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FRP COMPOSITE FAÇADE Using FRP on high-rise buildings

William Kreysler Kreysler & Associates bill@kreysler.com

ABSTRACT

Fiber Reinforced Polymer (FRP), in this case glass fiber reinforced polyester resin composite with a polymer concrete face coat, was used in the USA for the first time as exterior cladding on a Type 1 multi-story building on the SFMoMA addition. This 11-story addition, completed in May of 2016, makes SFMoMA the largest museum of modern art in the USA with the largest architectural FRP facade application in the USA to date.

FRP was chosen to mimic the rippling water of the nearby San Francisco Bay on the east and west elevations. Although recognized by the IBC (International Building Code) in 2009 as an accepted building material (INTERNATIONAL CODE COUNCIL 2009), any FRP material used must pass the same code requirements as other combustible materials. The most difficult of these requirements is the NFPA 285 test (NFPA285). Until this and other requirements are met, no combustible material, including FRP, is allowed.

The design for the SFMoMA project called for over 700 unique, individual, constantly curving panels (Fig. 1).

Figure 1: SFMOMA façade of Snøhetta expansion (photo Jon McNeal, © Snøhetta.jpg).

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Although it is possible to construct such panels with metal, the only practical option was to mold the 710 unique panels, thus suggesting precast concrete or the lighter UHPC or GFRC. The less familiar FRP was listed as an alternative by the façade consultant in part because of its more widespread use in European construction (Fig.2).

Figure 2: Photo of Heydar Aliyev Cultural Center in Baku, Azerbaijan (Photo by Interfase, https://en.wikipedia.org/wiki/File:Heydar_Aliyev_Cultural_Center.jpg, public domain).

Although used sparingly on US buildings for decades, FRP has dominated other industries such as corrosion resistant ducting and chemical storage tanks, wind energy, marine, and heavy truck components (Fig. 3).

Figure 3: Photo of wind power plants in Xinjiang, China (Photo by Fred J, https://commons.wikimedia.org/wiki/File:Wind_power_plants_in_Xinjiang,_China.jpg, CC BY-NC-SA 2.0).

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However, it has seen no extensive use on Type 1 buildings. This has been partly because of codes and partly because its primary advantages over other materials are its high strength-to-weight ratio and its ability to be formed into complex shapes. Neither of these characteristics has been very important in construction until recently.

After successfully passing a rigorous evaluation process, FRP was chosen because it offered solutions to several problems presented by the use of other systems. Its primary advantages were its light weight and formability, the very features exploited by other industries in the past, and now increasingly relevant in contemporary design and construction.

KEYWORDS

Composites, FRP, Facades, Rainscreen, NFPA 285, Fiberglass, FRP, sculpture, curtain wall, unitized panels

INTRODUCTION

Aside from curiosity about something new, several factors are pushing building designers towards sometimes radical departures from traditional means and methods. This shaking up of the status quo, in an otherwise risk-averse industry, is leading to the startlingly rapid deployment of fundamentally new building systems, including to a large degree the building envelope itself. Environmental concern for building materials as well as building operations, health and safety issues relating to building construction and occupancy, rapidly changing regulations and code modifications are driving these new approaches. Additionally, jobsite labor costs, time to delivery, and an evolving design ethic brought about by 3D computer modeling are leading designers to consider an array of new ideas, methods, and materials. FRP composites represent one of these “new materials” that offer a fundamentally new approach to building construction. Although still some ways off, there is technically no reason why FRP cannot be used to create entire building structures as well as complete envelopes (Lambrych n.d.). Indeed, such composites structures are common in other industries such as aerospace, transportation and marine where monocoque structures are routine (Fig. 4).

Figure 4: Photo of Boeing 787 (Photo by MilborneOne, https://commons.wikimedia.org/wiki/File:B787-2109.jpg, CC BY-SA 3.0), photo of racecar (Photo by TyH22,

https://commons.wikimedia.org/wiki/File:HCastronevesIndy.jpg, CC BY-SA 3.0), photo of yacht (Photo by Berthold Werner,

https://commons.wikimedia.org/wiki/File:Yacht_Lady_Moura_in_Monaco.jpg, public domain).

Meanwhile, FRP composites will find increasing use in non-load-bearing architectural applications in AEC due to its formability, high strength-to-weight ratio, durability, and minimal maintenance requirements.

Designers, engineers, builders, owners, and even fabricators of FRP products, need reliable information about the proper use of FRP in construction. This paper is an attempt to improve the understanding of one of these materials and to address questions, concerns, and misconceptions relating to the proper use of Fiber Reinforced Polymers on building facades.

BACKGROUND

FRP has found limited acceptance in construction despite it’s proven success in other industries. Its principle advantages are its high strength-to-weight rations compared to other materials (Fig. 5a & 5b) and the ability to consolidate what would otherwise be assemblies of other materials such as wood or metal into a single molded part. For example, on the Boeing 787, a primary benefit of composites was to significantly reduce part count.

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Figure 5a: Strength to Weight Ratio of FRP compared to other materials (Diagram courtesy of Jordan Composites, Inc.).

Figure

5b: Strength to Weight Ratio of FRP compared to other materials (Diagram courtesy of Jordan Composites, Inc.).

Recent changes in building codes and design are opening the door to more widespread use of composites in architectural and even structural applications. The American Concrete Institute has adapted a design standard for the use of FRP in concrete structures (American Concrete Institute 2008, 440.2R-16), including a design standard for FRP composite rebar in structural concrete (ACI Committee 440 2015). AASHTO has published a standard for pedestrian bridge designs using composite structures (American Association of State Highway and Transportation Officials 2008). DOT initiatives throughout the USA and other countries have had experimental bridges and other structures in place for decades and are beginning to publish results indicating successes (American Association of State Highway and Transportation Officials 2009).

METHDOLOGY

The use of FRP cladding on the San Francisco Museum of Modern Art provides a case study for the use of FRP as cladding on a multi-story Type V commercial building. Initial prototypes, cost estimation, design assist procedures, code compliance strategies, and engineering and installation methods were developed to meet the design intent, budget, project schedule, code requirements, and environmental constraints.

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PR OT OT Y PE FAB R I CAT I ON

The architectural façade design was modeled originally by the architect in Rhino 3D (Mc Neel Associates) (Robert McNeel & Associates n.d.) using a “Grasshopper” script to alter the wavelength, amplitude, and frequency of the façade “ripples” over the curved and tilting East and West elevations of the building. Rhino models can be reliably imported into software used to guide CNC cutting tools (in this case PowerMILL by Delcam) (Delcam n.d.) which can be used to cut the shape of the part or it’s mirror image out of a block of material, thus creating a female mold directly from the architect’s model. Once made, this rapidly and inexpensively created mold can serve to fabricate a full scale model of any portion of the façade (Fig. 6).

Figure 6: Full scale mockup of SFMoMA panel, (Photo courtesy of Kreysler & Associates).

Easily fabricated mock-ups serve as a rapid verification of material durability, process fidelity, and panel weight. By early fabrication of a full-scale mock-up such things as material cost per square foot, overall weight, repair ability and strength are determined. This step improves the quality of the production cost estimate as well as the architect’s and client’s confidence in the material option.

COST E ST I M AT I ON

Although no two of the 710 façade panels were the same shape, the use of 3D computer modeling and CNC mold fabrication made cost estimating reliable. Rhino provided accurate surface area and such key characteristics as panel center of gravity. Knowing the materials required on a per square foot basis allowed accurate material cost prediction. PowerMILL includes algorithms that predict milling time for each mold (Fig. 7).

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Figure 7: Screen shot of surface area and milling time in PowerMILL software, (Screenshot courtesy of Kreysler & Associates).

Thus, despite the highly complex and variable shapes, accurate cost estimation and scheduling was possible for the tooling phase. Through the use of digital fabrication tools and conventional material, labor, and manufacturing overhead allocation methods, a reliable cost could be predicted.

D E SI GN ASSI ST

An element of contemporary construction is the ever increasing need for collaboration between the design team and specialty contractors. Although the traditional “design, bid, build” method is still dominant, it frequently leads to wasted time on the part of design professionals who attempt to produce a plausible “construction document” based often on insufficient knowledge of materials or fabrication methods. Expecting an architect, or even a façade consultant, to be an expert in composite fabrication can lead to erroneous assumptions, insufficient and inaccurate documentation, and faulty conclusions. At best he or she might propose a solution that does not optimize current technology, which in turn leads to costlier and lower quality solutions (Fig. 8).

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Figure 8: Early design concept for SFMoMA façade, (Screenshot courtesy of Snøhetta).

Expensive hours are spent attempting to develop plausible construction systems in hopes of achieving a “low” cost proposal. Too often such approaches fail to leverage the current fabrication best practices and can lead to inaccurate and higher risk results than would be the case with a negotiated contract with pre-qualified vendors based on pre-agreed budget targets. More enlightened approaches utilize design assist (Hart 2007) services, but this method continues to suffer when the selection process is based on responses to “conceptual” fabrication strategies, typically delivered to the vendors as 2D drawings for contract compliance purposes (Turner Construction n.d.). Such documents when they attempt to describe complex shapes, regularly lead to impossible construction details when applied to the actual 3D environment. Too often these irregularities don’t reveal themselves until after the vendor selection process, leading to change orders and wasted time. Solutions to this rapidly growing problem are beyond the scope of this paper, but they must be addressed as soon as possible. These solutions must, among other things, allow a shift to the use of 3D models as construction documents. They must also insist that vendors who participate in complex architectural projects be vetted and fully conversant with mutually compatible software (Miller 2012); they must be fluent in the use of the latest digital tools.

COD E COM PLI ANCE

Since 2009, the International Building Code (IBC) has recognized FRP as Fiber Reinforced Plastics, Fiberglass Reinforced Polymers in Section 2612. This section of Chapter 26 recognizes FRP as a combustible material allowing its use when the product can demonstrate the ability to meet the code requirements applied to similar architectural products. Since FRP can be formulated with a wide variety of mechanical and physical properties, formulations are available that meet most requirements. For building facades, the code allows any façade made of combustible material to be used below 40 feet provided it can pass ASTM E-84 with a class 2 rating – or better --for flame spread and also meet the appropriate structural

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requirements. This is a relatively easy standard for FRP materials. Above 40 feet the code becomes more rigorous. Although passing ASTM E-84 continues to be a requirement, any coverage over 20% of the total surface area means the material must meet the Class 1 requirements of ASTM E-84 for flame spread and smoke density and also pass, among other tests, the rigorous NFPA 285 test (NFPA285). For the SFMoMA addition, this was a major hurdle which had to be cleared before FRP could be seriously considered as the façade material. The specific formulation for the test panel is confidential and is in fact now patented by the panel fabricator. However the specifically fabricated panel did pass the test and as a result, composite material was selected for use based on the projection of significant cost and time savings.

Code requirements for engineering of the panels to meet wind, seismic, and dead load requirements, including the fixing designs, were met by following standard engineering principles and test standards for the design of similar façade products, with shop drawings, stamped by engineers duly licensed to practice in the jurisdiction.

E NGI NE E R I NG

FRP has long been the focus of reliable engineering techniques. Indeed, the development of modern FEA (Finite Element Analysis) engineering was driven to a large degree by the need to engineer complex aircraft forms made possible by composites. Aerospace and military uses of composites starting in the 1940’s, followed by the large compound curved shapes found in marine applications. These applications generated a vast array of ASTM and other standard tests procedures, many of which can be used for architectural composite design. The American Composites Manufacturers Association (ACMA) recently published Guidelines and Recommended Practice for Architectural FRP which contains examples and relevant material properties, engineering examples, and test procedures for the proper use of FRP in construction (American Composites Manufacturing Association 2016).

FAB R I CAT I ON PHASE

As we have mentioned, one of the unique characteristics of FRP is its very high strength-to-weight ratio (Fig. 9).

Figure 9: Comparison of Mechanical Properties of Conventional Materials and FRP Composites. Source: (American Composites Manufacturers Association 2016, 10).

This feature led to panels whose weight was approximately 3 pounds per square foot (~15 kg/M^2), making them light enough to be affixed to the front of the aluminum unitized panels used to form the waterproof barrier of the building. This was convenient for several reasons. It eliminated the need for any penetrations of the waterproofing. It allowed the FRP to be fastened to the unitized panels off site, which meant that the FRP rain screen was installed simultaneously with the unitized wall. This simultaneous installation eliminated the need for a back-up support system and reduced the construction time by replacing an original design that required three trips around the building by three different trades with a design that required one trip around by one contractor. Additional benefits involved less tower crane time, fewer crane moves, easier cleaning, higher quality damage tolerance, superior reparability, and lower overall cost. Comparative life cycle studies done by Stanford University graduate students in a non-peer reviewed LCA comparison (Stanford University 2009) also suggested the FRP had significantly less impact on the environment compared to the alternative system using GFRC or UHPC.

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Another unique characteristic of FRP is that the shape and configuration can be economically customized. Conventional unitized panel systems are most economical and reliable when creating flat walls.The SFMoMA façade was anything but flat. Resolving the problem created by these two seemingly incompatible features presented a unique challenge. How do you make a flat back on an ever varying front surface? Not only was the front wavy, it tilted forward and back as it rose higher and curved in plan through a wide variety of irregular radii. The solution lay in the use of digital tools to create asymmetrical return edges which were different on virtually every panel (Fig. 10).

Figure 10: Diagrams courtesy of Kreysler & Associates

As the façade diverged from a conventional flat wall, the edges of the panels were molded with edges that varied between 4 and 34 inches. This allowed for considerable design flexibility before running up to one of these edge dimension limits. When the curve diverged beyond these limits, a custom unitized panel was fabricated to “twist” the flat wall into a new facet (Fig. 11).

Figure 11: SFMoMA twisted panel and frame, (Photo courtesy of Kreysler & Associates).

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Again assisted by digital tools and relying on skillful craftsmanship and valuable collaboration between the FRP façade fabricator and the aluminum unitized wall manufacturer, calculation of the balance between the additional cost of these special twisted unitized panels and the cost of fabricating asymmetrical FRP panels determined the 4-to-34-inch edge tolerance. The contractor was able to minimize cost while retaining the original architect’s shape within a tolerance of less than 1.5 inches throughout the entire 11-story elevation.

DATA

Figure 12a: NFPA 285 Test Summary chart screenshot, (Courtesy of Kreysler & Associates).

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Figure 12b: NFPA 285 Test Summary chart screenshots, (Courtesy of Kreysler & Associates).

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Figure 13: E-84 Flame Spread and Smoke Density Chart screenshot, (Courtesy of Kreysler & Associates).

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Figure 14: Finite element analysis (FEA) of typical panel screenshots (Courtesy of Jordan Composites, Inc.)

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Figure 15: Connection capacity testing – aluminum rod to FRP laminate (Courtesy of Western Carolina University).

Figure 16: Calculation of connection components (Courtesy of Martin and Martin, Inc. Engineering).

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Figure 17: List of composites test methods (American Composites Manufacturers Association 2016, 79).

Mechanical  Properties Method

Bearing  Load   ASTM  D1602ASTM  D695ASTM  D6641ASTM  D3410ASTM  C365ISO  844ASTM  D638ASTM  D3039ASTM  D5083ASTM  C297DIN  53455ASTM  D638ASTM  D3039ASTM  C297DIN  53457ASTM  D638ASTM  D3039ISO  1922ASTM  D790ASTM  D6272ASTM  C393ASTM  D7249ASTM  D7250

Punch  Shear  Test   ASTM  D732

ASTM  D3518ASTM  D3846ASTM  D3914ASTM  D5379ASTM  D4255ASTM  D7078ASTM  C273ASTM  C393ISO  1922

Lap  Shear  Strength   ASTM  D3164

Short  Beam  Strength   ASTM  D2344

Izod  Impact   ASTM  D256

Charpy  Impact   ASTM  D256ASTM  D953ASTM  D5961

Compressive  Strength  and  Modulus

Bearing  Strength

In-­‐plane  Shear  Strength  and  Modulus

Tensile  Strength

Tensile  Modulus

%  Elongation

Flexural  Strength  and  Modulus

Flexural  Strength  and  Stiffness

Fire Method

ASTM  E84

ASTM  D162

Oxygen  Index ASTM  D2863

NBS  Smoke  Test   ASTM  E662

Multi-­‐Story  Building  Test NFPA  285

Room  Corner  Test   NFPA  286

Ignitability  by  Radiant  Panel   NFPA  268

Potential  Heat  of  Building  Materials   NFPA  259

Cone  Calorimeter  ASTM  E1354

Surface  Burning  Characteristics

Suface  Testing Method

Gravelometer SAE  J-­‐400

Gardner  Gloss  Meter Gardner

Stain  Resistance ANSI  Z124

Barcol  Hardness ASTM  D2583

Physical  Properties Method

Specific  Gravity ASTM  D792

Water  Absorption ASTM  D570

Glass  Transition ASTM  D7028

CTE ASTM  E289

Heat  Distortion ASTM  D648

Material  Properties Method

Brookfield  Viscosity ASTM  D2196

Ignition  Loss  of  Cured  Reinforced  Resins ASTM  D2584

Gel  Time ASTM  D2471

Glass  Fiber  Strands ASTM  D578

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EXPLANATION

The data show that FRP can meet the IBC acceptance criteria for architectural products. Standard test methods for fire and durability can be applied. ASTM tests exist for composite materials; these tests have been in existence for many years and have proven to be reliable in assisting engineers in designing structures as well as architectural products. In addition, FRP products, in large part because of their high material efficiency, often compare favorably to conventional materials in environmental assessments such as LCA (Life Cycle Assessment) studies.

CONCLUSION AND FUTURE WORK

Although somewhat new to the construction industry, FRP is a proven material with decades of successful use in demanding applications throughout the marine, aerospace, and transportation industries. To date, FRP composites have been used only infrequently in construction, mainly in remote and extreme environments where the need for prefabrication, light weight, and easy assembly have warranted their use.

Although offering many advantages, care must be taken to use industry standard design principles. As with any new material, the specifier of composite materials will be greeted with a wide variety of options and prices. Since quality is a function of fabrication, not unlike concrete, it is incumbent on the designer to beware of and to exercise caution in selecting a fabricator. Conflicting information needs to be reconciled and verified. Engineers must recognize this is a highly specialized discipline. Being an anisotropic material there are virtually limitless options in terms of fiber orientation, fiber volume, number of layers, type of resin, resin filler options, sandwich and single-skin construction techniques, and cure options. Engineers have control over a dizzying array of material properties, including even thermal expansion and contraction CTE which will vary from carbon fiber and it’s negative CTE to resins that have higher CTE’s than aluminum.

Use of FRP on the SFMoMA and other façade projects in Europe and Asia demonstrates that properly executed work can result in successful outcomes. However, there are ample examples of less successful outcomes. Although FRP has been proven for decades in applications at least as demanding as building facades and often in those that are much more demanding, making decisions based solely on cost is risky and almost certain to yield poor results. With care, appropriate formulation, and proper quality control, FRP can not only provide the structural properties to compete favorably with alternatives, but can also meet fire and other code requirements.

Similar to concrete, the mechanical and other critical properties are largely determined during the fabrication process. Stringent quality assurance is essential and close collaboration with a reliable and properly certified fabricator is critical. The IBC code requires any FRP part delivered to a jobsite to have affixed to it an ICC recognized independent test agency label certifying that it is manufactured in compliance with the code and subject to third-party inspection. Such a label is the first line of defense in the proper selection of FRP products for buildings.

Future study will need to explore structural opportunities for composites in construction. Engineering examples and ideally an LRFD model for FRP tailored to the construction industry should be developed. Durability case studies need to be assembled from the wide variety of existing examples to improve documentation. Such studies should rely on properly documented empirical evidence and science of which there are numerous examples (Pauer 2016).

ACKNOWLEDGEMENTS

The author wishes to acknowledge the following persons for providing information used in this paper: John Busel, P.E., American Composites Manufacturers Association Dr. Robert Steffen, PhD, P.E., North Carolina State University school of Construction Management Dr. Nicholas Dembsey, PhD, FPE, P.E., Professor, Wooster Polytechnic Institute, Wooster MA Jesse Beitel, P.E., FPE, Jensen Hughes Consultants, Baltimore MD Kevin Lambrych, CE, Ashland Chemical Emily Guglielmo, S.E., Martin & Martin Consulting Engineers Kurt Jordan, M.E., P.E., Jordan Composites

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—. 2008. Design Guidelines for FRP Pedestrian Bridges. D.C.: American Association of State Highway and Transportation Officials.

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American Concrete Institute. 2008. Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures. Farmington Hills: American Concrete Institute.

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