«R. SHANKAR NAIR R. Shankar Nair R. Shankar Nair, Ph.D., P.E., S.E. is a principal and senior vice president of Teng & Associates, Inc. in Chicago. In ...»
Simply stated, the vulnerable element is removed and the structure should not collapse. The structure must have another load path to prevent collapse. Conditions from the structure which are considered “fundamental” by the GSA in new designs and upgrades are the “double span” design of beams and girders. Beams and girders need to be continuous through columns so that if a column is removed, the resulting structure can develop an alternate load path and carry the existing loads. Clearly most ordinary buildings could not meet this guideline even though a progressive collapse consideration does exist in some codes. In addition the GSA guideline recommends connection resilience similar to that developed with the AISC seismic standards for connections.
From a lateral system consideration the guidelines would develop designs with uniformly distributed moment frames on all the column grids as a first approach to robustness and redundancy. Bracing systems, which could be severely impacted by local blast effects are less robust than uniform moment frames and would be discouraged or combined with uniform moment frames. A perimeter moment frame strengthened on the first level above grade is also recommended.
As demonstrated clearly in the World Trade Center collapse, serviceability (wind stiffness) and redundancy can provide considerable reserve strengths for unexpected demands on a structure. The lateral system designed for wind and gravity had the strength and robustness to provide an alternative load path for a severely disrupted gravity system.
A brief list of some of the guidelines noted in the General Services Administration document is listed below:
• Continuity of floor members to produce alternate load paths.
• Redundancy for alternate load paths.
• Provide extra strength on first supported level above grade.
• Moment frames on the perimeter for protection of exterior elements. Keep girders same, oversize connections.
• Tie everything together – beneficial effects of composite construction.
• Provide extra reinforcing in first supported slab – to strengthen membrane effect.
• Consider longer effective lengths for first tier columns considering a local loss of floors.
• Consider loss of localized lateral system – bracing most vulnerable, uniformly distributed moment frames best.
EXISTING STRUCTURE - EVALUATION
After the events of a 9/11, many owners have requested a vulnerability analysis of their buildings. As an example of a structure not designed for any specific threat, a 39 story tower was investigated for the specific threat of losing one or two of its main vertical supports at the street level. Of interest here is not damage but collapse potential. The purpose of the analysis is not an “exact” simulation of a structural collapse, but to provide the analyst with useful information for assessing the performance of the structure and to make judgments on its safety. Capacity evaluation should be done without the usual factors of safety and member strength modifications.
The 39 story structure under consideration was comprised of typical structural steel construction with metal deck and concrete floors, steel beams and columns and several transfer girders at the lowest level. The lateral system is a fully welded perimeter moment frame of beams and columns. Because of transfers on several levels above the ground level the frame possessed additional strength in these levels. This is recommended in the guideline but came about because of gravity transfers.
An analysis of the structure was carried out incorporating the GSA load conditions of 2 (DL &.25L) with the columns removed. This analysis used a “push down” methodology (similar to the “push over” analysis) to capture the non-linear behavior of the perimeter frames as the full load was applied. Figures 1 through 4 show the results of analysis for two different locations of missing columns. The perimeter frame, originally controlled by wind drift design, indicated a capacity to redistribute the load to adjacent columns. As seen from the “push down “ diagrams yielding did not begin until well above the original design load levels. An important consideration in this study was the investigation of the actual connection capacity to transfer the loads in the new load path. This analysis is approximate in that the membrane action of the slab is not considered which would provide additional resistance to the structure. More detailed programs such as RAM’s PERFORM – COLLAPSE are now available which do include the important membrane effect.
By considering the recommended design guidelines of the current GSA progressive collapse and DOD AntiTerrorist standards one can apply design standards to existing structures to evaluate their vulnerability. The following items are a brief summary of essential features.
• Defensive Design – Keep blast away from structure
• Element resistance – not available, capacity demand, accept local failure
• Local analysis unnecessary – Remove element from structure and prevent progressive collapse Progressive Collapse
• Alternate load paths, imperative
• Redundancy (goes hand in hand with above)
• Resilient connections.
• Over strength connections to create alternate load paths.
• Overall design continuity vs. local element resistance.
1. “Progressive Collapse Analysis and Design Guidelines for New Federal Office Buildings and Major Modernization Projects”, General Services Administration, June 2003.
2. “DoD Minimum Anti-Terrorism Standards for Buildings”, Department of Defense, UFC, 4-010-01, July 31, 2002 3. “Structural Design for Physical Security”: State of the Art Practice, ASCE Tech Committee, SEIASCE, 1999 <
CONSIDERATIONS FOR RETROFIT OF
EXISTING STEEL BUILDINGS FOR RESISTING
BLAST AND PROGRESSIVE COLLAPSE
BY WILLIAM J. FASCHAN, RICHARD B. GARLOCK, AND DANIEL A. SESIL
Evaluation and subsequent strengthening of existing structures for extreme loading cases, such as blast, require a realistic and pragmatic design approach. Effective communication between Owner, Structural Engineer, Architect, Risk Analyst, Insurance Providers, and other stakeholders is paramount to a finished project that is satisfactory to all. The benefits of structural steel for use in the renovation of existing buildings are well documented and are applicable to the type of retrofitting required for resistance to blast and progressive collapse. The performance of steel construction during the 1993 bombing of The World Trade Center is further evidence. Combination of the existing conditions of the structure and the nature of the threat leads to strengthening techniques that may not be the first choice in the case of new construction. Less intrusive types of strengthening are favored. The general approach for strengthening of existing buildings starts with researching the original construction documents and then performing a condition assessment of the building. Vulnerability analysis is a multi-step process where there is constant dialogue about the possibilities of non-structural methods to decrease the threat on the building. With the goal of enhancing a building’s performance under an extreme event, we have provided a range of upgrades from enhanced perimeter protection to structural hardening.
INTRODUCTIONBalancing cost and vulnerability is a challenge for the security of any new building. Add considerations of an existing structure, building occupants, decades-old design criteria, and you have begun the first step in the evaluation of the hardening prospects for an existing building. Today, many building owners who are developing new buildings with a high risk profile are considering extreme loading criteria for their building designs. The pool of existing buildings, however, is much greater, and owners of these buildings are questioning how their structures would perform under similar criteria.
Evaluation and any subsequent strengthening of existing structures for extreme loading cases, such as blast, require a realistic and pragmatic design approach. Effective communication between Owner, Structural Engineer, Architect, Risk Analyst, Insurance Providers, and other stakeholders is paramount to a successful finished project. With the goal of enhancing a building’s performance under an extreme event, we have provided a range of upgrades from enhanced perimeter protection to structural hardening. The benefits of structural steel for use in the renovation of existing buildings are well documented and are applicable to the type of retrofitting required for resistance to blast and progressive collapse. This paper discusses the aspects of blast hardening specific to existing facilities. Lessons learned from the 1993 bombing of The World Trade Center are described. Following a discussion on a general approach for the hardening of existing structures, we discuss common goals and situations. Due to the confidential nature of the work, project names and identifying photos are not provided.
ASPECTS OF BLAST HARDENING SPECIFIC TO EXISTING FACILITIES
Effectively protecting an existing facility by blast hardening is a relatively difficult task. Realistically, the built environment has a number of inherent weaknesses when considering the possible effects of an extreme event. Rare is the facility that has systems designed for improved performance in an extreme event. Cladding, site planning, stairs, power systems, and structures are planned to deal with more common environmental conditions. Structures are typically constructed without specific consideration of redundancy or robustness in an extreme event.
While risk analysis and vulnerability assessment are essential first steps in any security project, these steps take on a special importance for an existing facility. Due to the particular difficulties of effectively hardening an existing building, it is important that the risk analysis and vulnerability assessment result in a clear understanding by the client of the potential vulnerabilities and of the scale of construction work that may be required to mitigate or prevent damage from the identified threats.
Since the costs of hardening an entire existing facility are often so high, clients commonly choose to focus their efforts on specific locations or functions within a facility where risks are highest. They establish limited hardening objectives. Frequently, non-structural security measures prove to be the client’s most practical and cost-effective alternative. Preventing or re-routing pedestrian or vehicular traffic, instituting operational changes, providing redundancy in the building’s critical power and sprinkler systems and other similar measures are often the most effective techniques for enhancing the performance of an existing facility.
Perhaps the single most important aspect of existing building security projects is the identification of practical alternatives, the best of which may not involve structural hardening.
Where a decision is made to harden some part of an existing facility or a specific structural system or element, the
design approach is influenced by a series of factors that include the following:
• Information about relevant existing conditions is often limited;
• Structure to be renovated is commonly hidden or obstructed by existing architectural or building services systems that are difficult or costly to remove;
• Structural renovation work is typically constrained by the need for continuity of building operations;
• Generally, renovation of a steel-framed structure is more economical than the renovation of a concrete structure;
• The use of steelwork in a renovation is generally more cost-effective than the use of reinforced concrete;
• The level of ductility of the existing construction may limit its strength.
These factors lead to fundamental differences in the approach to blast hardening between new and existing construction.
Uncertainty about existing construction may limit the sophistication of blast analysis that is appropriate; there may be no point in a precise determination of the presumed behavior where no equally precise understanding of the existing structure or its connections is available.
Conversely, the high cost of renovating an existing structure may justify a more sophisticated blast analysis where reliable detailed information is available and where there is reason to believe that substantial savings may be achieved in the construction cost of the strengthening project.
The approach that one takes with the analysis, or the design, is a matter of effectively relating the scale of the enormous, but transient, blast pressures to the effective resistance of the structure.
In considering the construction cost of retrofitting an existing facility, it is axiomatic to consider the total construction cost, not simply the structural costs. Often, the non-structural costs will equal or exceed the structural costs; therefore, the true costs of a retrofit project relate more to the number of locations of work than to the amount of work done in each location. This relationship should influence structural design and analysis decisions. For example, sophisticated analysis that reduces, but does not eliminate, the reinforcement of an inaccessible column may have little real benefit.
In new blast-resistant construction, ductile structural systems are designed to deform inelastically under large blastinduced forces. In many instances, existing construction will have limited post-elastic dynamic capability. Often, performance is limited by the shear capacity of critical structural elements. Further, wide-spread strengthening of the construction may be precluded by the costs of removing and replacing the enclosing construction.