In light of recent tragedies, mitigation of progressive collapse has become an important consideration in the design of certain kinds of buildings.
A summary of the developed guidelines
In light of recent tragedies, mitigation of progressive collapse has become an important consideration in the design of certain kinds of buildings. While many types of buildings essentially are exempt from such considerations, high profile or government buildings, for example, may require an analysis to determine if progressive collapse is likely.
This article examines current national code requirements on structural integrity and progressive collapse, and summarizes the guidelines developed by the General Services Administration (GSA) and the Department of Defense (DoD) to mitigate progressive collapse.
No quantifiable or enforceable requirements to mitigate progressive collapse are contained in the International Code Councils International Building Code or any of the legacy codes. General structural integrity requirements are given in Section 1.4 of the American Society of Civil Engineers Minimum Design Loads for Buildings and Other Structures (ASCE 7-02) and the following definition of progressive collapse is given in the commentary of that section: Progressive collapse is defined as the spread of an initial local failure from element to element, eventually resulting in the collapse of an entire structure or a disproportionately large part of it.
Direct and indirect design methods also are specified in the commentary of ASCE 7-02. In the Direct Design Method, resistance to progressive collapse is considered by using either the Alternate Path Method or the Specific Local Resistance Method. In the Alternate Path Method, local failure of a structural member is permitted to occur and alternate load paths are provided so that major collapse is avoided. Design for a specific threat or abnormal load is not considered. In contrast, structural members are sized and detailed for specific threats or loads in the Specific Local Resistance Method.
The Indirect Design Method stipulates minimum levels of strength, continuity, and ductility to mitigate progressive collapse. No threats or abnormal loads are identified and no structural members in the building are removed notionally. The structural integrity reinforcement requirements in Section 7.13 of the American Concrete Institutes Building Code Requirements for Structural Concrete and Commentary (ACI 318-05) are an example of this type of method. The intent is to improve redundancy and ductility by prescribing minimum amounts of continuous reinforcement and specific types of detailing of certain members in a reinforced concrete structure.
The ASCE 7-02 provisions on progressive collapse are more qualitative than quantitative. The commentary contains a number of different design concepts and details to meet general structural integrity requirements. Providing sufficient continuity, redundancy, and/or ductility in the structural members of a building is key to mitigating progressive collapse.
As part of their overall building security requirements, the General Services Administration (GSA) and the Department of Defence (DoD) have developed guidelines that specifically address the vulnerability of buildings to progressive collapse. Each of these guidelines is examined below.
The latest GSA criteria for progressive collapse, which were published in June 2003, are in Progressive Collapse Analysis and Design Guidelines for New Federal Office Buildings and Major Renovation Projects (GSA Guidelines). The purpose of the GSA Guidelines is to assist in the reduction of and assessment of the potential for progressive collapse in new and existing buildings, respectively. The methodology and performance criteria are prescriptive, and the potential for progressive collapse is examined independently of any specific threat. Vertical load-bearing members are removed to help introduce redundancy and resiliency into the structure; this approach does not necessarily replicate the effects of any specific abnormal load. The criteria in the GSA Guidelines are not part of a blast design or analysis, and the results based on these criteria should not be substituted for those obtained from a substantiated blast analysis.
Flowcharts are used to determine whether a building is exempt from detailed consideration for progressive collapse. A number of important attributes (including occupancy, type of framing system, number of stories, and standoff distance) are used in the exemption process. If it is determined that further consideration for progressive collapse is required, then either a linear or nonlinear static/dynamic analysis procedure must be used. A linear analysis approach is limited to low- to mid-rise buildings that are 10 stories or less in height with relatively simple structural layouts. A more sophisticated nonlinear analysis usually is used for taller buildings and/or buildings that have atypical structural configurations (see Appendix A of the GSA Guidelines). Only qualified structural engineers who are experienced in interpreting the results should perform such an analysis. The discussion here will focus on the linear static analysis approach.
As a minimum, the building must withstand the loss of one primary vertical load-bearing member without causing progressive collapse. Both exterior and interior scenarios must be examined for typical structural configurations. In the exterior scenario, a framed building is to be analyzed separately for each of the following cases: 1) Instantaneous loss of a column in the first story located near the middle of the short side of the building; 2) Instantaneous loss of a column in the first story located near the middle of the long side of the building; and 3) Instantaneous loss of a column in the first story located at the corner of the building.
Buildings with underground parking areas or uncontrolled ground floor areas (for example, retail areas that have no security countermeasures in place) must be analyzed for the instantaneous loss of an interior column as well (interior scenario). Similar analyses are required for buildings with exterior and interior walls.
When performing a static analysis for the scenarios outlined above, the following vertical load is to be applied to the structure:
Load = 2(DL + 0.25LL) where DL = dead load, and LL = live load.
Since the probability is small that full live load is present during a possible progressive collapse event, 25 percent of the live load is used in the load combination. Also, the factor of 2 is a simplified way to account for dynamic effects that amplify the response when a column or wall is instantaneously removed from a building.
Design material strengths may be increased by a strength-increase factor. Table 1 summarizes the strength-increase factors for various construction materials. For structural steel components, the factors for tensile strength and yield strength depend on the grade of the steel and the year it was fabricated.
After the analysis has been performed, a demand-capacity ratio (DCR) is computed for each of the structural members in the building:where QUD = acting force (demand) determined in a component or connection/joint from the analysis (bending moment, axial force, shear force, and possible combined forces); and QCE = expected ultimate unfactored capacity of the component and/or connection/joint (bending moment, axial force, shear force, and possible combined forces).
Failure of a structural member depends on the magnitude of the DCR. In typical structural configurations, structural elements and connections in reinforced concrete buildings that have DCR values for bending moment, axial force, shear force, or a combination thereof that exceed 2.0 are considered to be damaged severely or collapsed. In such cases, it is unlikely that members and/or connections will have additional capacity for redistributing loads. In steel-framed buildings, the maximum DCR values range between 1 and 3, depending on the structural component (see Table 5.1 in the GSA Guidelines).
Once the DCRs have been computed after the required number of analysis runs, it is possible to determine the extent of collapse, if any. The maximum allowable extent of collapse resulting from the instantaneous removal of an exterior column or wall must be confined to the smaller of the following two areas: 1) the structural bays directly associated with the instantaneously removed column or wall; or 2) 1,800 square-feet at the floor level directly above the instantaneously removed column or wall. Similar limits are given for allowable collapse areas based on the removal of interior columns or walls. A high potential for progressive collapse is assumed to exist when collapse areas that are determined from analysis are greater than the appropriate limiting values prescribed in the GSA Guidelines.
In new construction, several iterations in the design of key structural members may be needed before the acceptance criteria are satisfied. Although not required by the GSA Guidelines, procedures to obtain preliminary member sizes are given in Appendix B. Existing facilities undergoing modernization are to be upgraded or retrofitted to meet new construction requirements.
The latest DoD requirements for progressive collapse are in Design of Buildings to Resist Progressive Collapse (UFC 4-023-03), which was published in January 2005. This Unified Facilities Criteria (UFC) applies to new construction, major renovations, and leased buildings and must be used in accordance with Minimum Antiterrorism Standards for Buildings (UFC 4-010-01). According to UFC 4-010-01, all new and existing buildings three stories or more in height must be designed to avoid progressive collapse. Even if a structure has been designed to resist a specific abnormal load or threat, the progressive collapse requirements of UFC 4-023-03 must still be satisfied.
The level of progressive collapse design for a structure is correlated to the Level of Protection (LOP) assigned to the building as determined by the Security Engineering Facility Planning Manual (UFC 4-020-01)—see Table 2.
A design requirement not in Table 2 that must be satisfied for all LOPs is that the floor system at all levels, including the roof, must be able to withstand a net upward load equal to 1.0D + 0.5L, where D is the dead load based on self-weight only, and L is the live load. This uplift load, which accounts for the effects that may be caused when a building is subject to abnormal loading, is applied to each bay in the structure, one bay at a time. The members and connections are designed for this load using the appropriate criteria in Chapters 4 through 8 in UFC 4-023-03.
Tie force method—In the Tie Force method, a building is mechanically tied together. Minimum tie forces, which vary with construction type and location in the structure, typically are resisted by the structural members and connections that are designed for gravity and lateral loads. The purpose of the horizontal and vertical ties is to enhance continuity and ductility, and to develop alternate load paths in the structure. Internal and peripheral horizontal ties must be provided, along with ties to external columns and walls. Required tie strengths are given in Chapters 4, 5, 6, 7, and 8 of UFC 4-023-03 for reinforced concrete, structural steel, masonry, wood, and cold-formed steel construction, respectively. In all cases, the paths of ties must be straight and continuous; no changes in direction are permitted.
In a reinforced concrete structure, the flexural reinforcement in slabs, beams, and girders can be used to satisfy the horizontal tie force requirements. Similarly, longitudinal steel in concrete columns can be used to satisfy vertical tie force requirements. Steel beams/girders are used as horizontal peripheral ties and column ties in a steel structure, while either steel beams or the reinforcement in the concrete slab are used as internal ties. Keep in mind that the connections between the steel members also must be designed to transfer the required tie forces. Steel columns are used as vertical ties. Providing ductile detailing at connections and joints usually satisfies tie force requirements in masonry construction. In such cases, seismic details can be used to develop tie forces (see Appendix E of UFC 4-023-03 for some typical seismic details). Requirements for internal horizontal ties, external wall ties, peripheral floor ties, and vertical ties in wood-framed construction can be satisfied using a combination of member strength and supplemental mechanical ties (straps, hangers, and other manufactured connection products).
Structural members that do not provide the required horizontal tie force capacity must be redesigned or retrofitted in new and existing construction, respectively. The Alternate Path (AP) method, which is described below, is not to be used in these types of situations. However, if a vertical structural member cannot provide the required vertical tie force capacity, either the member must be redesigned or the AP method must be used, where that particular member is removed from the structure.
Alternate path method—The AP method, which is applicable to buildings assigned to medium and high LOPs, is similar to the method in the GSA Guidelines in that vertical load-bearing elements are removed at various locations in the building. However, unlike the GSA Guidelines where the members are removed at only the first floor level, in the DoD AP method the vertical elements are removed at each floor level, one at a time. For example, if there are three exterior columns that must be investigated and there are five stories in the building, 15 AP analyses must be performed. The details of the analysis are given in Chapters 4 through 8 for the different construction materials; including overstrength factors for material strength, which are analogous to the GSA strength-increase factors.
In a linear or nonlinear static analysis, apply the following amplified factored load to those bays immediately adjacent to the removed element and at all floors above the removed element:
Load = 2[(0.9 or 1.2)D + (0.5L or 0.2S)] + 0.2W
where D = dead load, L = live load, S = snow load, and W = wind load.
For the rest of the structure, the following load is applied:
Load = (0.9 or 1.2)D + (0.5L or 0.2S) + 0.2W
Subsequent to the analyses, acceptability criteria must be satisfied. This consists of satisfying strength design requirements of the applicable material standards and deformation limits (ductility and rotation) for all of the members in the structure.
When an external column or load-bearing wall is removed, the collapsed area of the floor directly above the removed element must be less than the smaller of 750 square feet or 15 percent of the total area of that floor. Also, the floor directly beneath the removed element should not fail, and collapse must not extend beyond the structure tributary to the removed element. The damage limits for interior columns and walls is two times that of exterior ones. Any collapse must not extend into the bays immediately adjacent to the removed element.
Additional ductility requirements—The main goal of the additional ductility requirements, which are applicable to all types of construction in structures assigned to Medium and High Levels of Protection, is to ensure that the failure mode for all external columns and walls in the ground floor is flexural (ductile) rather than shear (brittle).
There is debate about whether the progressive collapse critetia in the GSA and DoD Guidelines or criteria developed by other sources are the most suitable to use to mitigate progressive collapse. While this debate may continue for some time until progressive collapse criteria are codified in the United States, keep in mind that continuity, redundancy, and ductility are essential attributes to provide in any building structure. Keep in mind that continuity, redundancy, and ductility are essential attributes to provide in any building structure.
David A. Fanella, Ph.D., S.E., P.E., is a project manager at Graef, Anhalt, Schloemer & Associates, Inc., headquartered in Chicago. He can be reached at firstname.lastname@example.org.