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1. Where ue Change in micro strain k Gauge Calibration Constant Fy Base reading in f 1000 units F Current reading in f 1000 units Please note when Fj is positive the resultant strain is tensile The calculation of Load in a member using data from strain gauges is often complex The fundamental problem is determining the composite Young Modulus E of the member since it is often difficult to accurately determine the properties of the insitu materials Once a Young Modulus is calculated the following equations can be used to calculate the loading on the structural member at the location of the Strain Gauge Force F 2 Stress S x Area A Where A Cross sectional area in m Where F units Newton s Where S units N m Stress S Young Modulus of Elasticity E x Strain e Where E units N m 16 User Manual Example calculation Steel pipe outside diameter 1 016m Steel pipe inside diameter 0 984m Calculated change from the strain gauges 54 688 e Young Modulus of Elasticity of the steel pipe 200 000 000 000 N m2 Stress Exe 200 000 000 000 x 0 000054688 10937600 N m2 Area rr TX outside diameter 2 2 x x inside diameter 2 mt X 1 016 2 2 1 x 0 984 2 mt X 0 508 2 a x 0 492 mt X 0 258064 m x 0 242064 0 810732 m 0 760466 m 0 050266 m Force SxA 10937600 x 0 050266 549789 4N User Manual 17 Section 9 Temperature Effects It is best pract2. construction site environment and is therefore relatively robust However it is essential that the equipment covered by this manual is both operated and maintained by competent and suitably qualified personnel They must READ AND UNDERSTAND the procedures outlined in this manual before attempting installation or operation of the equipment on site Soil Instruments will not accept for repair under guarantee instruments that have been neglected or mishandled in any way User Manual Section 2 Introduction Vibrating Wire Rebar Strain Gauges are designed primarily for monitoring the stresses in reinforcing steel in concrete structures such as bridges concrete piles and diaphragm walls The strain meter is comprised of a length of high strength steel bored along its central axis to accommodate a miniature vibrating wire strain gauge Readout of load or stress is achieved remotely using a portable readout or datalogging system The Model ST5 16 to 40 Vibrating Wire Rebar Strain Gauge consists of a short length of high strength steel welded between two sections of reinforcing bar It is designed to be welded between sections of structural concrete reinforcing bar The cable exits from the strain meter via a compression fitting See Figure 1 Rebar Strain Gauge Body Stress Gauge Electromagnetic Coil Heat Shrink Thermistor Compression Fitting Instrument Cable Figure 1 Model ST5 16 to 40 Rebar Strain Gaug
3. the swept frequency excitation settings correct e Is there a source of electrical noise nearby Most probable sources of electrical noise are motors generators and antennas Make sure the shield drain wire is connected to ground whether using a portable readout or datalogger e Does the readout work with another strain meter If not the readout may have a low battery or be malfunctioning Symptom Strain Gauge Fails to Read e Is the cable cut or crushed This can be checked with an ohmmeter The nominal resistance between the two gauge leads usually red and black leads is 1700 100 Remember to add cable resistance when checking 22 AWG stranded copper leads are approximately 48 50 km multiply by 2 for both directions If the resistance reads infinite or very high megohms a cut wire must be suspected If the resistance reads very low lt 20Q a short in the cable is likely e Does the readout or datalogger work with another strain meter If not the readout or datalogger may be malfunctioning Symptom Thermistor resistance is too high e Is there an open circuit Check all connections terminals and plugs If a cut is located in the cable splice according to instructions in Section 2 3 Symptom Thermistor resistance is too low e Is there a short Check all connections terminals and plugs If a short is located in the cable splice according to instructions in Section 2 3 12 User Manual User Manual 13 Section 7 T
4. units to strain using Linear Factors ye G R R K T T Where G is the Linear Gauge Factor from the calibration sheet Rt is the current F 1000 reading R is the initial baseline F 1000 reading K is the thermal factor T is the current temperature in degrees C E T is the initial temperature in degrees C Equation 2 To reduce F 1000 units to strain using Polynomial Factors ye AR BR C K T T9 Where A is the A Polynomial Factor from the calibration sheet B is the B Polynomial Factor from the calibration sheet C is the C Polynomial Factor from the calibration sheet Rt is the current F 1000 reading K is the thermal factor T is the current temperature in degrees C T is the initial temperature in degrees C 10 User Manual 5 02 Temperature Correction Rebar strain Gauges are usually embedded in concrete and strained by the concrete the assumption being that the strain in the meter is equal to the strain in the concrete When the temperature changes the concrete may expand and contract at a slightly different rate to that of the steel of the vibrating wire The coefficients of expansion are Steel 12 2TM 9C Concrete 10to 14M 0 C Difference c 2 2 to 1 8 M 9 C Table 1 Thermal Coefficients Hence a correction is required to the observed strains equal to the difference of these two coefficients See Equation 3 Equation 3 Thermal Correction
5. 4AW 18 User Manual
6. C T To Where is the resultant from Equation 1 or 2 To is the initial temperature recorded at the time of installation T4 is the current temperature C is the thermal coefficient from Table 1 NOTE For most practical purposes the temperature effects on the embedded gauges are considered to be the same as those on the concrete 5 03 Environmental Factors Since the purpose of the strain meter installation is to monitor site conditions factors which may affect these conditions should be observed and recorded Seemingly minor effects may have a real influence on the behaviour of the structure being monitored and may give an early indication of potential problems Some of these factors include but are not limited to blasting rainfall tidal or reservoir levels excavation and fill levels and sequences traffic temperature and barometric changes changes in personnel nearby construction activities seasonal changes etc 11 User Manual Section 6 Troubleshooting Maintenance and troubleshooting of Vibrating Wire Rebar Strain Gauges are confined to periodic checks of cable connections Once installed the meters are usually inaccessible and remedial action is limited Consult the following list of problems and possible solutions should difficulties arise Consult the factory for additional troubleshooting help Symptom Strain Gauge Readings are Unstable e If using a datalogger to record readings automatically are
7. e The Model ST5 12 Vibrating Wire Rebar Strain Gauge or Sister Bar consists of a short length of high strength steel welded between two long sections of reinforcing bar It is designed to be wire tied in parallel with the structural rebar The small diameter of the bar minimises its effect on of the sectional modulus of the concrete The cable exits from the strain meter through a small gland of protective epoxy See Figure 2 Rebar Strain Gauge Body Stress Gauge Heat Shrink Electromagnetic Coil ie Ei Thermistor encapsulated Protective Epoxy Instrument Cable Figure 2 Model ST5 12 Rebar Strain Gauge Both models of strain meters are robust reliable and easy to install and read and are unaffected by moisture cable length or contact resistance The long term stability of these instruments has proven to be excellent User Manual Section 3 Installation 3 01 Preliminary Tests It is always wise before installation commences to check the strain meters for proper function Each strain meter is supplied with a calibration sheet which shows the relationship between readout digits and microstrain and also shows the initial no load zero reading The strain meter electrical leads usually the red and black leads are connected to a readout box see section 3 and the zero reading given on the sheet is now compared to a current ze
8. hermistor Temperature Derivation Thermistor Type NTC 3000 ohm Resistance to Temperature Equation T 1 273 2 A B LnR C LnR Equation B 1 Convert Thermistor Resistance to Temperature Where T Temperature in C LnR Natural Log of Thermistor Resistance A 1 4051 x 10 coefficients calculated over the 50 to 150 C span B 2 369 x 10 C 1 019 x 10 User Manual 14 User Manual 16 60K 15 72K 14 90K 14 12K 13 39K 12 70K 12 05K 11 44K 10 86K 10 31K 9796 9310 8851 8417 8006 7618 7252 6905 6576 6265 5971 5692 5427 5177 4939 4714 4500 4297 4105 3922 3748 3583 3426 3277 3135 3000 2872 2750 2633 2523 Table B 1 Thermistor Resistance versus Temperature 15 Section8 Data Reduction 8 01 Converting from Hz to microstrain Data from strain gauges is generally presented in micro strain ue where strain is the ratio of the change in length per unit length Practical K factor 3920 Gauge calibration constant Conversion of Period and Linear Units to microstrain is carried out using either of the formulae detailed below Period Units 107 107 pe K AY Toy Where ue Change in strain in micro strain k Gauge Calibration Constant To Base reading in Period units x 10 T Current reading in Period units x 10 107 107 Pl note when ease note en cy ur is positive the resultant strain is tensile Linear Units pe K F Fo x 10
9. ice to record temperature when you record strain readings You can then use the temperature data as well as strain data to analyse the behaviour of the structure The steel used for the wire in the strain gauge has a thermal coefficient of expansion similar to that of structural steel Thus if the gauge and the steel have the same thermal coefficient of expansion and are subjected to the same temperature change corrections for temperature change are not required If the gauge is heated by direct sunlight so that its temperature increases faster than that of the structural steel you may see large changes in apparent strain It is difficult to correct for this so try to shield gauges from direct sunlight using thermally insulated covers If the steel in the structure has a thermal coefficient that is quite different from that of the gauge the following temperature correction might be appropriate Aue corrected Aue TCm TC x Temp Tempo Where Aue is the change in strain TC is the thermal coefficient of the member TC is the thermal coefficient of the gauge 11 0 ye C Temp is the current temperature Tempo is the initial temperature soil INSTRUMENTS Bell Lane Uckfield East Sussex t 44 0 1825 765044 e info itmsoil com TN22 1QL United Kingdom f 44 0 1825 744398 w www itmsoil com Soil Instruments Ltd Registered in England Number 07960087 Registered Office 5th Floor 24 Old Bond Street London W1S
10. ing within the concrete preferably using an epoxy based splice kit such as Soil Instruments Cable Jointing Kit User Manual Section 4 Taking Readings 4 01 Operation of the Vibrating Wire Readout Logger Connect the Readout using the flying leads or in the case of a Switchable Terminal Unit with a connector The red and black conductors are for the vibrating wire gauge the white and green conductors are for the thermistor 4 02 Measuring Temperatures Alternative Method Each Vibrating Wire Rebar Strain Gauge is equipped with a thermistor for reading temperature The thermistor gives a varying resistance output as the temperature changes Usually the white and green leads are connected to the internal thermistor 1 Connect an ohmmeter to the two thermistor leads coming from the strain meter Since the resistance changes with temperature are so large the effect of cable resistance is usually insignificant 2 Look up the temperature for the measured resistance in Table B 1 User Manual Section 5 Data Reduction 5 01 Strain Calculation The basic units utilised by Soil Instruments for measurement and reduction of data from Vibrating Wire Rebar Strain Gauges are Frequency Squared divided by 1000 F 1000 units Two types of calibration constants and a Thermal Factor are provided on the calibration sheet The Vibrating Wire Strain Meter is calibrated in kN and the constants calculated for strain Equation 1 To reduce F 1000
11. n tie wire to secure the cable as the cable could be cut Be sure when installing the strain meters to note the location and serial numbers of all instruments This is necessary for applying the proper calibration factors and determining strain characteristics when reducing data User Manual Diaphragm Wall a aj q Cg E A RR Lt T be d Rebar Strain Meters 2 places opposite Rebar Reinforcement Concrete Tieback installed after excavation Rebar Strain Meters 2 places opposite Tieback installed after excavation Rebar Strain Meters 2 places opposite Figure 3 Model ST5 16 to 40 Installation User Manual 3 02 2 Model ST5 12 Sister Bar The Sister Bar is usually installed using standard iron tie wire Normally ties near the ends and at the one third points are sufficient if the gauge is being wired to a larger section of rebar or to horizontal bars Wiring at the one third points alone is sufficient if the gauge is being wired in parallel to the structural rebar See Figures 4 and 5 Route the instrument cable along the rebar system and tie it off at metre intervals using nylon cable ties Avoid using the tie wire on the instrument cable as it could cut the cable Be sure when installing the strain meters to note the location and serial numbers of all instruments This is neces
12. ro reading The two readings should not differ by more than approx 25 digits after due regard to correction for temperature Checks of electrical continuity can also be made using an ohmmeter The resistance between the gauge leads should be approximately 1700 100 Remember to add cable resistance when checking 22 AWG stranded copper leads are approximately 14 70 1000 or 48 50 km multiply by 2 for both directions Between the green and white should be approximately 3000 ohms at 25 see Table B 1 and between any conductor and the shield should exceed 2 megohm NOTE Do not lift the strain meter by the cable 3 02 Rebar Strain Gauge Installation 3 02 1 Model ST5 16 to 40 The normal procedure is to weld the strain meter in series with the reinforcing steel that is to be instrumented on the site For a typical installation see Figure 3 The strain meter is long enough so that it may be welded in place without damaging the internal strain gauge element Figure 1 However care should still be taken to ensure that the central portion of the strain meter does not become too hot as the plucking coil and protective epoxy could melt In order to prevent this it may be necessary to place wet rags between the weld area and the coil housing Also take care not to damage or burn the instrument cable when welding After welding route the instrument cable along the rebar system and tie it off at metre intervals using nylon cable ties Avoid using iro
13. sary for applying the proper calibration factors and determining load characteristics when reducing date Instrument Cables EN i Pile V Y p I b Rebar Strain Meters xv gt lt sat A y 2 or 3 places y 4 AAA Rebar Reinforcement or Hoo RD a Concrete Sy lt EY RARA N gt S Y y SS Rebar Strain Meters D 2 or 3 places 2 Rebar Strain Meters MM Figure 4 ST5 12 Sister Bar Installation User Manual Instrument Cables Reinforcing Rebar or Strand Instrument Cable Reinforcing Rebar or Strand Wire Tie 2 places Rebar Strain Meter Rebar Strain Meter 3 places 120 apart Wire Tie Tied to Reinforcing Rebar Tied to Reinforcing Rings Figure 5 Model ST5 12 Sister Bar Installation Detail 3 03 Cable Installation As noted in the installation sections route the instrument cables along the structural rebar and tie off using nylon cable ties every 1 metre to secure Outside of the instrumented structure the cable should be protected from accidental damage caused by moving equipment or other construction activity Cables may be spliced to lengthen them without affecting gauge readings Always waterproof the splice completely especially when embedd
14. soil INSTRUMENTS Vibrating Wire Rebar Strain Gauge User Manual Man154 3 0 2 06 08 2014 Kim Malcolm Phil Day Chris Rasmussen Manual No Revision Date Originator Checked Authorised for Issue User Manual Contents Section 1 Section 2 Section 3 3 01 3 02 3 02 1 3 02 2 3 03 Section 4 4 01 4 02 Section 5 5 01 5 02 5 03 Section 6 Section 7 Section 8 8 01 Section 9 User Manual gelm iu ele iii TI LITT 3 INY OCCU CN sd 4 Installation nos 5 PreliMmary Tests sa iii 5 Rebar Strain Gauge Mstala OM ais 5 Model S13 1610401 da de 5 Model ST 12 SIS DATA AAA A AAA IA A A 7 es mnkirsiliteln ETT TEILT HERE 8 Taking Reading Scan ci 9 Operation of the Vibrating Wire Readout Logger ccccccccccccnnccccncncnnncconnnnnnnoconnnnnnnnnnancnnnnnnos 9 Measuring Temperatures Alternative Method ccocccoocccccccccnncnnncccooncnnnnononnnnnnononanonnnononanonnss 9 Data Reduction mee er 10 Sa Calculado UE RS UU TU UU 10 Temperat re CorrecliOb sadi to ct ed eae i 2 eS sel ase reote Cau utei st ri te iot 11 EnvironmentalFaebof Sans 11 Troubleshoot aura sata 12 Thermistor Temperature Derivation 1 eese ceres rennen nennen 14 Data Reduccion 16 Converting from Hz to Microstr Mini di 16 Temperature ETMe coso iaa 18 2 Section 1 Foreword Soil Instruments Vibrating Wire Rebar Strain Gauge as with all our equipment has been designed to operate consistently in a
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