McCowen and Kivela, 2011). Photo Credit: SGH

Figure 5: An image of the CNA Tower in Chicago, IL.(Dschwen, 2011)

Case Study: CNA Tower Chicago, IL

On October 8th, 1999 a jagged shard of glass fell from the sky from the CNA Tower in Chicago, IL, seen in Figure 5. Tragically, a woman was killed a from the falling piece of glass. The window which fell had been cracked for several months without being replaced. This was also not the first time a pedestrian had been struck by falling glass. Since the building's construction in 1973, cracked windows have always been a problem; within the first three years after the building erection, approximately 100 windows were reported cracked, and as time passed, more windows showed damage. These cracks were developed from thermal stresses in the glass. After the incident, CNA decided to replace all 2900 windows in the tower with a heat strengthened alternative, at an approximate final cost of 5 million dollars (Kiernan, 2000).

Design and Installation Issues

Unfortunately, another common type of failure in curtain walls is design and installation errors. Engineers make decisions on curtain walls without understanding the full consequences or failure scenarios on the curtain wall or construction teams do not fully understand how to properly install the curtain wall. In order to prevent these failures, a structural engineer or other design professional should be fully informed and experienced on site. Also, construction teams should have a full understanding of the importance of proper installation of curtain walls. These types of failures can vary from improper installation of sealant which causes the building to leak, to collapses of curtain wall sections.

Case Study: 1983 Hurricane Alicia, Houston, TX

This case study incorporates two types of failures mentioned in this article, damage from impact and design considerations which were neglected. In August of 1983 Hurricane Alicia hit downtown Houston. After the storm passed, thousands of glass units in curtain walls had shattered. One theory which arose was excessive deflections and pressure from the wind; however, this confused many engineers because the wind speeds of the hurricane were believed not to have exceeded design speeds. Data from the weather station at the Houston airport confirms wind speeds probably did not exceed design speeds. The ensuing investigation showed the most probable cause of failure was small missile impact on the glass. The roof gravel from the surrounding buildings became projectiles in the wind, and shattered the glass. Small pieces of gravel found on upper floors on taller skyscrapers indicate the gravel as the most likely projectile (Beason, et al, 1984). In some locations, codes now control the size and use of roof gravel and roof ballast. This failure shows the importance of impact resistant glass, such as laminated glass as well as how design considerations for impact from projectiles, especially in hurricane areas, is necessary.

Case Study: Manhattan Curtain Wall Collapse

In February 2003 during a heavy snowstorm, a 30 ft by 30 ft louvered curtain wall fell from a circa 1960 construction curtain wall section. Thankfully, no one was injured. Thornton Tomasetti headed the investigation of the failure. Several causes of the collapse soon developed after the investigation. Since this was a relatively "early" curtain wall design, the need for locking connections was not established at the time of construction. Fatigue in the remaining standing facade showed these connections were initially installed, but became loose after years of wear. Nearly 2000 connections were inspected and replaced if necessary. After reconstructing the fallen section, engineers discovered bolts which had failed due to fatigue, but could not directly tie the failed bolts to the collapsed section. The final conclusion was the section failed because omitted mullion clips. The reason for the clips being omitted in this section is still not known. The most popular theories include field changes or misalignment during installation. Repairs were straightforward, rebuilding and ensuring proper installation techniques, and was completed by the end of 2003 (Gonchar, 2011).

Conclusions and Lessons Learned

From the descriptions of failures faced in curtain walls and case studies discussed in this report, several different conclusions can be inferred. First and foremost, an engineer or curtain wall design professional should be involved in all aspects of curtain wall design and construction. These professionals should be aware proper techniques to prevent failures such as waterproofing issues, glass failure issues, installation issues, poor visual performance, and poor thermal performance. Appropriate laboratory and field tests should be performed on all curtain wall sections to ensure proper performance. These tests also ensure any mistake is caught early on in the project, when changes to design to solve problems are less costly. Curtain wall failures can be prevented by proper consideration of potential failures and ensuring proper installation and maintenance of curtain wall sections.

Annotated Bibliography

Reference List
Beason, W. Lynn, Meyers, Gerald E. and Ray W. James. (1984.) "Hurricane Related Window Glass Damage in Houston." Journal of Structural Engineering, Volume 110 Issue 12. p. 2843-2857.
This article describes in detail the events of Hurricane Alicia in 1983 and how damage occurred to curtain wall structures, even though wind speeds were less than design wind speeds. The article mentions potential requirements for code changes to prevent future failures.

Chowdhurt, H., and Cortie, M.B. (2007). "Thermal stresses and cracking in absorptive solar glazing." Construction & Building Materials 21, no.2:464-468.
This journal article explains how thermal stresses occur in glazing sections. Specifically this experiment tested if absorptive coatings in glazing create more cracking potential than reflective coatings; however, it explains how thermal cracking happens in large glass sections.

Feld, Jacob and Carper, Kenneth L. (1997). "Nonstructural Failures." Construction Failures, 2nd ed., John Wiley & Songs Inc., p 377-378
This book provides an excellent mini-case study concerning water penetration, the Frank Lloyd Wright design of The Johnson Wax Tower in WI. Another case study concerning structural failures in a Houston Hurricane in 1983 may also be used. This book describes in detail the imroper use of sealants, and how thermal differences can cause sealants to fail. It also provides suggestions on how to properly prevent water damage.

Dschwen. (2011). "CNA Center's Unusual Red Exterior." Ookaboo: Free Picture of Everything on Earth. <> (Visited October 25, 2011)
This website provides a copyright free image of CNA tower which can be used in this report.

Gonchar, Joann. (2011). "Sleuthing the Mundane and the Catastrophic." Architectural Record Reading. The McGraw-Hill Companies, Inc.
The first part of this article provides an excellent case study of a installation error on a curtain wall in Massachusetts. The study was conducted by Thornton Tomsetti, and was determined a clip was omitted during construfction, which along with high wind loads, caused the structure to fail.

Gromowski, Kathryn. (2010). "Glass Breakage-Nickel SUlfide Inclusions." Failures Wiki Building, Architectural, and Civil Engineering Failures and Forensic Practices. <>(Visited September 19, 2011).
This wiki failure report covers failures in glasss from Nickel-Sulfide imperfections. It details glass manufacturing processes, where NiS failures are introduced, and how to prevent or test for potential failures. This report also has several, small, case studies on NiS failures.

Hodan, Jiri. (2011). "Glass Wall." Public Domain <> (Visited October 24, 2011).
This public domain website has a great picture of a curtain wall glass section, and does not require permission to use the photo.

Kiernan, Louise. (2000). "CNA Will Replace Tower Windows." Chicago Tribune.30 September 2000. <> (Visited September 19, 2011)
This newspaper article, from 200, provides a background on the CNA Tower, located in Chicago, which had glass fall-out from thermal cracking. It also goes on to explain how the windows were replaced with heated treated windows, which do not crack.

Kosis, Alex. (2011). "Waterproofing 101." Simpson, Gumpertz, and Heger Presentation to AE 537: Building Failures and Forensic Techniques Course at Penn State University. September 29, 2011.
This presentation by Alex Kosis has excellent figure on water-proofing of curtain walls. His presentation also showed the best configureation for curtain walls to eliminate water penetration.

McCowan, Derek B. and Kivela, Joshua B. (2011) "Lessons Learned From Curtain Wall Failure Investigations" Simpson, Gumpertz, and Heger Report, 2011.
This report from two SGH employees has excellent examples and illustration of curtain wall failures, specifically thermal cracking in an IGU.

Schwartz, Thomas A. (2001). "Curtain-Wall Fundamentals." Association for Preservation Technology Internation (APT) Bulletin, Vol. 32, No.1. p37-45.
This article provides a brief summary of the types of failures faced in curtain walls, with the most detail on water-damage and thermal stresses. It also provides interested case studies. Most importantly, this article provides a summary of prevention techniques for curtain wall failures.

WJE. (2005). "St. Claire's Hospital: Condensation Investigation and Repair Design." WJE Project Summary. Wiss, Janney, Elstner Associates, Inc. 2004-2011.
A case study provided by WJE, which provides basis for a mini-case study about condensation failures in curtain walls.

Vigener, Nik and Brown, Mark A. (June 7, 2010). "Building Envelope Design Guide-Curtain Walls." Whole Building Design Guide. <> (Visited September 19, 2011)
This article provides a basic definition of a curtain wall, as well as the purpose of a curtain wall. Design considerations concerning condensatioin and water penetration are also mentioned. Structural elements of the curtain wall are detailed in the appendix of this webpage.