Key Project Summaries

 

 

Characterizing the Response of a Signature Bridge

 

Situation:  A Southeastern state DOT elected to conduct in-depth monitoring to characterize the structural response of its most valuable bridge.  The bridge, a six year old cable-stayed design, is subject to several major non-live load forces — seismic, wind, and thermal.  Increased knowledge of the structural response will lead to better decision making for non-routine maintenance, inspection effectiveness, and maximizing asset life.

System Installation:   The owner decided that LifeSpan’s Progressive Diagnostic Protocol approach would best meet its needs.  While both system controllers were sized to accommodate a future doubling of sensing devices, only 24 PeakStrain and six temperature sensors were initially installed, providing the owner with a reasonable and cost effective starting point for understanding the structural response.  Of particular interest are peak strain values, both tensile and compressive, in major floor beams, stay girders and two cable stays.  The project was delivered on-schedule and on-budget.

Discussion:   The owner frequently checks the values returned via the cellular network to the network operations center from his iPad.  While the expected live load strains are relatively minor, the entire bridge system is experiencing significant thermal loading.  The data will be reviewed over time to quantify the actual effects of temperature change on the structure and additionally, the operation of the expansion joints.  Monitoring is ongoing and expected to last for years.  Additional sensing devices may be added at key locations as initial diagnostic conclusions are made at the end of the first thermal cycle.

 

 

Reducing Bridge Postings with Maximum Strain Data

 

Situation:   Load postings on bridges increase commercial user costs by higher fuel use and lower driver productivity.  Postings affect other bridges and local residents with collateral damage, congestion and increased air pollution.  A state DOT recently decided to implement an AASHTO-approved alternative process to calculate more accurate and hopefully less severe load postings as a means to enhance local economic competitiveness and spur economic development, while reducing the negative effects caused by detours.

The Process:   The 2011 version of AASHTO’s Manual for Bridge Evaluation (MBE) states: “The actual performance of most bridges is more favorable than conventional theory dictates. When a structure’s computed theoretical safe load capacity or remaining fatigue life is less than desirable, it may be beneficial to the Bridge Owner to take advantage of some of the bridge’s inherent extra capacity that may have been ignored in conventional calculations.” The MBE explains how to use maximum strain values obtained from sensors to calculate a correction factor that can substantially increase the safe load rating. 

Discussion:   The state DOT has supplied sensor kits to each bridge inspection team as an additional tool to capture not only maximum strain for load posting calculations, but also field monitoring of cracks, load distribution studies, out-of-plane bending, and proper operation of bridge bearings.  The expectation is that the bridge inspection personnel will be more closely aligned with engineering and maintenance objectives, supporting difficult decisions on load postings, rehab or replacement actions.  Fifty percent of bridges evaluated so far have avoided a restrictive load posting.     

 

 

Confirmation of a Unique Repair Methodology 

 

Situation:   A state DOT was experiencing concrete cracking between superstructure beams and deck on a number of bridges across the state.  Although the beam-deck joint was assumed to be integral, undetermined forces were causing the deck to move horizontally and vertically, creating cracks along a fillet and shearing reinforcement bars.  After several expensive bridge replacements, the DOT decided to try a new repair methodology to reduce the expense and outage time from wholesale replacement of these deficient structures.

System Installation:   The state DOT decided to install sixteen dual channel PeakStrain sensors at various locations across existing cracks in a manual data capture configuration.  As the photograph shows, some sensors were installed at right angles to capture both horizontal and vertical displacement at the same location.  The installation was accomplished at night, to avoid traffic issues; and was completed safely, on-schedule and on-budget.

Discussion:   LifeSpan worked with the state DOT to develop a data capture scheme to determine the effectiveness of the proposed repair.  This cost-effective manual solution was possible because LifeSpan PeakStrain sensor can capture maximum displacement without power.  The state DOT used a LifeSpan handheld reader to determine maximum displacements before the repairs started and again, after repairs were completed –comparing the displacements to confirm that the unexpected lateral and vertical movement had stopped.  LifeSpan’s sensors confirmed the repair technique; saving this state DOT millions of dollars in unnecessary bridge replacements.

 

 

Safe Extension of the Operating Life for a Deficient Bridge

 

Situation:   A state DOT was concerned about two long-span bridges that showed degradation due to heavy section loss from age and in one case, tidal salt water.  Both bridges were eligible for replacement due to low sufficiency ratings, but only one was possible to replace due to funding constraints.  Heavy truck traffic required consideration of structural strengthening for one bridge being replaced while the new bridge was under construction.  It was determined to defer the $875,000 project in favor of monitoring to assure safe operations.  The owner decided to monitor the other structure also for assurance of safety while a replacement project was deferred in favor of higher priority projects; a fundamental tenant of good transportation asset management.

System Installation:   LifeSpan installed less than twenty of its PeakStain sensors at various locations on each bridge.  Sensor locations were selected by the bridge owner to capture strain at periods of maximum and minimum loading, for both tension and compression members.  Because of their unique design, LifeSpan’s PeakStrain sensors are capable of capturing peak strain events even without power, so important data is not missed.  Several temperature sensors were also installed to determine the influence of temperature on observed strains.  A system controller and solar power source were part of the solution for each bridge.  Installation was completed on-schedule and on-budget.

Discussion:   The state DOT periodically reviews information in graphical format on smartphones and personal computers over the Internet via LifeSpan’s network operations center (NOC).  The NOC is completely secure and password protected, allowing only the state DOT to access the structural information.  Changes in the performance of one bridge led the DOT’s enforcement unit to apprehend overloaded logging trucks illegally using the bridge.  In addition to assuring safety, this state DOT sees a robust return on investment as replacement capital expenditures are deferred.

 

 

Confirmation of a Unique Repair Methodology

 

Situation:   A major U.S. city was faced with replacing fifteen short span bridges to upgrade their load carrying capacity.  Because of unique design standards required by a channel crossing, replacement bridges were estimated to cost about four million dollars each; a total of $60 million for the entire replacement program.  Just prior to signing a contract for the first bridge replacement, a national engineering firm contacted LifeSpan to see if we could monitor structural response after installation of a CFRP (carbon fiber) strengthening solution.  The system needed to be installed before rising water in the channel made access to the structure impossible.  LifeSpan delivered and installed the custom configured system ahead of schedule, less than thirty days after receipt of the order on Christmas Eve.

System Installation:   LifeSpan worked with the engineering firm to install a comprehensive solution, consisting of nineteen dual channel PeakStrain sensors, four temperature sensors, and an on-site controller for data capture and communication with LifeSpan’s Internet Network Operations Center.  The controller was powered by a battery pack with solar recharge installed on a concrete pad near the bridge.  To protect against vandalism all wiring was installed in conduit, with chain link fencing and razor wire surrounding the equipment cabinets.

Discussion:   The most interesting facet of the LifeSpan solution was the embedment of four sensors in the deck, with two sensors mounted on top of the CFRP rods that were buried in the concrete.  The engineering objective was to measure load induced strains and compare strains on the CFRP rods with strains in the concrete deck.  Also, a series of sensors were installed underneath the deck, some mounted on CFRP soffit mats and the remainder on the concrete soffit.  The system returned data flawlessly for several years.  The city is projecting savings of nearly $45 million dollars by using this repair technology while confirming repair effectiveness with a LifeSpan monitoring solution.  The project received the prestigious Engineering Excellence Grand Award from the state’s ACEC.

 

 

Out-of-Plane Bending on a Signature Bridge

 

Situation:   A national engineering firm needed to assess the impact of fatigue cracking, severe section loss, and potential out-of-plane bending on a series of stringer webs on a signature bridge.  They contacted LifeSpan Technologies and discussed how such monitoring could be implemented.  In particular, the bridge owner was concerned about future limitations of loads due to continued degradation of this signature bridge and had a seventy million dollar repair project ready to remediate this problem.

System Installation:   LifeSpan installed eighteen dual channel PeakStrain sensors at locations chosen by the owner’s engineering consulting firm.  In particular, LifeSpan’s unique sensors were mounted to capture web deflection in both tensile and compressive modes.  The installation of LifeSpan’s sensors was accomplished on-schedule and on-budget.

Discussion:   LifeSpan’s PeakStrain sensor is extremely versatile, allowing measurement of a wide range of variables under demanding conditions.  In this project, mounting was by means of customized extension rods installed on both sides of the stringer flanges.  Stiffness of these rods had to be assured, which was accomplished by use of a novel backer magnet that prevented bending toward the web.  The LifeSpan solution continues to report data with no interruption of peak displacement capture even with intermittent power loss by the third-party systems integrator’s data capture electronics.  The ability to capture peak data without power was a major factor in deciding to use LifeSpan’s PeakStrain sensor.  And, the owner has successfully deferred implementation of the seventy million dollar repair project, saving well over three million dollars in interest cost every year.

 

 

Load Monitoring for a Major Retailer

 

Situation:   A major U.S. retailer built a new store with two levels; one for their traditional format, the other for a warehouse format.  The load carrying capacity of the second floor beams was questioned before completion, since point loading was expected, given the inevitable movement and concentration of palletized inventory.

System Installation:   A forty channel strain sensor monitoring system was installed with appropriate control hardware and software.  Displacement data was captured on a regular schedule and data uploaded to LifeSpan’s Network Operations Center (NOC).  A local overload annunciator panel was also installed and activated for alerting in-store personnel.  The project was delivered on-budget and within thirty days of receipt of order (two weeks ahead of schedule).

Discussion:   The owner was concerned about controlling point loading to avoid high levels of stress/strain on the second floor beams.  Internet capture and distribution of the sensor derived information was made available to store personnel, the building contractor and structural engineers.  The sensor information clearly showed store loading (initial inventory build) and subsequent additional loading in certain areas prior to the Christmas holiday season.  After reviewing the information captured for six months, the structural engineering firm established limits of displacement/strain and these limits were programmed into the local alarm software.  Store personnel received alarms when loading was over established limits and took corrective action to alleviate potential overloaded conditions.  This solution saved the owner millions of dollars in unnecessary repair work and a substantial delay in the store opening.

 

 

Effectiveness of the Repair Program for an Older Turnpike Bridge

 

Situation   An Eastern toll authority developed plans to repair an older deck truss bridge structure on their system.  Prior to letting the repair contract, the authority decided to install a structural monitoring system that would monitor key compressive and tensile stresses for up to six months, establishing a baseline of strains/stresses to compare against similar information captured after the repairs were completed.  In essence, the owner wanted to determine if the recommended repair, based on an NBIS visual inspection, was necessary and effective. 

System Installation:   A twelve dual channel PeakStrain sensor monitoring system was installed with appropriate hardware and software, including a battery power source with solar recharge capability.  Displacement/strain data was captured several times per day and relayed via cellular modem to LifeSpan’s Network Operations Center (NOC).  Four sensors are measuring current and peak compression; eight are measuring current and peak tension.  The installation was completed on-budget and ahead of schedule.  Data flowed immediately to the owner.

Discussion:   The owner has the ability to download captured data from the NOC into convenient Excel spreadsheets for subsequent analysis.  The owner’s engineering consultant also noted differences in strain due to temperature differentials between a truss side exposed to the sun vs. the shade.  Monitoring is ongoing and a finite element model was being developed using data from the structure, confirming the repair was not necessary.  The system has been fully operational for approximately seven years on its original set of solar recharged batteries with no loss of data.

 

 

Damage from Adjacent Construction Activity

 

Situation:   In addition to conducting condition assessment of older structures, LifeSpan’s configured solutions can be used to assure asset condition during adjacent construction operations.  The City of Raleigh, North Carolina used a LifeSpan solution to assess the potential damage caused by adjacent deep excavation and foundation construction adjacent to their newly opened parking deck.

System Installation:   Twenty dual channel PeakStrain sensors and a wireless monitoring system were installed.  The LifeSpan sensor values were reported several times daily, during peak load and unloaded conditions.  Alarm conditions were reported immediately.  Uploaded to the LifeSpan Network Operations Center, the data was available for analysis at any time.  The project was completed on-schedule and on-budget.

Discussion:   If damage did occur, LifeSpan’s monitoring solution would automatically alert the City’s third party engineering firm.  Their structural engineers could then evaluate the information, assess the damage, and work with the construction team to resolve any issues before the parking deck was damaged beyond repair.  The cost of LifeSpan’s monitoring solution was a small investment to insure Raleigh’s new ten million dollar parking deck survived undamaged during adjacent construction.

 

 

FRP Reinforced Concrete Structures

 

Situation:   An older concrete bridge pier suffered severe corrosion damage.  In some places, the corroded rebar expanded and cracked the overlying concrete creating a spalling condition.  A composite, carbon-fiber wrap was applied to strengthen the pier and safely extend its life.

System Installation:   A single, PeakStrain sensor was attached to the outside of each composite wrap to detect changes in hoop-strain.  Since this was a manual system, no additional equipment, such as a power supply or data acquisition system, was required for this application.

Discussion:   As the rebar continued to corrode and expand, it created hoop strain in the composite wrap.  When this strain gets too large, it can damage the wrap, so that the composite wrap may not provide the additional strength required in this application.  This condition can occur without any visible damage to the outside of the wrap as the column is now completely enclosed, and it is impossible to detect continued corrosion by visual inspection.  Since the deterioration is quite slow, it is not necessary to monitor the sensors continuously.  The inspector simply uses a LifeSpan handheld reader to capture data manually whenever he performs a routine inspection.  The sensor provides a precise, quantitative indication of continued corrosion and warns of damage to the wrap that might otherwise go undetected.

 

 

Facade Cracks on the Washington Monument

 

Situation:   During renovation of the Washington Monument, a number of large facade cracks were discovered.  These cracks penetrated through the stone veneer and extended vertically through multiple veneer stones.  The cause of cracking was unknown and the US Park Service wanted assurance more severe cracking would not appear.

System Installation:   An assortment of fifty sensing devices was installed to monitor crack width, temperature, tilt, humidity and wind force.  Three groupings of sensors were located outside the Monument at approximately 200′ and 465′ above the ground.  Additional sensors were mounted inside the monument at positions near those on the outside.

Discussion:   Data from all sensing devices was collected every five minutes.  The sensor controller transmitted this data once each day.  The data was analyzed to correlate crack width with the other measured parameters in order to determine the probable cause of the cracking.  The data was placed on a secure web site that incorporated a 3D model of the Monument, graphical data presentation and photographs of the sensors.  Although the cracks did not pose an immediate structural concern, it was prudent to monitor to ensure that no additional remedial action was necessary.

 

 

Crack Monitoring of Steel Girders

 

Situation:   Large cracks were propagating in a steel girder bridge where the transverse cross-bracing attached to the girders.  The owner needed assurance that stress relief measures were working.

System Installation:   Several PeakStrain sensors were mounted between the girders and the cross-brace attachments.  A data-acquisition system collected the outputs and transmitted the readings via cellular data modem to the owner’s offices.

Discussion:   LifeSpan’s peak displacement sensor provides a precise record of how far the crack opens under varying traffic loads and temperature differentials.  By monitoring the peak-displacement on a daily basis, it was possible to determine how quickly the cracks were growing  

 

 

Differential Settlement of a Public Structure

 

Situation:   A large, public stadium needed to be expanded beyond its original size.  The proposed new construction used mat foundations.  The original construction was built on piles driven to bedrock.  The stadium is in an alluvial, river basin within a zone-four earthquake area.  Differential settling of the independent, reinforced concrete structure was considered possible and a potential problem if settling was not uniform.

System Installation:   LifeSpan’s PeakStrain sensors were installed to monitor differential movement between the old and new stadium structures.  Other sensors were installed to monitor acceleration of the upper decks and strain in several cantilevered beams.  The sensors were located in three groups, linked to a central data collection location at the stadium.  Combined data was transmitted to the owner via a telephone modem.

Discussion:   This multi-purpose system collected data once a day to monitor the rate of differential settlement.  This data was used to document structure settlement over time and to schedule modifications to utility connections before major damage occurred.  PeakStrain sensors also captured peak displacements during events which stressed the structure, such as earthquakes and rock concerts.

 

 

Crack Monitoring in Concrete Box-Beams

 

Situation:   A large, concrete box-girder bridge used on a curved roadway was cracking as a result of torsional stress.  Numerous surface cracks in the diaphragms inside the box girder were visible and extended around the entire diaphragm.  Visual inspection was difficult and provided minimal detailed information on crack growth.

System Installation:   A number of PeakStrain sensors were mounted across the largest cracks to monitor displacement as the cracks opened under traffic loads and differential temperatures.  Cables were run inside the box girder to connect the sensors to a data acquisition system.  Data was collected once per day for peak and current displacement.

Discussion:   Increases in peak displacement under load are a direct and quantitative measure of crack propagation.  Increases in crack width when thermal differentials are factored out can prove that irreversible damage has occurred and is progressing.  Data was collected and reported each day and trends plotted to determine if remedial action was necessary.

 

 

Monitoring Shipping Damage

 

Situation:   Bulb-T, pre-stressed concrete beams were to be used for construction of a new bridge.  Previous beams of this type exhibited large surface cracks in the girder and some of the girders were noticeably damaged during shipping to the construction site.

System Installation:   A single PeakStrain sensor was mounted on the girder at mid-span to ascertain maximum strain experienced during shipment.

Discussion:   After the girder arrived at the bridge site, the sensor was connected to a hand-held device to display the peak-strain it experienced during shipment.  In applications like this, the sensor can be left in place after installation and checked manually during regular bridge inspections.  Depending upon the cost of inspection and data frequency required, these sensors can also be connected at a later date to LifeSpan’s controller for remote, near real time monitoring.