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Multi-hazard Design of Facades: Important Considerationsof Wind and Seismic Interaction with Blast Requirements
Aldo E. McKay1, Cliff A. Jones2, Ed Conrath3, and CarrieDavis4
1Protection Engineering Consultants, Principal and SeniorEngineer, 14144 Trautwein Road, Austin, Texas 78737; PH(512) 380-1988 x6; email: [email protected] 2Protection Engineering Consultants, Sr. AssociateEngineer, 14144 Trautwein Road, Austin, Texas 78737; PH(512) 380-1988 x319; email: [email protected] 3Protection Engineering Consultants, Senior Principal,Omaha, Nebraska; PH (512) 380-1988 x4; email:[email protected] 4Protection Engineering Consultants, Project Engineer,14144 Trautwein Road, Austin, Texas 78737; PH (512) 380-1988 x305; email: [email protected]
ABSTRACTThis paper provides architects and engineers with anoverview of key factors to consider when designing facadeenvelope components against wind, seismic and blastloads. Examples and situations that may be encountered inthe design of curtainwall, storefronts and other façadecomponents are presented along with “lessons-learned”recommendations on how to effectively deal with or avoiddesign problems. “Typical approaches” for designingenvelope components are illustrated and existinggovernment and industry criteria for wind, seismic andblast loads are reviewed and discussed.
INTRODUCTIONMulti-hazard design is complex due to competing and, attimes, conflicting design requirements. The designapproaches for blast are typically quite different fromthose used for gravity, wind and seismic design, andoftentimes, different load cases and load combinationswill control different aspects of the façade design.Design teams are recognizing that significant time,effort and money can be saved by considering the fullspectrum of design requirements concurrently rather thandesigning for each load case independently. This paperprovides a general overview of key factors to considerwhen coordinating blast with gravity, wind and seismicdesign requirements for facades. Common conflicts thatarise in multi-hazard design projects and recommendationsfor efficient and effective details are also discussed.
DESIGN CODES AND GENERAL REQUIREMENTSTo efficiently combine and coordinate between multipleload cases, it is important to first understand therequirements for each. The following sections brieflysummarize the design requirements and codes typicallyused to perform gravity, wind, seismic and blast design.
Traditional Loads (Gravity, Wind, Seismic)In the United States, gravity and combinations ofgravity, wind and seismic design of façade components istypically performed per the International Building Code(IBC) using the load combinations and approachessummarized in ASCE 7: Minimum Design Loads for Buildingsand Other Structures. These loads are referred to as“Traditional Loads” in this paper. Gravity design requirements are described in Chapter 3:Dead Loads, Soil Loads and Hydrostatic Loads. Componentsare designed for their self-weight and the weight of anyattached components. Design is static, components aredesigned to remain elastic and lower bound material
properties are assumed. Connections are designed fortheir tributary gravity load.Wind design requirements are described in Chapter 30:Wind Loads – Components and Cladding (C&C). Componentsare designed statically for the equivalent peak windforce in combination with other traditional loads such asgravity, snow, rain, seismic, etc. They are designedelastically assuming lower bound material properties andmust meet strength and serviceability requirements.Connections are designed for their tributary wind load.Additional impact requirements may be necessary inhurricane and tornado zones. Seismic design of façade components is summarized inChapter 13: Seismic Design Requirements for NonstructuralComponents. Even though seismic loading is dynamic,design for this load case is typically performed usingequivalent static loads. Similar to other traditionalloads, components are typically designed to remainelastic and lower bound material properties are used.However, the goal is to prevent falling façade componentsduring seismic events that could present hazards tooccupants and first responders. Therefore, seismic designalso carries additional detailing requirements andfactors to ensure connections have additional resistanceto accommodate “overloading” during a seismic event.Connections are also designed for seismic deformations ofthe structure.
Blast LoadsA much wider variety of criteria is used in designingfaçade components for blast loading. These include, butare not limited to, Department of Defense (DoD) UFC 4-010-01: DoD Minimum Antiterrorism Standards forBuildings, Interagency Security Committee (ISC) PhysicalSecurity Criteria for Federal Facilities, ASCE 59-11:Blast Protection of Buildings and Design of BlastResistant Buildings in Petrochemical Facilities.
Additionally, client- and project-specific blast-resistant requirements may also be specified. Blast is typically considered independently from allother loads except gravity. In contrast to design fortraditional loads, blast design considers expecteddynamic material strengths by factoring the lower boundstrength by a static increase factor (SIF) to account fortypical material strength over lower bound values, anddynamic increase factor (DIF) to account for materialrate effects. Since the goal of blast design is typicallyto prevent loss of life and/or protect valuable assets,components are allowed to undergo significant inelasticdeformation (generally). In order to accommodate thislevel of response, connections are typically designed forthe out-of-plane ultimate flexural capacity of theattached components.Table 1 provides a summary of the main contrasting designparameters for blast and traditional loads.
Table 1: General Design Parameters Matrix
Traditional Loads Blast Loading
Design LoadType Static Pressure Dynamic Pressure-
Impulse
LoadCombinations IBC Independent Load
Case
Design Methods Static (LRFD, ASD) Dynamic
AllowableResponse Elastic Inelastic
MaterialStrengths Lower Bound Expected (SIF and
DIF)
THE DESIGN PROCESSWhen designing facades for multiple hazards, the order inwhich the design requirements from each hazard areincorporated is important in minimizing duplicate work
and consequently the design time and cost. The followingsections discuss a typical approach and known issuesassociated with it along with a recommended moreefficient approach based “lessons learned” from realprojects.
Common Approach and Associated Issues Typically, façade design is compartmentalized by loadcase. One engineer may size a façade for blast, anotherfor wind and gravity and yet another for seismic. Bycompartmentalizing the design in this fashion, each loadcase is considered independently and synergies andconflicts that exist between the load cases are oftenoverlooked and not considered. As a result, the designprocess can undergo one or more unnecessary iterationshad all criteria been considered during each step of thefaçade design.In addition, where blast design is required, engineersoften expect it to control the size of all members andconnections. Thus, blast design is often sub-contractedto a specialty engineer and may be performed separatefrom the design for traditional loads. In this case, onceblast design is complete, the traditional designrequirements for loads such as gravity, wind and seismicmay cause changes to the design. This often results insignificant re-design of the system for blast loadingparticularly the connections. This common approach isillustrated in Figure 1.
Figure 1 Flow Chart – Example of Typical InefficientDesign Process
Recommended Design ApproachFor efficiency, it is best to take a holistic approach todesign, considering all criteria and their interactionduring each step. Therefore, it is recommended that thedesign be broken up by task rather than by discipline.The flow chart shown in Figure 2 provides an example ofan efficient façade design process that considers blast,wind and seismic requirements.
Figure 2 Flow Chart – Example of Recommended EfficientDesign Process
When coordinating requirements for all load cases,designers must be aware of common design conflicts andcoordination issues that if addressed early, can lead toefficient design of façade systems. Some common examplesare summarized in the following section.
COMMON PITFALLS IN COORDINATING AND IMPLEMENTING MULTIPLEREQUIREMENTS As discussed above and summarized in Table 1, traditionalloads design requirements for façade components candiffer significantly from blast requirements. Thus, whendesigning for multiple hazards there are importantcharacteristics and aspects of the system that must becoordinated and defined early in the design process aschanges, even slight modifications, to these cansignificantly affect one or more design load cases andresult in wasted time and higher design costs. Thefollowing sections summarize several importantcharacteristics of the façade systems that must beconsidered early when coordinating design requirementsfor multiple hazards.
Weight of the SystemThe weight of the façade components has a significantimpact on the flexural response of blast loadedcomponents and in determining seismic connection forces.For blast resistant facades, heavy systems such as studswith brick veneer, reinforced CMU and precast concretepanels are beneficial because the added mass providesincreased inertial resistance to blast loads. Incontrast, these heavy systems can be problematic inseismic applications where the connection forces aredirectly proportional to the mass of the system. Inseismic applications, light weight systems such as studswith EIFS, and/or metal panels are more beneficial asthey result in lower connection forces and lower imposedloads on the supporting structure that can be more easilyaccommodated.
Flexural Strength and StiffnessThe flexural strength and stiffness of façade componentscan have substantial and opposite effects on wind andblast-resistant requirements. Facade components designedfor high wind areas or for long span conditions are oftencontrolled by serviceability requirements (deflectionlimits). In these cases, additional flexural stiffness isincorporated into the design by increasing the moment ofinertia of the cross-section. In glazed aluminum systemthis is often done by adding steel reinforcement insidethe vertical mullions since often times the depth of thesystem cannot be altered due to architecturalconstraints. In other types of systems such as metalstuds, heavier gauge studs may be used. This additionalreinforcement also increases the flexural strength of thecomponent. In these cases, and when dealing with blast-resistant requirements, it is important to provide justenough additional stiffness to meet the serviceabilitydemands and avoid overly conservative reinforcement sinceit can lead to unnecessary high blast reactions, whichmay cause difficulties if the substrate has not beenproperly selected or retrofitted in the case of anexisting structure.
Impact ResistanceMany projects in coastal regions combine blast-resistantrequirements with wind-borne debris requirementsnecessitating the use of laminated glass and/orpolycarbonate interior panes in order to resist impactfrom a wind-borne debris missile. These types of glazinghave high strength and in certain blast environments cantransfer extremely high reaction forces into thesupporting structural elements. In this type ofapplication, it is very important that the structuralsupports for glazing systems be adequately selected toaccommodate these atypically high blast reactions fromthe glazing system. This is particularly important forretrofit applications of older buildings.
RECOMMENDED TYPICAL DETAILSThe following sections provide a few basic examples oftypical detailing recommendations for façade componentsdesigned for blast and traditional loading applicationscommonly used in current construction.
Metal Stud Walls and Components Metal stud walls are very popular in today’s constructionbecause of their light weight and relatively quick speedof installation. Metal studs typically used ontraditional applications (wind for example) often haveenough flexural strength to resist blast loads, but theircapacity is typically limited by their connections(screws into a metal track that is fastened into thefloor slabs) which do not allow the component to achievesignificant plastic deformations. Thus, by improving theconnections of the metal studs significant blastresistance can be achieved.The most efficient way to provide increased inboundconnection capacity is by providing bearing connectionsfor the stud. At intermediate slab edges, this isaccommodated by allowing the stud to span continuouslypast the stab edge as is typically done in “balloonconstruction”. At the bottom ends of the stud, bearingcan be provided by notching the slab edge as shown inFigure 3.
Figure 3 Bearing Type Supports for Metal Stud WallIndividual stud clip connections designed for theflexural capacity of the stud can also be used. Anexample of this connection type is provided in Figure 4.However, these connections often costly and laborintensive to install so bearing connections arerecommended where feasible.
Figure 4 Clip Angle Connection for Metal Studs
Cantilever Knee Walls Supporting Façade ComponentsFor traditional loading, glazed storefronts, curtainwallsand other façade elements are often designed to frame ontop of cantilever knee walls made from metal studs orunreinforced masonry (see Figure 5). While these detailsmay be adequate for common wind loads, they do not offeradequate support for blast. In blast-resistantapplications, the lateral reactions from the supportedfaçade components cause high flexural and rotationaldemands at the base of the knee wall. Thus, the preferredmethod is to use reinforced CMU walls doweled with steelreinforcement in to the foundation or concrete walls castmonolithically with the foundation as shown in Figure 6.Alternatively, the glazing systems could also be designedto connect directly into the floor slab or an alternatecontinuous support can be provided behind thearchitectural knee wall.
Figure 5 Typical Unrestrained Metal Stud Knee Wall
Figure 6 Reinforced and Doweled Knee wall
Punched Openings in Retrofit ApplicationsIt is common to upgrade the façade of older buildings toimprove the level of protection provided to theoccupants. Often, in these applications, the structuralconfiguration of the existing façade is unknown or isfound to be deficient for blast loads. A typical approachfor providing adequate support for windows and doors isto use a metal frame placed behind the existing façade asshown in Figure 7. Metal frames are generally designedwith cold-formed metal studs anchored directly to thefloor and/or main structural framing. The glazing systemor door is then connected directly to the frame. Thus, noblast reactions are transferred into the existing façadecomponents.
Figure 7 Metal frame Support for Punched Opening
Impact Resistant ConstructionWhere wind-borne debris is a design consideration, theglazing is generally required to resist the impact from a9 pound 2x4 piece of lumber with impact speeds varyingbetween 34- and 54.5-mph depending on the specificationsand location on the building. To provide this type ofprotection, glazing systems are often designed usinglaminated single panes of at least 5/8-in. with laminatethickness (typically polyvinyl butyral, PVB) of 0.09-in.or 0.1-in. Double panes (IGU) are also typical (generallybetween 1-in. and 15/16-in. overall thickness) where theinterior pane has similar configurations as describedabove for the single pane. On occasion, for largeopenings, the glazing configuration may includepolycarbonate interior panes. Attachments into supportingstructural elements are generally verified throughtesting. Thus, when dealing with design for blast andwind-borne debris impact requirements, the structuralsupports must be considered during early coordinationbetween the architect, structural and specialty blastengineer disciplines to ensure proper flexural and shear
strength. Some examples of typical structural supportsystems that can be designed to meet both sets ofrequirements are steel or heavy aluminum frames, 8-in.thick reinforced CMU or 8-in. thick concrete panels. Whenchoosing these systems, sufficient edge clearances shouldbe planned to allow for the installation of post-installed anchors.
CONCLUSIONSThe design requirements and approaches for blast aretypically quite different from those used for traditionaldesign of façade elements. In many cases when all must beconsidered, different load cases and load combinationswill control different aspects of the façade design. Itis common practice for each of these requirements to beaddressed by different contractors separately and withminimal coordination. This often results in additionaldesign cycles and increased design cost as many of thesynergies and conflict between disciplines areoverlooked. A more efficient design can be obtained by consideringall criteria and their interaction during each step. Todo this, it is recommended that the design of the façadebe separated by task and not be discipline. The flexuralstrength and stiffness requirements of the framingmembers should be finalized for all disciplines prior tocommencing the design of the connections. Finally, when dealing with blast loads, the goal is toprovide a ductile mode a failure and ensure that thecomponent will remain attached to the supports in thecase of an overload. As a result, façade components aretypically designed to sustain significant deformation.Thus, connections become critical as they must bedesigned and detailed to allow components to reach theselevels of deformation. The details presented in the paperprovide typical approaches for connection design offaçade elements commonly used construction today.
REFERENCESAmerican Society for Testing and Materails (ASTM).
(2014). ASTM E1996-14a: Standard Specificationfor Performance ofExterior Windows, Curtain Walls, Doors, and Impact Protective SystemsImpacted by Windborne Debris in Hurricanes. WestConshohocken, PA: ASTM International.
American Society of Civil Engineers (ASCE). (2010). ASCE7-10: Minimum Design Loads for Buildings and Other Structures.Reston: ASCE.
American Society of Civil Engineers (ASCE). (2010). Designof Blast-Resistant Buildings in Petrochemical Facilities (2nd ed.).(W. L. Bounds, Ed.) Reston: ASCE.
American Society of Civil Engineers (ASCE). (2011). ASCE59-11: Blast Protection of Buildings. Reston: ASCE | SEI.
Department of Defense. (9 February 2012 with Change 1, 1October 2013). Unified Facilities Criteria (UFC) 4-101-01: DoDMinimum Antiterrorism Standards for Buildings. DoD.
Federal Emergency Management Agency (FEMA). (2008). FEMAP-361: Design and Construction Guidance for Community Safe Rooms.FEMA.
Interagency Security Committee. (12 April 2010). PhysicalSecurity for Federal Facilities (FOUO). ISC.
International Code Council (ICC). (2011). InternationalBuilding Code (IBC) 2012. Country Club Hills:International Code Council, Inc.
Sammarco, E. L., Jones, C. A., Williamson, E. B., &Sprague, H. O. (2014, March). Design for Blast andSeismic: Acknowledging Differences and LeveragingSynergies. Structure Magazine, pp. 10-14.
U.S. Army Corps of Engineers (USACE) Protective DesignCenter (PDC). (2014). PDC-TR 06-01: Methodology Manual forthe Single-Degree-of-Freedom Blast Effects Design Spreadsheet(SBEDS). USACE PDC.
