During the mid-eighties, the Amoco Building--formerly known as Standard Oil and now known as the Aon Center--suffered a major failure of its Italian Carrara marble facade. The eighty-story, 1,136-foot Chicago building is supported by a steel structure and is entirely clad in white Carrara marble. The marble was shipped from Italy precut into 50"x45" panels which were bolted to the steel structure.

The Amoco/Aon Center [Photo by J. Crocker]

Photo Credit - J Crocker

Shortly after the building's completion in 1974, engineers noticed that the marble panels were starting to buckle outward. By 1988, thirty percent of the 44,000 panels had deflected outward more than 1/2", with some bowing out as much as 1-1/2". Aside from the bowing, a few panels actually detached from the steel frame, with one of them even punching through the roof of a nearby building.

Thorough inspection revealed widespread cracking and bowing throughout the facade of the tower. It was concluded that the 1-1/4" thick panels were simply too thin for Chicago's infamously severe weather conditions. Fluctuating temperatures had caused the panels to expand and deflect permanently, leading to significant reductions in the marble's strength. Though it was concluded that the anchoring system was not to blame for the cladding failure, it was upgraded anyway during the eventual renovation of the tower's facade.

The cladding's major safety and aesthetic issues finally prompted an $80 million renovation, approximately half of the original cost of the building. The problematic marble panels were replaced with Mount Airy white granite, which can still be seen on the building today. Amoco's litigation against the original designers and contractors of the building was eventually settled with the results kept confidential.

Design Team

Amoco Properties, Inc.

Wiss, Janney, Elstner Associates, Inc.
Chicago, Illinois
Schal Bovis
Chicago, Illinois
CONTRACTOR: Stone Setter
Dayton, Ohio
CONTRACTOR: Insulation
Insulation Services
Chicago, Illinois
CONTRACTOR: Hoist Towers
USA Hoist
Countryside, Illinois

Background: Thermal Hysteresis

Amoco Building's New Granite Facade [Photo by J. Crocker]

In his book Failed Stone, Patrick Loughran describes hysteresis as the "changed response to an object due to a given influence, which leaves a permanent record of how the object was influenced." Put in simpler terms, it is the permanent altering of an object's physical properties due to a certain influence (usually some kind of force). In the case of thermal hysteresis, this influence is exposure of the object to large temperature variations.

An appropriate example of such variation is the fluctuating temperatures building facades experience as they undergo solar heating throughout the day. When the cladding involved consists of thin-stone veneers (such as white Italian marble), the effect of the differential thermal cycle is permanent deformation of the material. This deformation is magnified by the fact that the two faces of thin-stone panels experience different temperature gradients, as one side is facing the sun and the other is facing the interior of the building--which is normally air-conditioned. Aside from the effect on the facade's appearance, thermal hysteresis also negatively affects the material's strength. According to Loughran, stone panels undergoing years of thermal cycles can "degrade to the point of becoming a block of loose granular material," not exactly the type of material suited for resisting the high wind loads associated with Chicago skyscrapers [Loughran].

Exterior Cladding

Original Design

Edward Durell Stone, the Amoco Building's architect, had insisted that white Italian marble be used for the entire facade of the building. The panels themselves were only 1-1/4" thick--a relatively thin panel for the extreme weather conditions experienced in the city of Chicago. As for the anchorage into the structural system, the panels were attached using two galvanized steel clip angles and a stainless steel bent plate angle running along the width of each panel. The marble panels were connected to the angles via non-continuous lateral connection tabs.

Original & New Panel Designs [AutoCAD Drawing by Dan Navarrete]

New Design

After the failure of the original Italian marble, the Amoco Building was reclad in 2" thick Mt. Airy granite. Aside from the increased thickness, the new facade was made of a more durable material: the granite has more strength than the original marble. With the extra weight of the new cladding, the steel connection pieces used to attach the granite to the main structure were upgraded to continuous stainless steel angles with continuous lateral connection tabs. A continuous connection allows for even distribution of the load along the whole edge and eliminates stress concentrations that result from a non-continuous connection.

Causes of Cladding Failure

As a proactive measure, Amoco decided to start periodically checking the marble cladding for signs of wear and tear. In 1985, even before any problems were noticed, Chicago-based firms Wiss, Janney, Elstner Associates (WJE) and J.M. Logan and Associates (JML) began inspections of the Carrara marble panels, but did not find any major bowing or cracking. However, after two years of regular inspection, they discovered that many of the panels had deflected out as much as 1-1/2" from the original flush position.

At this point, the bowed panels were mainly an aesthetic problem, says Ian Chin of WJE. Nevertheless, according to Chin, the bowing and cracking might have meant a reduction in the strength of the marble. And so, each of the 44,000 marble panels were secured with two stainless steel straps as a precautionary--and temporary--measure until a more permanent solution could be developed and agreed upon. According to Chin, the calcite crystals that compose marble expand and contract differently in all directions as they undergo extreme temperature changes like the kind commonplace in Chicago [Hook]. On the south and east elevations, and somewhat on the west, this effect is magnified by the direct sunlight that warms the panels to an even greater extent. As the exterior face of the marble heats in the sun, it expands more than the interior face, causing the substantial deflections seen during inspection. So great was the extent of the bowing, that the crystalline structure of the marble permanently deformed, thus preventing the panels from returning to their original shape--an effect known as thermal hysteresis.

Thermal Gradient & Deflection in Marble Cladding [Diagram by Dan Navarrete]

Various field and laboratory tests undertaken by WJE confirmed what Ian Chin had hypothesized: the marble panels had lost nearly half of their flexural strength due to weathering over the 15-year life of the Amoco Building. This decrease in strength was greater than the loss assumed by the Marble Institute of America when settling on their factor of safety. The tests also found that the greater the deflection on the panel, the higher the loss in its strength [Loughran].

Recommendations and Remedies

After extensive testing and assessment, WJE recommended that Amoco replace every single marble panel with a stronger material. From their experience with the original facade, it was evident that Carrara marble was simply not suited for the Chicago climate--specially with the 1-1/4" thickness. The owners and the engineers collaborated in evaluating different materials as replacement cladding for the building. They looked at everything from aluminum panels to other types of stone, such as limestone and granite. Finally, they decided on a certain type of white granite from North Carolina called Mt. Airy granite, named after the area where it was quarried.

The white granite was a fitting aesthetic replacement for the white Italian marble. As a material, Mt. Airy granite has a higher initial strength than marble; furthermore, it does not lose as much of its strength over time. But as an added precaution, the designers specified a 2" thick granite panel as opposed to the 1-1/4" thickness of the marble. Although tests revealed that the cladding failure was in no way due to shortcomings in the anchoring system, engineers decided to upgrade the connectors anyway because of the additional weight of the granite panels. The re-cladding of the Amoco Building increased the weight of the facade from 10,960 kips (kilopounds) to 18,670 kips--almost an 8,000 kip increase. As such, 20 of the tower's columns were reinforced with welded steel plates [Hook].


The recladding of the Amoco tower cost $80 million, more than half of the building's original cost. Upon completion of the renovation, Amoco was the tallest building in the world to ever be reclad, and it was "the biggest recladding job in history" [Hook]. Improper use and detailing of the facade material lead to a disastrous failure and a very costly repair. Amoco pursued legal action against the facade's original designers, and the litigation was eventually settled. The results of the settlement were kept confidential from the public.

Aside from the negative aspects of the Amoco case, this particular failure also warned designers of the inherent fragility of thin-stone veneers. It lead to the development of standardized weathering and stone cladding connection tests by the American Society for Testing and Materials (ASTM). And generally, it also gave designers and engineers a better overall understanding of thin stone as a cladding material. Unfortunately, it took a major failure for the industry to learn these lessons, but such is the case for most failures. The best thing the industry can do is to learn all it can from a failure to avoid repeating the same mistakes in the future.


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