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NISTNCSTAR1-2 Federal Building and Fire Safety Investigation of the World Trade Center Disaster Baseline Structural Performance and Damage Aircraft Impact Analysis of the World Trade Center Towers Fahim Sadek National Institute ofStandardsandTechnology • TechnologyAdminisirofion • U.S. DeportmentofCommerce NISTNCSTAR1-2 Federal Building and Fire Safety Investigation of the World Trade Center Disaster Baseline Structural Performance and Damage Aircraft Impact Analysis of the World Trade Center Towers Fahim Sadek Building andFire Research Laboratory National Institute ofStandards and Technology September 2005 U.S. DepartmentofCommerce Carlos M. Gutierrez, Secretary TechnologyAdministration Michelle O'Neill, Acting UnderSecretaryfor Technology National Institute ofStandards and Technology William Jeffrey. Director DisclaimerNo. 1 Certain commercial entities, equipment, products, or materials are identified in this document in ordertodescribe a procedure orconcept adequately ortotrace the history ofthe procedures and practices used. Such identification is not intended to imply recommendation, endorsement, or implication thatthe entities, products, materials, or equipmentare necessarilythe bestavailable forthe purpose. Nordoes such identification implyafinding offaultor negligence bythe National Institute ofStandards and Technology. DisclaimerNo. 2 The policy ofNIST isto use the International System ofUnits (metric units) in all publications. In thisdocument, however, units are presented in metric units orthe inch-pound system, whichever is prevalent in the discipline. DisclaimerNo. 3 Pursuantto section 7 ofthe National Construction SafetyTeam Act, the NIST Directorhas determined thatcertain evidence received by NIST in the course ofthis Investigation is "voluntarily provided safety-related information"that is "not directly related to the building failure being investigated" and that"disclosureofthatinformation would inhibitthe voluntary provision ofthattype of information" (15 USC 7306c). In addition, a substantial portion ofthe evidence collected by NIST in the course ofthe Investigation has been provided to NIST under nondisclosure agreements. Disclaimer No. 4 NISTtakes no position asto whetherthe design orconstruction ofa WTC building was compliantwith anycode since, duetothe destruction oftheWTC buildings, NIST could notverifythe actual (oras-built) construction, the properties and condition ofthe materials used, orchangesto the original construction made overthe life ofthe buildings. In addition, NIST could notverifythe interpretations ofcodes used byapplicable authorities in determining compliance when implementing building codes. Where an Investigation report stateswhethera system was designed or installed as required by a codeprovision, NIST has documentaryor anecdotal evidence indicating whetherthe requirementwas met, orNIST has independently conducted tests oranalyses indicating whetherthe requirementwas met. Use in Legal Proceedings No partofany report resulting from a NIST investigation into a structural failure orfrom an invesfigation underthe National Construction SafetyTeam Act may be used in any suitoraction fordamages arising outofany matter mentioned in such report (15 USC 281a; as amended by P.L. 107-231). National Institute ofStandards and Technology National Construction Safety Team Act Report 1-2 Natl. Inst. Stand. Technol. Natl. Constr. Sfty. Tm. Act Rpt. 1-2,458 pages (September2005) CODEN: NSPUE2 U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 2005 Forsale bythe Superintende—nt ofDocuments, U.S. Gov—ernment Printing Office Internet: bookstore.gpo.gov Phone: (202) 512-1800 Fax; (202) 512-2250 Mail: Stop SSOP, Washington, DC 20402-0001 Abstract Thebaseline structuralperformance and aircraft impact damage analysis ofthe National Institute of Standards and Technology (NIST) Investigation ofthe World Trade Center (WTC) disasterhad two WTC primary tasks: (1) to develop reference structural models ofthe towers and use these models to establish the baselineperformance ofeach ofthe towers undergravity and wind loads, and (2) to estimate the damage to the towers due to aircraft impacts and establish the initial conditions forthe fire dynamics modeling and the thermal-structural response and collapse initiation analysis. This reportprovides the technical approach, methodology, and results related to both tasks. WTC Forthe first task, the baseline performance ofthe towers under gravity and wind loads was established in orderto assess the towers" ability to withstand those loads safely and to evaluate the reserve capacity ofthe towers to withstand unanticipated events. The baseline perfonnance study provides a measure ofthe behaviorofthe towers underdesign loading conditions, specifically: (1) total and inter- story drift (the sway ofthe building underdesign wind loads), (2) floor deflections under gravity loads, (3) the stress demand-to-capacity ratio forprimary structural components ofthe towers such as exterior walls, core columns, and floorframing, (4) perfonnance ofexteriorwalls under wind loading, including distribution ofaxial stresses andpresence oftensile forces, (5) performance ofconnections between exterior columns, and (6) resistance ofthe towers to shear sliding and overturning at the foundation level. Wind loads were a governing factor in the design ofthe structural components that made up the frame- tube steel framing system. Wind load capacity was also a key factor in determining the overall strength ofthe towers and was important in determining not only the ability ofthe towers to withstand winds but also the reserve capacity ofthe towers to withstand unanticipated events such as major fire or impact damage. Accurate estimation ofthe wind load on tall buildings is a challengingtask, given that wind engineering is still an evolving technology. For example, estimates ofthe wind-induced response WTC presented in two recent independent studies ofthe towers differed from each otherby about 40 percent. In this study, NIST developed refined estimates ofwind effects by critically assessing information obtained from the CermakPeterka Peterson, Inc. (CPP) and Rowan Williams Davis and Irwin. Inc. (RWDl) reports and by bringing to bear state-of-the-art considerations. Furthermore, the available prescriptive codes specify wind loads on tall buildings that are significantly lower than wind tunnel-based loads. This case study provided an opportunity to assess effectively current design practices and various code provisions on wind loads. Forthe purpose ofestablishing the baseline performance ofthe towers, various wind loads were considered in this study, including wind loads used in the original WTC design, wind loads based on two recent wind tunnel studies conducted in 2002 by CPP and RWDI for insurance litigation concerningthe towers, and refined wind load estimates developed by NIST. In orderto develop the reference models and conduct the baselineperformance analyses, the following steps were undertaken: WTC WTC • Develop structural databases forthe primary structural components ofthe 1 and 2 towers from the original computerprintouts ofthe structural design documents. NISTNCSTAR 1-2. WTC Investigation HI Abstract • Develop reference structural analysis models that captured the intendedbehaviorofeach of the two towers using the generated databases. These reference models were used to establish the baseline perfoiTnance ofthe towers and also served as a reference formore detailed models for aircraft impact damage analysis andthermal-structural response and collapse initiation analysis. The models included: (1) two global models (one for each tower) ofthe major structural components and systems ofthe towers, and (2) floor models ofa typical truss-framed floor and a typical beam-framed floor. • Develop estimates ofdesign gravity (dead and live loads) and wind loads on each ofthe two towers for implementation into the reference structural models. The following three loading cases were considered: - Original WTCdesign loads case. Loads included dead and live loads as in original WTC design, in conjunction with original WTC design wind loads. - State-of-the-practicecase. Loads included dead loads; currentNew York City Building Code (NYCBC 2001) live loads; and wind loads from the RWDI wind tunnel study, scaled in accordance withNYCBC 2001 wind speed. - RefinedNISTestimatecase. Loads included dead loads; live loads from the American Society ofCivil Engineers (ASCE) 7-02 Standard (a national standard); and refined wind loads developedby NIST. • Perform structural analyses to establish thebaseline performance ofeach ofthe two towers under design gravity and wind loads. Forthe second task related to aircraft impact, the aircraft impact damage to the exteriorofthe WTC towers could be visibly identified from the video and photographic records. However, no visible information couldbe obtained forthe extent ofdamage to the interiorofthe towers, including the structural system (floors and core columns), partition walls, and interiorbuilding contents. Such information was needed forthe subsequent fire dynamics simulations and post-impact structural analyses. In addition, forthe fire dynamics modeling, the dispersion ofthejet fuel and the location ofcombustible aircraft debris were required. The estimate ofthe extent ofdamage to the fireproofing on the structural steel in the towers due to impact was essential for the thermal and structural analyses. The aircraft impact damage analyses were the primary tool by which most ofthe information on the tower damage could be estimated. WTC The focus ofthe analysis was to analyze the aircraft impacts into each ofthe towers to provide the following: (1) estimates ofprobable damage to structural systems, including exteriorwalls, floor systems, and interior core columns; (2) estimates ofthe aircraft fuel dispersion during the impact; and (3) estimates ofdebris damage to the building nonstnactural contents, includingpartitions and workstations. The results were to be used to estimate the damage to fireproofing based on the predicted path ofthe WTC debris field inside the towers. This analysis thus estimated the condition ofthe two towers immediately following the aircraft impacts and established the initial conditions forthe fire dynamics modeling and the thermal-structural response and collapse initiation analysis. The impact analyses were conducted at various levels ofcomplexity including: (1) the component level, (2) the subassembly level, and (3) the global level to estimate the probable damage to the towers due to aircraft impact. NISTNCSTAR WTC IV 1-2, Investigation Abstract In orderto estimate the aircraft impact damage to the WTC towers, the following steps were undertaken: • Constitutive relationships were developedto describe the behaviorandfailure ofthe materials underthe dynamic impact conditions ofthe aircraft. These materials included the various grades ofsteels used in the exteriorwalls, core columns, and floortrusses ofthe towers, weldment metal, bolts, reinforced concrete, aircraft materials, and nonstructural contents. • Global impact models were developed forthe towers and aircraft. The towermodels includedtheprimary structural components ofthe towers inthe impact zone, including exteriorwalls, floor systems, core columns, and connections, alongwith nonstructural building contents. A refined finite element mesh was used for the areas in the path ofthe aircraft, and a coarsermesh was used elsewhere. The aircraft model included the aircraft engines, wings, fuselage, the empennage, and landing gear, as well as nonstructural components ofthe aircraft. The aircraft model also included a representation ofthe fuel, using the smooth particle hydrodynamics approach. • Component and subassembly impact analyses were conducted to support the development of the global impact models. The primary objectives ofthese analyses were to (1) develop an understanding ofthe interactive failure phenomenon ofthe aircraft and tower components, and (2) develop the simulation techniques required forthe global analysis ofthe aircraft impacts into the WTC towers, including variations in mesh density andnumerical tools for modeling fluid-structure interaction for fuel impact and dispersion. The component and subassembly analyses were used to detennine model simplifications forreducingthe overall model size while maintaining fidelity in the global analyses. WTC • Initial conditions were estimated forthe impact ofthe aircraft into the towers. These included the aircraft speed at impact, aircraftorientation and trajectory, and impact location ofthe aircraft nose. The estimates also included the uncertainties associatedwith these parameters. This step utilized the videos and photographs that captured the impact event and subsequent damage to the exteriorofthe towers. • Sensitivity analyses were conducted at the component and subassembly levels to assess the effect ofuncertainties on the level ofdamage to the towers due to impact and to determine the most influential parameters that affect the damage estimates. The analyses were used to reduce the number ofparameters that wouldbe varied in the global impact simulations. • Analyses ofaircraft impact into WTC 1 and WTC 2 were conducted using the global tower and aircraftmodels. The analysis results included the estimation ofthe structural damage that degraded their strength and the condition and position ofnonstructural contents such as partitions, workstations, aircraft fuel, and otherdebris that influenced thebehaviorofthe subsequent fires in the towers. The global analyses included, for each tower, a "base case" based on reasonable initial estimates ofall inputparameters. They alsoprovided a range of damage estimates based on variations ofthe most influential parameters. This range included more severe and less severe damage cases. NISTNCSTAR 1-2. WTC Investigation v Abstract • Approximate analyses were conducted to provide guidance to the global finite element impact analyses. These included: (1) analysis ofthe overall aircraft impact forces and assessment ofthe relative importance ofthe airframe strength and weight distribution, (2) evaluation ofthepotential effects ofthe energy in the rotating engine components on the calculated engine impact response, (3) influence ofthe static preloads in the towers on the calculated impact damage and residual strength predictions, and (4) analysis ofthe load characteristics required to damage core columns compared to the potential loading from impact ofaircraft components. Keywords: Aircraft impact, finite element analysis, floor system, load, model, structural, truss, wind loads. WorldTrade Center. NISTNCSTAR WTC 1-2, Investigation 11 Table of Contents Abstract iii List ofFigures xiii List ofTables xxi List ofAcronyms and Abbreviations xxiii Preface xxvii Acknowledgments xxxvii Executive Summary xxxix Chapter 1 Introduction 1 , 1.1 Background 1 1.2 Reference Models and Baseline PerformanceAnalysis 1 1.3 AircraftImpact Damage Analysis 4 Chapter2 Development of Reference Structural Models 9 2.1 Introduction 9 2.2 Development ofStructural Databases 10 2.3 Global Models ofthe Towers 1 2.3.1 ExteriorWall Modeling 14 2.3.2 Core Columns Modeling 22 2.3.3 Hat Truss Modeling 22 2.3.4 Flexible andRigid FloorDiaphragm Modeling 24 2.3.5 Boundary Conditions 26 2.3.6 Results ofModal Analysis 26 — 2.4 Typical Truss-Framed Floor Model Floor 96 ofWTC 1 29 2.4.1 Primary Trusses 3 2.4.2 Bridging Trusses 32 2.4.3 Concrete Slab and Metal Deck 33 2.4.4 Viscoelastic Dampers 34 2.4.5 Strap Anchors 34 — 2.5 Typical Beam-FramedFloorModel Floor 75 ofWTC 2 34 2.5.1 Composite Beams 35 NISTNCSTAR 1-2. WTC Investigation vii Table ofContents 2.5.2 Horizontal Trusses 35 2.5.3 Concrete Slab and Metal Deck 36 2.5.4 Viscoelastic Dampers 36 2.6 Review ofthe Structural Databases and Reference Models ofthe Towers 37 2.6.1 Structural Databases 37 2.6.2 Reference Structural Models 37 2.7 Summary 38 2.8 References 39 Chapter3 Wind Loads on the WTC Towers 41 3.1 Introduction 41 3.2 Original WTC Design Wind Loads 43 3.3 State-of-the-Practice Wind Loads 44 3.4 Refined NIST Estimate ofWind Effects 45 3.4.1 Summary Comparison by WeidlingerAssociates, Inc., ofCPP andRWDI Estimates 46 3.4.2 Review ofCPP Estimates 46 3.4.3 Review ofRWDI Estimates 47 3.4.4 Comments by Third Party Reviewer (Skidmore, Owings & Merrill LLP- SOM)- Appendix D 49 3.4.5 Summary 50 3.5 Comparisons ofWind Loads, Wind Speeds, and Practices 50 3.5.1 Wind Loads 50 3.5.2 Wind Speeds 53 3.5.3 Wind Engineering Practices Pertaining to Tall Buildings 55 3.6 References 57 Chapter4 Baseline Performance ofthe WTC Towers 59 4.1 Introduction 59 4.2 Baseline Performance ofthe Global Models 59 4.2.1 Analysis Methodology 59 4.2.2 Total and Inter-Story Drift 62 4.2.3 Demand/Capacity Ratios 63 4.2.4 Exterior Coluirais Axial Loads and Stresses 77 4.2.5 ExteriorColumns Splice Connection 84 NISTNCSTAR WTC viii 1-2, Investigation