FORENSIC ENGINEERING 2012 © ASCE 2013
`
`1015
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`Non-Destructive Radiographic Evaluation and Repairs to Pre-Stressed
`Structure Following Partial Collapse
`
`
`
`E. M. Reis, Ph.D., P.E.1 and U. Dilek, Ph.D., P.E.2
`
`
`1Siemens Energy, Inc., 110 MacAlyson Court, Cary, NC 27511; PH (919) 463-8757;
`FAX (919) 463-8731; engin.reis@siemens.com
`2Independent Consultant, udilek@gmail.com
`
`ABSTRACT
`
`This article presents use of radiographic imaging (X-Ray) in evaluation of existing
`reinforcing steel configuration of structures and development of steel retrofit and
`carbon fiber reinforced polymer (CFRP) repairs. A collapse of the driving surface in a
`precast concrete parking deck prompted an engineering evaluation and survey of the
`whole deck for damage assessment and repairs to distressed members. Distress was
`identified in decking members and perimeter spandrel beams. Repairs to the decking
`members involved supporting the distressed decking using supplemental steel
`brackets installed through the double-tee stems containing pre-stressing tendons. The
`precise location of the tendons in the stems needed to be identified to implement this
`repair in order not to damage the tendons during drilling. Radiographic X-ray
`imaging in this application enabled locating and avoiding the tendons in the stems to
`support and strengthen the decking member. The supplemental steel bracket also
`enabled continued operation of an existing expansion joint in the area of repair. The
`same technique was also used to identify steel reinforcement configuration in the
`spandrel beams exhibiting cracking at bearing locations for evaluation of existing
`steel configuration and implementation of CFRP repairs .
`
`INTRODUCTION
`
`
`The subject parking structure is a multi-level precast concrete parking garage.
`A typical level of the parking garage is constructed of 10 ft (3 m) wide by 60 ft (18.3
`m) long precast, pre-stressed, lightweight aggregate concrete double-tee members that
`are simply supported on spandrel beams at either end. Column spacing, and
`therefore, the length of the spandrel beam is 30 ft (10 m) and each spandrel beam
`collects the end reactions of three 10 ft wide double tees. Each double tee bears on
`the spandrel beam with its two stems, through either 6 recessed bearing points or
`protruding corbels on the spandrel beam.
`
`
`
`OVERVIEW OF THE FAILURE
`
`The consecutive flanges of decking forming the driving surface are connected for
`membrane action by way of embedded hairpins or plates at matching points on each
`double tee. Embedded hairpins are connected using a short piece of bar stock welded
`on either side of the joint. This connection is concealed by the field placed concrete
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`Metromont Ex-1011, p.1
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`FORENSIC ENGINEERING 2012 © ASCE 2013
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`
`1016
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`topping slab serving as a traffic wearing course. Connection of the decking enables a
`smooth ride and the decking members to deflect simultaneously as vehicles traverse
`over the joints. This connection however did not exist at the main expansion joint of
`the parking deck. The expansion joint was equipped with a commercially available
`neoprene expansion joint to accommodate lateral movement, but no connection to
`deal with vertical movement. This detail caused the double-tees on either side of the
`joint to deflect independently. As a wheel would traverse over the joint it would make
`the first member deflect and impact the flange of the second member causing distress
`over time.
`A failure of the decking occurred adjacent to the main expansion joint in the
`parking deck. (see Figure 1) The flange of one double tee sagged while a heavy truck
`traversed the joint and subsequently collapsed to the lower level. Post-collapse
`investigation revealed that this flange was cracked which was perceived as non-
`structural and waterproofed by sealing from the top (see Figure 2). Lack of support
`and lack of load sharing from adjacent double-tee and the impact of wheels over the
`service life of the deck was identified as the primary cause of the failure.
`
`
`
`
`
`Figure 1. Decking failure at expansion joint and subsequent repairs.
`
`An engineering evaluation of the structure was requested by the owner to
`identify the overall condition of the deck and areas exhibiting similar or other
`conditions of concern and to evaluate necessary repairs or rehabilitation for continued
`safe operation of the parking garage. The scope of the engineering evaluation
`consisted of two parts; survey of the parking deck, non-destructive evaluation and
`design and implementation of repairs.
`
`SURVEY OF THE DECK
`
`
`The collapse of decking occurred at an expansion joint at which connection of
`consecutive members was non-existent and repeated wheel impact due to members
`deflecting individually had distressed the decking portion of the double-tees. A
`
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`Metromont Ex-1011, p.2
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`FORENSIC ENGINEERING 2012 © ASCE 2013
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`1017
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`survey of the deck identified distress similar to the section that failed at other
`expansion joints, as well as distress at regular joints due to failed connections
`between consecutive decking members (see Figure 2).
`
`
`
`
`
`Figure 2: Distress at other expansion joint locations and regular D/T connections
`
`The survey also identified distress at the spandrel beams. The spandrel beam
`collects end reactions of three double tees through the two stems of each double tee
`resulting in either 6 recessed bearing points or protruding corbels. Transverse cracks
`indicative of shear failure were noted at the bearing points (see Figure 3). The cracks
`were particularly pronounced at the outermost bearing locations (bearings 1 and 6),
`where the shear force on the spandrel beam is the highest.
`
`
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`Figure 3. Shear cracking at recessed bearing and corbel locations of spandrel
`
`DEVELOPMENT OF REPAIR OPTIONS
`
`Type I: Spandrel Beam Cracking: The repair methodology involved identification
`of existing reinforcing configuration using radiography and subsequent design of
`CFRP repairs to externally supplement the spandrel beam.
`
`
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`Metromont Ex-1011, p.3
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`FORENSIC ENGINEERING 2012 © ASCE 2013
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`1018
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`Type II: Expansion Joint Location with Failed Section: The first step of repairs
`involved restoring the failed decking by forming and anchoring a new concrete
`section into place (see Figure 1). The decking and the field placed topping slab had a
`total thickness of 10 cm (4 in.) making it challenging to anchor with any substantial
`capacity. Furthermore the working of the existing expansion joint as-is was already
`conducive to cause failures. The design therefore involved supporting the decking
`once restored, using steel profiles supported from other non-distressed segments of
`the double tees. The design would support the expansion joint vertically so that
`decking would deflect together while allowing horizontal movement. Successful
`implementation of this repair required precisely locating the tendons in the stem
`portion of the double tee using radiography, so that tendons could be avoided during
`drilling.
`
`Type III: Expansion Joint Location without Distress to the Decking: This repair
`was a simpler version of the Type II repair above, which did not require supporting
`the decking from the stems due to lack of distress. The repair did not involve
`radiography to locate tendons, but a dual channel repair, with each channel connected
`to one side, allowing horizontal movement but providing decking to work together
`vertically.
`
`Type IV: Severed Regular D/T Joint Connection With or Without Distress to
`Decking: This repair was a combination of Types II and Types III, however the main
`difference was the steel channel at the joint was anchored on both sides since no
`horizontal or vertical movement is required at this type of joint. In some instances a
`simple re-instatement of the connection was performed while in other instances
`distressed decking also needed to be supported.
`
`NON-DESTRUCTIVE EVALUATION
`
`
`Radiographic exposures are similar to medical X rays. Radiographic
`exposures have found widespread use in the construction industry both in assessment
`cases to identify the presence and configuration of steel in the member, as well as
`locating steel or prestress/post-tension cables prior to removal of cores or cutting
`openings in slabs for various reasons (Dilek, 2009).
`The technique is advantageous when compared to covermeters or ground
`penetrating radar (GPR) such that it provides a clear picture of the steel inside
`concrete, rather than an indication-based on electrical or physics principles. In simple
`terms, it involves a radioactive source placed on one side of the member and a
`radiographic film placed on the other side of the member for the duration of a
`predetermined exposure time. The exposure time is determined based on the
`thickness and density of the member and the remaining half-life or strength of the
`radioactive source in curries. The technique requires a safe boundary (generally a 2
`mR/hr boundary) to be evacuated during the exposure when the source is engaged.
`This generally is a 75 to 100 feet distance depending on the strength of the source,
`which creates complications particularly in occupied buildings or downtown settings.
`
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`Metromont Ex-1011, p.4
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`FORENSIC ENGINEERING 2012 © ASCE 2013
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`1019
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`The technique involves placement of lead letters or templates in the frame for
`reference in order to identify the position of objects in the picture with reference to a
`known location such as a corner or end of the member. Figure 4 shows developed
`films photographed over a light table. Upon developing the film, the measured
`locations of reinforcing steel on the film need to be corrected for geometric skew (see
`Figure 4) for identification of true location. Lead letter templates (see Figure 4)
`provide the reference for these corrections.
`
`
`Figure 4 Lead letter templates for reference and correction for geometric skew
`
`
`To implement the Type II repairs a precise location of the tendons in the stems
`and determination of the harping pattern was necessary to not damage the tendons
`while drilling for the support assembly. Record drawings were not available. The
`stems of the double-tees were X-Rayed at multiple points using a Cobalt radioactive
`source and radiographic exposure film and a precise tendon layout plan and harping
`pattern was determined to be able to install steel support brackets without damaging
`the tendons (see Figure 5 ). Radiographic X rays in this application served the
`purpose of locating and skipping the tendons in an undamaged member to support
`and strengthen a damaged member.
`
`
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`Figure 5. Radiographic Exposures to determine tendons and harping pattern
`
`Figure 6 shows the harping pattern of the double tees and how the drilled
`through bolt locations are modified along the stem to avoid the tendons.
`
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`Metromont Ex-1011, p.5
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`FORENSIC ENGINEERING 2012 © ASCE 2013
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`1020
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`Figure 6. Tendon harping pattern and through bolt pattern based on X rays
`
`Radiographic exposures were also performed at select spandrel beam bearing
`locations for the the purpose of identifying the steel configuration in the damaged
`spandrel beams both in terms of number and size of steel and configuration of any
`anchored bearing plates in the bearing areas.
`
`DESIGN AND IMPLEMENTATION OF REPAIRS
`
`Type I: Spandrel Beam Cracking: The existing reinforcing steel configuration was
`determined using radiographic exposures. Drawings were not available for
`comparison with the in-situ configuration. A cursory structural review indicated that
`steel configuration at the bearing areas on spandrel beams was slightly deficient for
`the shear at ends of the beam. A pronounced pattern of cracking was noted at a
`majority of the outermost bearing locations (bearing 1 and 6) where the shear load
`was highest (see Figure 7). A retrofit scheme using carbon fiber reinforced polymer
`(CFRP) material was designed in general accordance with ACI 440 (2007) guidelines
`to supplement the distressed bearing areas externally. The repair design included 1
`ply, 8 in. (203 mm) wide U-wraps around the spandrel beam at the bearing locations
`and 1-ply, 8 in. (203 mm) wide, 24 in. (610 mm) long longitudinal strip of CFRP
`sheet along the cracked area. The design developed was a secondary system per
`requirements of ACI 440 (2007) to supplement the primary reinforcing.
`
`Type II: Expansion Joint Location with Failed Section: Prior to implementation
`of supplemental steel repairs the driving surface was restored (see Figure 1). Upon
`restoring the decking, through-bolt holes were drilled at locations determined based
`on radiography and steel profiles were installed to support the decking.The design
`would support the expansion joint vertically so that decking would deflect together
`while allowing horizontal movement. This was accomplished by having dual
`members that were bolted only on one side of the joint.(see Figure 8). Steel profiles
`were pre-measured and welded together prior to installation. Steel shim blocks and
`noeprene bearing pads cut to fit steel-to-concrete interface were used to account for
`differences in surface tolerances as well as to dampen the noise (see Figure 9).
`
`
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` Forensic Engineering 2012
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`
`Metromont Ex-1011, p.6
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`FORENSIC ENGINEERING 2012 © ASCE 2013
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`1021
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`Figure 7. Cracking locations after preparatory grinding and CFRP repairs
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`Figure 8. Steel bracket repair at expansion joint when decking is distressed
`
`Type III: Expansion Joint Location without Distress to the Decking: The repair
`at expansion joint locations that did not exhibit decking distress was simpler. It did
`not involve radiography or supporting the decking from the stems. The repair was a
`dual channel repair, with each channel connected to one side, allowing horizontal
`
`
`
`
`
` Forensic Engineering 2012
`
`Downloaded from ascelibrary.org by Christopher Kelly on 05/31/20. Copyright ASCE. For personal use only; all rights reserved.
`
`Metromont Ex-1011, p.7
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`FORENSIC ENGINEERING 2012 © ASCE 2013
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`1022
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`movement but providing decking to work together vertically at the expansion joint
`(see Figure 9).
`
`
`
`Figure 9. Steel bracket repair at expansion joint when decking is not distressed
`
`Type IV: Severed Regular D/T Joint Connection With or Without Distress to
`Decking: This repair was a combination of Types II and Types III. However, the
`main difference was the steel channel at the joint was anchored on both sides since no
`horizontal or vertical movement is required at this type of joint. In some instances a
`simple re-instatement of the connection was performed with channels bolted to both
`sides. In other instances distressed decking also needed to be supported.(see Figure
`10)
`
`
`
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`
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`Figure 10. Restored D/T Connection without and without decking distress
`
`
`CONCLUSIONS
`
`
`
`
`In the absence of drawings for the double-tees in a parking garage,
`radiographic imaging was used to locate the tendons in the double tee stems. The
`purpose of the radiographic imaging was to precisely locate the tendons to enable safe
`
`
`
` Forensic Engineering 2012
`
`Downloaded from ascelibrary.org by Christopher Kelly on 05/31/20. Copyright ASCE. For personal use only; all rights reserved.
`
`Metromont Ex-1011, p.8
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`FORENSIC ENGINEERING 2012 © ASCE 2013
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`1023
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`drilling without damage to the tendons during steel retrofit installation. Radiographic
`exposures in this application served the purpose of locating and avoiding the tendons
`in an undamaged member to support and strengthen a damaged member. The
`engineering evaluation also identified shear cracks in the exterior spandrel beams at
`the bearing points to support perpendicular double-tees. In this case, radiographic
`imaging helped identifying the steel reinforcement configuration in the spandrels
`exhibiting distress for the purpose of developing supplemental CFRP repairs.
`
`REFERENCES
`
`
`Dilek, U. (2009). “Chapter 4: Condition Assessment of Concrete Structures.” Failure
`Distress and Repair of Concrete Structures, Ed. Norbert Delatte, Woodhead
`Publishing, CRC Press, New York
`ACI 440 (2007) Report on Fiber Reinforced Polymer (FRP) Reinforcement for
`Concrete Stuctures
`
`
`
`
`
`
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`
` Forensic Engineering 2012
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`Downloaded from ascelibrary.org by Christopher Kelly on 05/31/20. Copyright ASCE. For personal use only; all rights reserved.
`
`Metromont Ex-1011, p.9
`
`