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Progressive Collapse Analysis and Design Guidelines - PDF
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1 Progressive Collapse Analysis and Design Guidelines for New Federal Office Buildings and Major Modernization Projects June 2003
2 TABLE OF CONTENTS TABLE OF CONTENTS Section Page Preface... iii Section 1. General Requirements Purpose Applicability Guideline Philosophy How To Use This Document Documentation Requirements Section 2. Definitions General Terms Frangible/Non-frangible Façade Alternate Analysis Techniques Section 3. Exemption Process Section 4. Reinforced Concrete Building Analysis and Design New Construction Design Guidance Analysis Analysis Techniques Procedure Analysis Considerations and Loading Criteria Typical Structural Configurations Atypical Structural Configurations Analysis Criteria Material Properties Modeling Guidance Redesign of Structural Elements Procedure Existing Construction Section 5. Steel Frame Building Analysis and Design New Construction Design Guidance Local Considerations Global Considerations Analysis Page i
3 TABLE OF CONTENTS Analysis Techniques Procedure Analysis Considerations and Loading Criteria Typical Structural Configurations Atypical Structural Configurations Analysis Criteria Material Properties Modeling Guidance Redesign of Structural Elements Procedure Existing Construction Section 6. Resources Appendix A. Atypical Structural Configurations... A-1 Appendix B. Design Guidance...B-1 Appendix C. Example Calculations...C-1 Appendix D. Structural Steel Connections... D-1 Page ii
4 PREFACE Preface The U.S. General Services Administration (GSA) developed the Progressive Collapse Analysis and Design Guidelines for New Federal Office Buildings and Major Modernization Projects to ensure that the potential for progressive collapse is addressed in the design, planning and construction of new buildings and major renovation projects. Mr. Bruce Hall, P.E., of the Office of the Chief Architect, initiated this work in 1999 and served as the GSA Project Manager. The Guidelines, initially released in November 2000, focused primarily on reinforced concrete structures. GSA subsequently identified the need to update the November 2000 Guidelines to address the progressive collapse potential of steel frame structures. Preparation of the updated Guidelines was performed by Applied Research Associates, Inc. with assistance provided by Myers, Houghton & Partners, Inc., Simpson, Gumpertz and Heger, Inc., the U.S. Army Corps of Engineers, and the U.S. Department of State. Page iii
5 SECTION 1 General Requirements Section 1. General Requirements 1.1 Purpose The purpose of these Guidelines is to: Assist in the reduction of the potential for progressive collapse in new Federal Office Buildings Assist in the assessment of the potential for progressive collapse in existing Federal Office Buildings Assist in the development of potential upgrades to facilities if required To meet this purpose, these Guidelines provide a threat independent methodology for minimizing the potential for progressive collapse in the design of new and upgraded buildings, and for assessing the potential for progressive collapse in existing buildings. It should be noted that these Guidelines are not an explicit part of a blast design or blast analysis, and the resulting design or analysis findings cannot be substituted for addressing blast design or blast analysis requirements. The requirements contained herein are an independent set of requirements for meeting the provisions of Interagency Security Committee (ISC) Security Criteria regarding progressive collapse. The procedures presented herein are required for the treatment of progressive collapse for U.S. General Services Administration (GSA) facilities. The previous guidelines, Progressive Collapse Analysis and Design for New Federal Office Buildings and Major Modernization Projects, November 2000 focused primarily on analysis and design for progressive collapse of reinforced concrete structures. This update includes lessons learned and adds a separate section pertaining to structural steel buildings. 1.2 Applicability These Guidelines should be used by all professionals engaged in the planning and design of new facilities or building modernization projects for the GSA. It applies to in-house Government engineers, architectural/engineering (A/E) firms and professional consultants under contract to the GSA. The primary users of the document will be architects and structural engineers. While mandatory for GSA facilities, these Guidelines may also be used and/or adopted by any agency, organization, or private concern. The exemption process contained in these Guidelines applies to the majority of the construction types currently in the GSA building inventory as described in Section 3. The analysis and design guidance for considering non-exempt, new or existing facilities is described in Sections 4 and 5. The use of a simplified analysis approach (hereafter referred to in these Guidelines as a Linear Procedure ) should typically be limited to consideration of low-to-medium-rise facilities. A Linear Procedure implies the use of either a static or dynamic linear-elastic Page 1-1
6 SECTION 1 General Requirements finite element analysis. Typically such facilities consist of buildings and specialty structures that are nominally 10 stories above grade or less. However, when analyzing buildings that have more than 10 stories above grade, and/or exhibit an atypical structural configuration (as defined in Section 2.1 and elaborated on in Appendix A), project engineers should consider using a more sophisticated analysis method hereinafter referred to as a Nonlinear Procedure. A Nonlinear Procedure implies the use of static or dynamic finite element analysis methods that capture both material and geometric nonlinearity. Special attention should be taken for facilities that contain atypical structural configurations and/or high-rise buildings that may exhibit complex response modes for the case where a primary vertical element is instantaneously removed. If a complex structural response to the analysis process contained in these Guidelines is anticipated, a Nonlinear Procedure may be required. It should be noted that if a Nonlinear Procedure is utilized, the approach must be based on the intent of these Guidelines and use the same allowable extents of collapse area as that presented in Section 4 and Section 5 in the evaluation of the potential for progressive collapse. As such, the assessment team will require experience and demonstrated expertise in structural dynamics, abnormal loading, and nonlinear structural response. Additionally, the applied procedure will require approval by the project Contracting Officer Technical Representative (COTR). 1.3 Guideline Philosophy These Guidelines address the need to protect human life and prevent injury as well as the protection of Federal buildings, functions and assets. The Guidelines take a flexible and realistic approach to the reliability and safety of Federal buildings. The ISC Security Criteria requires all newly constructed facilities to be designed with the intent of reducing the potential for progressive collapse, regardless of the required level of protection determined in the facility-specific risk assessment. Similarly, existing facilities shall be evaluated to determine the potential for progressive collapse. The approach described below utilizes a flow-chart methodology to determine if the facility under consideration might be exempt from detailed consideration for progressive collapse, as illustrated in Figure 1.1. In other words, a series of questions must be answered that identify whether or not further progressive collapse considerations are required. This process is based on ascertaining certain critical documentation to ensure that resources are spent wisely regarding this issue. Critical documentation consists of identifying all of the following information: Building occupancy Building category (e.g., reinforced concrete building, steel frame building, etc.) Number of stories Seismic zone Detailed description of local structural attributes [discrete beam-to-beam continuity, connection redundancy, and connection resilience, as defined under Section 2.1] Page 1-2
7 SECTION 1 General Requirements Description of significant global structural attributes [single point failure mechanism(s), structural irregularities, etc.] The outcome of these answers leads to either (1) an exemption (no further consideration required) or (2) the need to further consider the potential for progressive collapse. The detailed analysis required in the latter case is intended to reduce the probability of progressive collapse for new construction and identify the potential for progressive collapse in existing construction. These Guidelines present the methodology and performance criteria for these determinations without prescribing the exact manner of design or analyses. As such, the architect/engineer may apply methods appropriate to the facility at hand. A threat independent approach is, however, prescribed as it is not feasible to rationally examine all potential sources of collapse initiation. The approach taken (i.e., the removal of a column or other vertical load bearing member) is not intended to reproduce or replicate any specific abnormal load or assault on the structure. Rather, member removal is simply used as a load initiator and serves as a means to introduce redundancy and resiliency into the structure. The objective is to prevent or mitigate the potential for progressive collapse, not necessarily to prevent collapse initiation from a specific cause. Regardless of other specific design requirements, (e.g., blast design, seismic design, impact design, fire design, etc.) there are always scenarios that will be capable of initiating a collapse. For example, say a building is rammed by an 18-wheeler taking out 3 columns and collapsing several structural bays over 2 floors. These Guidelines and the provisions made herein would undoubtedly reduce the extent of this initially collapsed area. More importantly, however, provisions of these Guidelines should serve to help arrest the progression of the collapse and should reduce the extent of the damage. The strategy places a premium on well designed continuity as well as post event capacity, ductility and robustness as compared with just using key element resistance. No special value is placed on using more robust columns that can survive a particular threat or the use of larger spans to avoid multiple column failure from a specific point threat. 1.4 How To Use This Document The intent of this document is to provide guidance to reduce and/or assess the potential for progressive collapse of Federal buildings, for new or existing construction, respectively. It should be noted that the use of a Linear Procedure, as provided for in these Guidelines, is not intended for and not capable of predicting the detailed response or damage state that a building may experience when subjected to the instantaneous removal of a primary vertical element. However, a Linear Procedure, albeit a simplified methodology, may, with proper judgment, be used for determining the potential for progressive collapse (i.e., a high or low potential for progressive collapse), providing the acceptance criteria accounts for the uncertainties in behavior in the form of appropriate Demand-Capacity Ratios. The owner, architect, and project engineer should be thoroughly familiar with the provisions of these Guidelines, as it applies to their specific facility. Page 1-3
8 SECTION 1 General Requirements Progressive Collapse Analysis and Design Guidelines Exemption Process (Facility Exemption Consideration) Section 3 No further consideration for progressive collapse is required Yes Is the facility exempt from further consideration for progressive collapse? Report New Construction Section 4.1 or Section 5.1 No New or Existing Construction? Existing Construction Section 4.2 or Section 5.2 Design The potential for progressive collapse is high. Analysis. linear static-dynamic. nonlinear static-dynamic Analysis. linear static-dynamic. nonlinear static-dynamic No Does the structure meet the requirements for minimizing the potential for progressive collapse? Does the structure meet the requirements for minimizing the potential for progressive collapse? Yes Yes No The potential for progressive collapse is low and the facility has met the requirements for minimizing the potential for progressive collapse. The potential for progressive collapse is high and the facility has not met the requirements for minimizing the potential for progressive collapse. Prepare report that documents findings, recommendations and costs. Prepare report that documents findings, recommendations and costs. Figure 1.1. Overall flow for consideration of progressive collapse. Page 1-4
9 SECTION 1 General Requirements The first step of the process is to evaluate the facility using the methodology outlined in Section 3 of these Guidelines to determine if the facility might be exempt from further consideration for progressive collapse. If the facility is determined to be exempt, the process concludes with documentation of the exemption process. For new construction, if the facility is determined not to be exempt from further consideration for progressive collapse, the methodology for new construction outlined in Section 4.1 or Section 5.1, as applicable, shall be executed. This process provides design guidance and evaluation guidance for determining the potential for progressive collapse. If the potential for progressive collapse is found to be high for a given design, a redesign must be executed. When the criteria are satisfied (i.e., the potential for progressive collapse is low), the process concludes with documentation of the analysis procedure and results. For existing construction, if the facility is determined not to be exempt from further consideration for progressive collapse, the methodology for existing construction outlined in Section 4.2 or 5.2, as applicable, shall be executed. The potential for progressive collapse determined in this process (whether low or high) must be quantified and the analysis procedure and results documented. 1.5 Documentation Requirements The entire evaluation process shall be documented to a level such that the conclusions can be independently verified by in-house designers or outside firms and professionals under contract to the GSA. This generally will involve providing adequate support for the answers to each question in the process in the form of a report. The STANDGARD (Standard GSA Assessment Reporter & Database) software program shall be used for preparing progressive collapse assessment reports for all existing GSA buildings, to ensure that such reports contain the same type of information for each building assessed, and can be easily compared to other assessed buildings as part of a common database. STANDGARD may also be used for reporting on new and upgraded building designs. Information on STANDGARD and access to the program may be obtained at the website For the exemption process, the answer to each question in the flow diagram should be supported by a written description and graphics (as may be appropriate) to fully describe the conclusion. For example, the initial consideration in the exemption process will generally require the following supporting information: (1) the site plan showing minimum defended standoff distances, (2) a description of the overall construction type, (3) a description of both the global and local structural attributes as they may affect progressive collapse and (4) the level of protection required. From this information, the conclusion as to whether or not adequate standoff is sufficient to justify an exemption from further consideration of progressive can be determined from Table 3.1. Answers to subsequent questions in the flow diagram will require either a similar or even a higher level of supporting detail, depending on the material category of a given building (e.g., reinforced concrete building vs. steel frame building), particularly as to structural considerations. Page 1-5
10 SECTION 1 General Requirements The written description of global attributes shall include column spacing, building height and number of stories, and any significant structural irregularities. The written description of local attributes shall include a detailed summary of essential connection elements and a detailed description and sketch of the geometry of those elements as they affect the ability to maintain structural continuity across a removed vertical element, and to achieve a resilient and robust design. In particular, the project engineer is required to explicitly describe how the essential attributes of discrete beam-to-beam continuity across a column, connection redundancy, and connection resilience (as defined in Section 2.1) are achieved in any proposed new or retrofit upgrade design, as well as how they are achieved or not achieved individually and collectively for assessments of existing buildings. In particular, for steel frame buildings, the reporting analyst shall first describe the beamto-column connection type, using a general description such as those used in Appendix D. The reporting analyst shall then describe those connection attributes (favorable or otherwise) that directly affect the connection s ability to maintain independent structural beam-to-beam continuity across a removed column, similar to the language used to characterize the term symmetric reinforcement in Section 2.1 for reinforced concrete buildings. Favorable attributes include the use of creative detailing of the geometry of connection elements (i.e., judicious configuring of weld orientations, selection of weld types, and/or orientation of bolt groups) to achieve discrete beam-to-beam continuity across a column, coupled with connection redundancy to ensure a multiplicity of clearly defined load paths, and with increased torsional strength and minor-axis bending strength to provide overall connection resilience. Page 1-6
11 Section 2. Definitions The following definitions apply to terms used throughout these Guidelines. General terms are defined in Section 2.1, frangible/non-frangible façade is described in detail in Section 2.2, and alternate analysis methods are discussed in Section General Terms Abnormal Loads Loads other than conventional design loads (dead, live, wind, seismic, etc.) for structures such as air blast pressures generated by an explosion or impact by vehicles, etc. Allowable Extent of Collapse (Exterior Consideration) The extent of damage resulting from the loss in support of an exterior primary vertical load-bearing member that extends one floor above grade (one story) shall be limited. Explicit limitations for damage to primary and secondary structural components are defined in Sections 4 and 5. Allowable Extent of Collapse (Interior Consideration) - The extent of damage resulting from the loss in support of an interior primary vertical load-bearing member that extends one floor above grade (one story) shall be limited. Explicit limitations for damage to primary and secondary structural components are defined in Sections 4 and 5. Alternate Analysis Techniques Sophisticated analysis methods (e.g., nonlinear, dynamic finite element analysis, etc.) that may be used to determine the potential for progressive collapse in a given facility. Requirements and further discussion of this topic is included in Section 2.3. Atypical Structural Configuration A structural configuration that has distinguishing features or details. A detailed discussion of atypical structural configurations is presented in Appendix A. Connection Redundancy A beam-to-column connection that provides direct, multiple load paths through the connection. Connection Resilience A beam-to-column connection exhibiting the ability to withstand rigorous and destructive loading conditions that accompany a column removal, without rupture. This ability is facilitated by the connection s torsional and weak-axis flexural strength, its robustness, and its primary use of proven ductile properties of a given construction material. Defended Standoff Distance The defended standoff distance is the range between a point along the defended perimeter and the nearest structural element. Defended Perimeter The defended perimeter is the line that defines the boundaries of defended standoff zones (Figure 2.1). Parking within this defended zone must be limited Page 2-1
12 Defended Perimeter to cleared employees or other controlled parking as defined by the ISC Security Criteria. In addition, security countermeasures (i.e., an automatic vehicle identification (AVI). system, a prescreening system, etc.) must be in place, if parking is allowed in this zone, to reduce the potential for the delivery of an explosive device into this defended area. In order for the perimeter to be considered defended, as a minimum, vehicle barriers capable of stopping the Medium Level Protection vehicle explosive threat (defined in the ISC Security Criteria) must be in place, unless a Higher Level of Protection is specified. Vehicle barriers such as bollards, planters, retaining walls, landscaping, etc., can be designed to stop a vehicle of the specified weight and speed consistent with the criteria. Minimum Defended Standoff Distance Defended Standoff Distance Facility Defended Standoff Distance Defended Standoff Distance Figure 2.1. Illustration depicting defended perimeter and defended standoff distances. Discrete Beam-to-Beam Continuity A distinct, clearly defined beam-to-beam continuity link across a column, for steel frame beam-to-column connection applications, that is capable of independently transferring gravity loads for a removed column condition, regardless of the actual or potential damage state of the column. Exemption Procedure - A facility exemption process is offered for both new and existing construction. This process presents the designer/analyst with an outlet to further consideration of progressive collapse if the facility possesses structural and/or site characteristics that enable the facility to be considered a low potential for progressive collapse. Frangible Façade - An exterior façade system (wall systems, window systems, etc.) that has an ultimate, unfactored flexural capacity that is less than 1.0 psi. Refer to Section 2.2 for a more detailed description. High Potential for Progressive Collapse The facility is considered to have a high potential for progressive collapse if analysis results indicate that the structural member(s) and/or connections are not in compliance with the appropriate progressive collapse analysis acceptance criteria. Page 2-2
13 ISC Higher Level Protection - Minor damage, repairable. The facility or protected space may globally sustain minor damage with some local significant damage possible. Occupants may incur some injury, and assets may receive minor damage. ISC Low and Medium/Low Level Protection - Major damage. The facility or protected space will sustain a high level of damage without progressive collapse. Casualties will occur and assets will be damaged. Building components, including structural members, will require replacement, or the building may be completely unrepairable, requiring demolition and replacement. ISC Medium Level Protection - Moderate damage, repairable. The facility or protected space will sustain a significant degree of damage, but the structure should be reusable. Some casualties may occur and assets may be damaged. Building elements other than major structural members may require replacement. Linear Procedure - A Linear Procedure is a simplified analysis approach, and implies the use of either a static or dynamic linear-elastic finite element analysis. Low Potential for Progressive Collapse The facility is considered to have a low potential for progressive collapse if analysis results indicate that the structural member(s) and/or connections are in compliance with the appropriate progressive collapse analysis acceptance criteria. Such facilities may be exempt from any further consideration of progressive collapse. Non-Frangible Facade An exterior façade system (wall systems, window systems, etc.) that has an ultimate, unfactored flexural capacity that is greater than or equal to 1.0 psi. Refer to Section 2.2 for a more detailed description. Nonlinear Procedure - A Nonlinear Procedure is a more sophisticated analysis approach, and implies the use of either static or dynamic elasto-plastic finite element analysis methods that capture both material and geometric nonlinearity. It is generally a more accurate analysis approach than are Linear Procedures to characterizing the damage state of a structure. When such procedures are used, less restrictive acceptance criteria (Table 2.1) are permitted, in recognition of the improved response information that can be obtained from such procedures when employed by highly trained analysts. Primary Structural Elements As defined by the ISC Security Criteria, the primary structural elements are the essential parts of the building s resistance to abnormal loads and progressive collapse, including columns, girders, roof beams, and the main lateral resistance system. Primary Non-Structural Elements As defined by the ISC Security Criteria, the primary non-structural elements that are considered are all elements (including their attachments) that are essential for life safety systems or elements that can cause substantial injury if failure occurs, including, but not limited to, ceilings or heavy suspended mechanical units. Page 2-3
14 Progressive Collapse - Progressive collapse is a situation where local failure of a primary structural component leads to the collapse of adjoining members which, in turn, leads to additional collapse. Hence, the total damage is disproportionate to the original cause. Qualified Blast Engineer/Consultant Consultant should have formal training in structural dynamics, and demonstrated experience with accepted design practices for blast resistant design and with referenced technical manuals (Figure 3.3). To be considered qualified, a blast consultant should have a minimum of 5 years of demonstrated experience in the design and assessment of facilities subjected to blast loads as well as in the testing and evaluation of hazard mitigating products. Robustness Ability of a structure or structural components to resist damage without premature and/or brittle failure due to events like explosions, impacts, fire or consequences of human error, due to its vigorous strength and toughness. Secondary Structural Elements As defined by the ISC Security Criteria, the secondary structural elements are all other load bearing members (not included in the primary structural elements category), such as floor beams, slabs, etc. Secondary Non-Structural Elements As defined by the ISC Security Criteria, the secondary non-structural elements are all elements not covered in primary non-structural elements, such as partitions, furniture, and light fixtures. Single Point Failure Mechanism A structural feature in which a localized structural failure can lead to a widespread collapse of the structure. A primary example includes the use of transfer girders (i.e., beams or girders that typically provide vertical support for intermediate columns or load bearing members located above, hence, transferring load to the load bearing members supporting the girder). Another example includes exposed perimeter columns. Symmetric Reinforcement Symmetric reinforcement is defined here as having continuous (i.e., no lap splices across a column) and equal amounts of main reinforcing steel in both the compressive and tension faces of a reinforced concrete girder or beam, across a column. Traditional Moment Connection A traditional moment connection is defined here as a steel frame moment-resisting beam-to-column connection that 1) typically joins beam or girder flanges directly to the face of a column flange in the field by using either a complete joint penetration (CJP) groove weld in a T-joint configuration, and/or 2) may significantly depend on panel zone participation from the column s web to achieve its rotational capacity. Page 2-4
15 Typical Structural Configuration A typical structural configuration consists of a structural layout that is generally simple and contains no atypical structural configuration arrangements. Ultimate, Unfactored Capacity The calculated flexural, shear, and axial capacities with no use of a capacity reduction factor (i.e., φ = 1.0). Uncontrolled Public Areas These are areas located at the ground floor or entry level that are utilized by retail and other users and have no, or inadequate, operational security countermeasures in place. The concern for this situation is that an explosive device could be brought into the facility and placed at a vulnerable location, such as next to a column. Uncontrolled Parking Public parking or a parking area located within the footprint of the building under consideration, where no operational security countermeasures are in place to screen vehicles that could enter this area with an explosive device. 2.2 Frangible/Non-frangible Façade Façade systems that constitute at least 25% of the wall area per structural bay shall be evaluated for flexural capacity. The façade system that has the largest capacity shall be used to specify the type of façade system (i.e., frangible or non-frangible). Any façade system that occupies less than 25% of the wall area per structural bay shall be disregarded for this consideration. A non-frangible façade system is quantified by having a static flexural capacity equal to 1.0 psi or greater, based on a uniform distributed load acting inward (towards the interior of the building). A frangible façade system is quantified by having a static flexural capacity that is less than 1.0 psi, based on a uniform distributed load acting inward. The procedure (i.e., action, boundary conditions, etc.) for determining the flexural capacity of the façade system should correspond with the construction details of the actual façade system. Unfactored, ultimate strengths should be used in the determination of the capacity. Examples of this process are shown in Appendix C. 2.3 Alternate Analysis Techniques Nonlinear Procedure A Nonlinear Procedure implies the use of either static or dynamic finite element analysis methods that capture both material and geometric nonlinearity. It is generally a more sophisticated analysis approach than are Linear Procedures in characterizing the performance of a structure. When such procedures are used, less restrictive acceptance Page 2-5
16 criteria are permitted, recognizing the improved results that can be obtained from such procedures. Caution, however, must be exercised when using Nonlinear Procedures because of potential numerical convergence problems that may be encountered during the execution of the analysis, sensitivities to assumptions for boundary conditions, geometry and material models, as well as other possible complications due to the size of the structure. Accordingly, it is imperative that only experienced structural engineering analysts with advanced structural engineering knowledge be allowed to implement these sophisticated analysis tools and judgment must be used in interpreting the results. The qualifications and experience of those proposed to perform the Nonlinear Procedure shall be reviewed and approved by the project manager, prior to starting the work. Empirically determined damage criteria must be utilized to predict the potential collapse of a structural element. One such set of damage criteria that may be utilized in conjunction with a nonlinear analysis approach is outlined in an interim Department of Defense Construction Standard (Department of Defense, Interim Antiterrorism/Force Protection Construction Standards, Guidance on Structural Requirements (Draft), March 2001) and is included in Table 2.1. Table 2.1 provides the maximum allowable ductility and/or rotation limits for many structural component and construction types to limit the possibility of collapse. The values listed are for typical elements in conventional construction (i.e., construction that has not been hardened to resist abnormal loading). At the time of this writing the Department of Defense was considering modifications to their guidance. Unless explicitly accepted by the GSA, the guidance and criteria in the March 2001 document, as included in Table 2.1, shall be used. Because of the inherent challenges, complexities and costs involved, Nonlinear Procedures have been used less frequently for progressive collapse analyses than have Linear Procedures. In addition, infrequent usage of Nonlinear Procedures was, until only recently, reinforced by limitations in computer hardware and analysis software. However, advancements in computer hardware and general-purpose analysis software packages over the past few years have now made it possible to employ sophisticated structural assessment techniques on large and complex structures, including dynamic time history nonlinear response of high-rise structures containing thousands of members and connections covering a wide range of inelastic constitutive relations for the purpose of practical design applications. Structural engineers, with proper experience and knowledge in structural dynamics, can now construct a global model of the whole structure to capture both material and geometric non-linearity, and to perform the required dynamic time-history non-linear analyses of the entire structure. Page 2-6
17 Table 2.1. Acceptance criteria for nonlinear analysis 1. COMPONENT ROTATION ROTATION DUCTILITY (µ) 3 Degrees %Radians (θ) 4 (θ) 4 Reinforced Concrete (R/C) Beam R/C One-way Slabs w/o tension membrane R/C One-way Slabs w/ tension membrane R/C Two-way slabs w/o tension membrane R/C Two-way Slabs w/ tension membrane R/C Columns (tension controls) R/C Columns (compression controls) 1 R/C Frames Prestressed Beams 2 Steel Beams Metal Stud Walls 7 Open Web Steel Joist (based on flexural tensile stress in bottom chord) 6 Metal Deck Steel Columns (tension controls) Steel Columns (compression controls) 1 Steel Frames Steel Frame Connections; Fully Restrained Welded Beam Flange or Coverplated (all types) 1.5 Reduced Beam Section 2 Steel Frame Connections; Proprietary to Steel Frame Connections; Partially Restrained Limit State governed by rivet shear 1.5 or flexural yielding of plate, angle or T-section Limit State governed by high 1 strength bolt shear, tension failure of rivet or bolt, or tension failure of plate, angle or T-section One-way Unreinforced Masonry (unarched) 1 One-way Unreinforced Masonry (compression membrane) 1 Two-way Unreinforced Masonry to 4.5 (compression membrane) One-way reinforced Masonry Two-way Reinforced Masonry Masonry Pilasters (tension controls) Masonry Pilasters (compression controls) 1 Wood Stud Walls 2 Wood Trusses or Joist 2 Wood Beams 2 Wood Exterior Columns (bending) 2 Wood Interior Columns (buckling) 1 * Notes provided on following page NOTES H/25 Max sidesway H/25 Max sidesway See Appendix D See Appendix D See Appendix D Page 2-7
18 1. COTR approval must be obtained for the use of updated tables. 2. Ductility is defined as the ratio of ultimate deflection to elastic deflection (Xu/Xe). 3. Rotation for members or frames can be determined using Figures 2.2 and 2.3 provided below. 4. Concrete having more than 2-degrees rotation must include shear stirrups per requirements of DAHSCWE Manual (See Reference 3, Page 6-1). 5. Proprietary connections must have documented test results justifying the use of higher rotational limits. Figure 2.2. Measurement of θ after formation of plastic hinges. Figure 2.3. Sidesway and member end rotations (θ) for frames. Page 2-8
19 Additionally, it is recommended that the following downward loads be applied when assessing the potential for progressive collapse as presented in this Guideline. Static Analysis Loading For static analysis purposes the following vertical load shall be applied downward to the structure under investigation: Load = 2(DL LL) (2.1) Dynamic Analysis Loading For dynamic analysis purposes the following vertical load shall be applied downward to the structure under investigation: where, Load = DL LL (2.2) DL = dead load LL = live load (higher of the design live load or the code live load). Page 2-9
20 SECTION 3 Exemption Process Section 3. Exemption Process The following procedure provides a process for evaluating the potential for progressive collapse for reinforced concrete and steel framed buildings, resulting from an abnormal loading situation. If the facility is at an extremely low risk for progressive collapse or if the human occupancy is extremely low (as determined in this process), the facility may be exempt from any further consideration of progressive collapse. This process does not preclude a building from being evaluated for progressive collapse potential by other well-established procedures based on rational methods of analysis that are approved, in writing, by the GSA on a case-by-case basis. The facility should be evaluated for the possibility of being exempt from further consideration of progressive collapse using the included computer program (an automated version of the exemption process) or by the following manual procedure. To begin the automated version of the exemption process, click on the Begin Exemption Process button (at right). Begin Exemption Process Procedure: Step 1. Follow the steps in Flowchart 1, depicted in Figure 3.1, to determine the potential for total exemption to the remaining methodology. Step 2. Using Table 3.1, determine the minimum defended standoff distance consistent with the construction type and required level of protection (as determined by the GSA) of the facility under consideration. If the type of construction is not listed in Table 3.1, go directly to Step 3. Otherwise, follow the steps in Flowchart 2, depicted in Figure 3.2, to determine the potential for total or partial exemption to the remaining methodology. Note that defended standoff is only considered as one factor in determining if a facility is exempt. If a facility is not exempt and an analysis is required, the analysis process is threat independent. Step 3. This step offers a more detailed consideration of the facility if the requirements set forth in Step 2 are not achievable or the construction type is not included in Table 3.1. The user shall begin with Flowchart 3 (Figure 3.3) to determine the potential for total exemption. The user will then continue to Flowchart 4 or 5 (Figures 3.4 or 3.5, respectively) for concrete structures, or Flowchart 4 or 6 (Figures 3.4 or 3.6, respectively) for steel frame structures as indicated. Step 4. The results determined in the exemption process shall be documented by the project engineer and submitted to the GSA Project Manager for review. This process is documented in all STANDGARD generated progressive collapse assessment reports. Page 3-1
21 SECTION 3 Exemption Process In newly constructed facilities, the project manager is ultimately responsible for verifying that the site and structural characteristics used in this procedure are consistent from conceptual through 100% plans (including architectural, structural and site drawings). Should the project characteristics change or if the GSA Project Manager disagrees with the assessment of the characteristics used in the exemption process, further progressive collapse consideration may be required. For existing facilities, the GSA Project Manager shall review the results of this procedure documented by the project engineer. If the GSA Project Manager disagrees with the assessment of characteristics used in the procedure, further progressive collapse consideration may be required. It should be noted that limited test data currently exists for steel frame beam-to-column connections subjected to the type of loading conditions that accompany removal of a column. As a result, the exemption process criteria have been designed to be conservative and therefore, there will be very few exemptions for steel frame structures. Page 3-2
22 SECTION 3 Exemption Process Table 3.1. Minimum defended standoff distances for various types of construction. Minimum Defended Standoff Distance (ft) Construction Type Reinforced Concrete Construction Rigid frame structure with a non-frangible facade (FEMA 310 Building Type: C1, C2) Rigid frame structure with a frangible facade (FEMA 310 Building Type: C1, C3, RM2) Flat slab structure with a non-frangible facade (FEMA 310 Building Type: C2) Flat slab structure with a frangible facade (FEMA 310 Building Type: C3, RM2) Shear wall structure (FEMA 310 Building Type: C2) Steel Construction Rigid frame structure with a non-frangible facade (FEMA 310 Building Type: S4) Rigid frame structure with a frangible facade (FEMA 310 Building Type: S1, S5, RM2) Lightweight steel framed structures (i.e., Butler style buildings, etc.) (FEMA 310 Building Type: S1A, S2, S2A, S3, S5A) Masonry Construction Reinforced masonry wall with steel or r/c concrete frame (FEMA 310 Building Type: C3, RM2) Masonry bearing walls with reinforced CMU pilasters (FEMA 310 Building Type: RM1, URM, URMA) Precast Construction Precast concrete frame (FEMA 310 Building Type: PC1, PC1A, PC2, PC2A) Wood Construction Wooden frame (FEMA 310 Building Type: C2A, C3A, W1, W1A, W2) ISC Required Level of Protection Low and Medium Higher Medium/low These distances are used in the progressive collapse exemption process only and are not directly related to general standoff distances cited in the ISC Security Criteria. Page 3-3
23 SECTION 3 Exemption Process Flowchart 1 Begin initial considerations Yes Is the building classifed for agricultural use, intended only for incidental human occupancy or occupied by persons for a total of less than 2 hours a day? No Yes Is the building a detached one- or twofamily dwelling? No Yes Is the building a special structure (i.e., bridge, transmission tower, hydraulic structure, etc.)? No Yes Is the building a one-story building of light steel frame or wood construction with an occupied area less than 280 m 2 (3000 ft 2 )? No Yes Has the remaining useful life of the building been determined to be less than 5 years? No Yes Is the building a one story, unreinforced masonry, wood, or adobe structure? No Has the building been designed and constructed to meet the progressive collapse requirements of either the GSA or ISC Security Criteria? No Yes The facility is a candidate for automatic exemption from further consideration of progressive collapse. Proceed to Step 4 of the Exemption Procedure Yes Is there adequate documentation as defined in Section 1.5 to confirm the adequacy of the structrual design concerning progressive collapse? No Go to Flowchart 2 (Figure 3.2) Figure 3.1. Flowchart 1. To be used with Step 1 of the exemption process. Page 3-4
24 SECTION 3 Exemption Process Flowchart 2 Begin initial considerations Go to Flowchart 3 (Figure 3.3) No Is the defended standoff distance > that required for the construction type under consideration? (Table 3.1) Yes No Does the structure contain single point failure mechanism(s) and/or atypical structural conditions and/or is it over ten stories? Yes Does the facility have public areas and/ or uncontrolled parking? Yes Are these areas controlled with adequate security measures? Yes No No Is the facility designed consistent with at least Seismic Zone 3 1 or Seismic Design Category D or E 2 requirements? Yes The facility is a candidate for automatic exemption from the consideration of progressive collapse. Proceed to Step 4 of the Exemption Procedure Further consideration regarding progressive collapse is required. Proceed with the analysis/design guidelines for the minimization of the potential for progressive collapse (Section 4 for Reinforced Concrete Buildings; Section 5 for Steel Frame Buildings) No Further consideration regarding progressive collapse is required. Proceed with the analysis/design guidelines for the minimization of the potential for progressive collapse (Section 4 for Reinforced Concrete Buildings; Section 5 for Steel Frame Buildings) 1 As defined in the 1997 Uniform Building Code 2 As defined in the 2000 International Building Code Figure 3.2. Flowchart 2. To be used with Step 2 of the exemption process. Page 3-5
25 SECTION 3 Exemption Process Flowchart 3 Begin additional considerations Go to Flowchart 4 (Figure 3.4) No Has a blast engineer/ consultant designed the primary and secondary structural members for blast loads (frame, roof, walls, foundation) consistent with the ISC Security criteria? Yes Structural Considerations If applicable, do the pre- or posttensioned members contain continuous standard symmetric steel reinforcement? (in addition to the preor post-tensioned reinforcement) Yes Does the structural system contain any pre- or posttensioned members? Yes No Yes Does the facility have public areas and/or uncontrolled parking? No Are all the perimeter bays part of a continuous moment frame designed consistent with at least Seismic Zone 3 1 or Seismic Design Category D or E 2 requirements? Are these areas controlled with adequate security measures? Yes No Is the facility design consistent with at least Seismic Zone 3 1 or Seismic Design Category D or E 2 requirements? Yes Yes (Concrete Only) No Yes (Steel Only). For Reinforced Concrete Buildings: Go to Flowchart 5 (Figure 3.5) Yes. For Steel Frame Buildings: Go to Flowchart 6 (Figure 3.6) No For Steel Frame Buildings: Are all perimeter bays and all affected interior bays part of continuous moment frames? No No Further consideration regarding progressive collapse is required. Proceed with the analysis/design guidelines for the minimization of the potential for progressive collapse. (Section 4 for Reinforced Concrete Buildings; Section 5 for Steel Frame Buildings) 1 As defined in the 1997 Uniform Building Code 2 As defined in the 2000 International Building Code Figure 3.3. Flowchart 3. To be used with Step 3 of the exemption process. Page 3-6
26 SECTION 3 Exemption Process Flowchart 4 Begin additional considerations Structural Considerations If applicable, do the pre- or post-tensioned members contain continuous standard symmetric steel reinforcement (in addition to the pre- or post-tensioned reinforcement)? Yes Does the structural system contain pre- or post-tensioned members? No Yes Yes Does the facility have public areas and/or uncontrolled parking? No Are all the perimeter bays part of a continuous moment frame designed consistent with at least Seismic Zone 4 1 or Seismic Design Category F 2 requirements? No Are these areas controlled with adequate security measures? Yes Is the facility design consistent with at least Seismic Zone 4 1 or Seismic Design Category F 2 requirements? Yes (Concrete Only) Yes No No Yes (Steel Only). For Reinforced Concrete Buildings: Go To Flowchart 5 (Figure 3.5) For Steel Buildings: Are all perimeter bays and all affected interior bays part of a continuous moment frame? Yes. For Steel Frame Buildings: Go to Flowchart 6 (Figure 3.6) No No Further consideration of progressive collapse is required. Proceed with the analysis/design guidelines for the minimization of the potential for progressive collapse. (Section 4 for Reinforced Concrete Buildings; Section 5 for Steel Frame Buildings) 1 As defined in the 1997 Uniform Building Code 2 As defined in the 2000 International Building Code Figure 3.4. Flowchart 4. To be used with Step 3 of the exemption process. Page 3-7
27 SECTION 3 Exemption Process Flowchart 5 Final Considerations (Reinforced Concrete) No Does the facility have all of the following structural features? (1) symmetric reinforcement in all primary and secondary structural members (if applicable), as defined in Section 2.1, (2) structural bay widths <= 30 ft, (3) Story heights <= 16 ft (20 ft for courts) Yes Yes Does the structure contain single point failure mechanism(s) and/or atypical structural conditions and/or is it over ten stories? No Yes Does the primary load bearing structure use one of the following construction types? precast concrete or gravity connections No Further consideration of progressive collapse is required. Proceed with the analysis/design guidelines for the minimization of the potential for progressive collapse. (Section 4) The facility is a candidate for automatic exemption from the consideration of progressive collapse. Proceed to Step 4 of the Exemption Procedure Figure 3.5. Flowchart 5. To be used with Step 3 of the exemption process. Page 3-8
28 SECTION 3 Exemption Process Flowchart 6 Final Considerations (Steel) No Does the facility have all of the following structural features, as defined in Section 2.1? (1) Discrete beam-to-beam continuity, (2) Connection redundancy, (3) Connection resiliance, (4) Structural bay width <=30 ft, (5) Story heights <=16ft (20 ft for courts) Yes Yes Does the structure contain single point failure mechanism(s) and/or atypical structural conditions and/or is it over ten stories? No Yes Does the primary load bearing structure use one of the following beam to column connections? (1) Partially restrained moment, (See Appendix D) (2) Pre-1995'traditional" (3) Riveted (4) Post-1995 without successful AISC cyclic testing (as defined in Section 5.1.1) No Further consideration of progressive collapse is required. Proceed with the analysis/design guidelines for the minimization of the potential for progressive collapse. (Section 5) The facility is a candidate for automatic exemption from further consideration of progressive collapse. Proceed to Step 4 of the Exemption Procedure Figure 3.6. Flowchart 6. To be used with Step 3 of the exemption process. Page 3-9