«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 ...»
In steel structures, common deficiencies include susceptibility to local buckling of outstanding flanges, and lack of connection ductility. Strengthening of a limited number of structural elements is usually practical, and, as with other types of renovations, there is a relative ease of working with steel construction.
In cast-in-place concrete structures, ductility and strength are often limited by the amount and the detailing of the existing reinforcement. For columns, fiber wraps, steel plate encasement and reinforced concrete encasement alternatives are practical. In most cases, wide-spread strengthening of a cast-in-place concrete structure will be impractical. Enhancement of a limited number of structural elements, however, is usually feasible.
Generally, precast concrete structures will exhibit the worst extreme event performance characteristics of either steel and concrete construction with potential weakness both at connections and in the detailing of the reinforcement.
Consequently, the potential for effectively strengthening precast concrete structures is relatively limited.
There are fundamental differences between new construction and structural renovation and these differences are equally fundamental for blast hardening projects. The best designs for blast hardening of existing facilities are based on a clear vision of the overall security goals of the project and on an equally clear understanding of the detailed limitations of that which exists.
LESSONS LEARNED FROM THE 1993 BOMBING OF THE WORLD TRADE CENTERAt 12:18PM on Friday February 26th 1993, a 1000 lb. TNT equivalent bomb, located within a van parked immediately adjacent to the columns of the south wall of the north tower, was detonated in the second basement of The World Trade Center complex.
The resulting explosion destroyed approximately 30,000 sq. ft. of the concrete flat slab floor construction located at the first and second basements and badly damaged over 25,000 sq. ft. of construction. The structural steel framing above suffered far less damage. A small opening was breached within one bay at street level and the plaza level framing was left with a prominent upward bow.
This destruction occurred outside of the footprint of the towers. The tower structures were largely unscathed, with the north tower suffering minor damage to one column and the loss of two diagonals located immediately adjacent to the blast. The primary structure of the south tower suffered no damage. At no time was the structural integrity of either tower significantly impacted.
Conversely, several steel columns that supported the north end of the 22-story Vista Hotel were rendered un-braced for a 68 foot height by the loss of the concrete flat slabs in the basements. In the days immediately following the explosion, one of the most critical structural repair initiatives was to temporarily stabilize these columns. This was accomplished through the installation of tubular steel bracing.
From the perspective of performance of existing structural systems in an extreme event, i.e. systems not explicitly
designed for blast resistance, two clear lessons can be learned from this tragic event:
1. The inherent blast resistance of an existing structure will be highly dependent on the scale of the building and on the scale of the wind or seismic forces for which the building has been designed; and
2. It is likely that steel construction will perform better than concrete construction because of its greater inherent ductility.
The structures of the 110-story World Trade Center towers were essentially unaffected by the blast, while the stability of the 22-story Vista Hotel was put into jeopardy. Fundamentally, this difference is a function of the relative scale of the buildings and the type of construction within the footprints of each building.
The inherent difference in blast-resistance between the steel and concrete construction was apparent to those of us who worked on the reconstruction. All around the blast site, one observed individual steel beams and columns remaining where the surrounding concrete slabs or beams were destroyed. Where steel beams failed, one observed in the dismembered pieces dramatic evidence of the ductile behavior that preceded failure.
Also, we observed large sections of concrete floor slab suspended in mid-air by one or two reinforcing bars, evidence of both the value of inherent tensile strength and the absence of this strength that is often the weakness of existing concrete construction not designed for extreme events.
GENERAL APPROACH TO HARDENING EXISTING STRUCTURES
Risk assessment and structural vulnerability assessment 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. The structural engineering vulnerability assessment of the existing facility needs establish the global strength of the lateral load resisting system relative to the magnitude of potential extreme events. Further, the particular vulnerabilities of specific structural systems and elements need be identified.
The importance of an overall vulnerability assessment of the structural systems of an existing facility needs to be communicated to the client at the beginning of a project. A certain prominent feature of the structure, such as a column, that the client wishes to protect may well be the reason an evaluation was sought; however, the loss of a related transfer truss or girder may precipitate a similar or larger collapse.
First, one needs begin with a condition assessment of the building. Ideally, this assessment should include review of the original construction drawings, shop drawings, subsequent renovation drawings, and maintenance records.
Commonly, however, the available documentation is limited. Where relevant information is lacking, a prudent assessment report may qualify the pertinent conclusions. Following a review of the documentation, one or more visits to the facility, complemented by probes at critical locations, are usually appropriate. In some circumstances, a consolidated set of drawings is created. An additional component of the assessment of the building is a load survey.
We sometimes conduct load surveys in existing buildings where we are transferring columns. When it comes time to study an element or a systems demand-to-capacity ratio the data from the survey can be very useful.
With the structural condition assessment of the existing building complete, a structural vulnerability assessment may be made for either an undefined threat or series of defined threats. Regardless, one should neither finalize the structural vulnerability assessment, nor proceed with a hardening design, until the risk assessment is completed.
Preliminary Analysis - Getting into scale Typically the risk analysis addresses the possibility of threats from explosives ranging in size from a pipe bomb up to a fully laden tractor trailer. Depending upon the project’s original design criteria, i.e. which natural hazards were considered, the height of the building, and the lateral load resisting system, it is possible that the global strength of a building’s structure may be overwhelmed by the larger threat scenarios.
For buildings where the global structural system may be easily compromised, alternatives to structural hardening are the preferred course of action. Often, by relocating high-risk functions, the building threat may be reduced to manageable proportions. For buildings were the global systems are able to withstand the range of forces triggered by the threat with modest intrusions, we then turn our attention to an evaluation of redundancy and specific potential weaknesses in the structure.
For the structure assessment, the following needs to be considered:
• local behavior of the items that make up the element, such as the flange of a column;
• global behavior of an element; such as large deflection of a column unable to restrain the resulting P-delta forces;
• local behavior of the connections; such as net section limitations at bolt hole lines;
• local behavior of transfer systems and trusses; and
• global behavior of lateral load resisting systems.
To analyze the effects of the blast pressures on the varying elements one of the more practical resources is the Army’s TM 5-1300 “Structures to Resist Accidential Explosions” (Army TM 5-1300, 1990). This reference, in combination with the ConWep Software (USAEWES/SS-R, 1992)1 guide you in the calculation of the pressure and related information necessary to perform an analysis for reinforced concrete construction and for structural steel construction. There is additional software and reference guides available, FRANG (NCEL, 1988), FRANG (NCEL, 1989), and even expanded code finite element analysis software like Weidlingers’ FLEX, but availability, cost and performance vary considerably. In some cases, a hand calculation is sufficient. At other times the use of advanced software is necessary. For those entering the field, a good reference is Structural Dynamics (Biggs, 1964).
Dr. Theodore Krauthammer provided an overview of available software during the “Modern Protective Structures Short Course” sponsored by SEAoNY in June 2003.
Consideration of Alternatives – Both Structural and Non-Structural Following the preliminary vulnerability analysis the design team meets to discuss the range of non-structural alternatives and the strengthening alternatives for the project. Often, the final criteria for the strengthening of the building are decided at this meeting.
Consideration must be given to the weight of the new structure that is being added to the building. A significant increase of weight to the building, such as will accompany the addition of concrete encasement or concrete walls, may require a supplementary reinforcement of the gravity or lateral load resisting systems of the structure. Care need be taken in checking with the local jurisdiction about the applicable building code to use in the case where you are reanalyzing the lateral load resisting system of the building. As with any renovation or retrofit project, the possibility of triggering a comprehensive upgrade of the existing building to the current building code needs to be considered. In almost all cases, accomplishing such an upgrade will not be economically or functionally feasible.
Detailed design and the impact of connections As with most renovations, the cost impact on the project is directly related to the number of locations within the building requiring work. For most hardening, steel elements provide more cost effective results than concrete. The connection of choice for field installation of steel work is welding. For detailed design, we typically turn to steel for its many advantages. As with most renovations, steel provides compact, high strength, and ductile elements which in turn bring an ease of installation and attachment.
When reviewing existing structural elements, we typically find that the connections are the first limitation to the elements performance under extreme loads. Strengthening of connections at beam-to-column joints afford another leap in capacity for the system.
COMMON GOALS AND SITUATIONS
With rare exception, both owners and architects are seeking a more secure building that does not take on the ‘bunker’ aesthetic. These goals can be significant hurdles until after the risk assessment has effectively communicated limited hardening objectives based on limiting low probabilistic threats.
The hardening goals may include hardening of the room/area used for mail handling/inspection, hardening of the security barrier at entry and hardening of specific structural elements that are deemed vulnerable.
Protecting specific structural elements The damage or loss of an individual member of a transfer system or truss may result in a disproportionate degree of damage to the building structure. Hardening of these elements takes the form of connection reinforcement, element reinforcement and stiffening of buckling-prone portions of the elements, e.g. individual plates of an unfilled built-up box column.
Where the design of the site layout or underground facilities provides for key elements, such as columns, to be accessible to attack, element hardening may depend upon the space available. Where possible the entire architectural envelope around the column may be utilized for increasing the strength of the element. In some locations, there also may be room within the depth of the ceiling to provide bracing to increase the buckling strength.
Providing redundancy to structural systems For most existing buildings strengthening of elements is costly. It can often be highly intrusive and provide only localized protection. Providing load paths or system redundancy to the critical structural systems is another means of strengthening. In this case, one assumes that a critical element of a system is lost in the event and its loads need be shed to an adjacent system. For some buildings this may take the form of simply interconnecting a series of existing trusses, girders and columns such that they act together to effectively redistribute loads. For other buildings a completely new system may need to be inserted.
Hardening a specific room or area Mail rooms are examples of rooms which can be subject to internal explosions. Strengthening for these areas includes the walls, floors, and ceilings. Venting systems are often an efficient method of mitigating the effects of blast in the hardened room and beyond. Strengthening in combination with a venting system is often your most efficient solution.
For some agencies, their building complex may include high risk functions that are directly adjacent to areas of public access. These include areas built below open air plazas, or buildings which are built with minimal setbacks from the public roads. External explosions outside of these areas are unable to be mitigated by increasing stand-off distance unless the local authority is willing to reroute traffic (highly unlikely). In these areas, the first response is to inquire about moving the high-risk agency or function away from the area. When that is not able to occur, one looks for blast curtains or sacrificial walls to shield the area from the direct pressures of a blast.