MAN-221 Concrete Surface Mount Vibrating Wire Strain
Contents
1. DR 3 QR V DRE n TR ED RR RC RR 12 Vibrating Wire Data ies kousE Ru eua REM au Ks RREEREU NE au an Ra RUOR RR RR RR RR Rad REOR QR Re OE RR RR RC ORARE RN ORE 13 Trouble Shooting Flowchart eee eeeeerrreee haha au nun nnn u aua uuuuuurrsssru uut 14 2 Section 1 Foreword The Soil Instruments Concrete Surface Mount Vibrating Wire Strain Gauge as with all our equipment has been designed to operate consistently in a 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 The Soil Instruments Concrete Surface Mount Vibrating Wire Strain Gauge is intended primarily for long term strain measurements of strain changes on concrete or rock surfaces The means of attachment is by using anchors grouted into drill holes The nature of the instrument dictates that it is used to measure changes in strain i e change in length per unit length Strains are measured using the vibrating wire principle a length of steel wire is tensioned between two mounting blocks that are welde2. as amended gabo EET Technical Documentation Issue 1 Octaber 2006 ibrating vw Vire Instruments Vie hereby certify that the apparatus described above conforms with the protection requirements of 2 23 EEC Low Voltage Directive as amended by BH3 BS EEC on the approximation of the laws of the Member States relating to Low Voltage Equipment i D i Signed hm en E a Signatory Chris Rasmussen Technical Director SCIl 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 4AW 15 User Manual
3. is likely that cable damage has occurred causing a short in the circuit Step 5 If the resistance is within the values quoted in Step 1 i e 800 to 1800 AND no continuity exists between conductor and earth screen and on checking the reading from the instrument it proves to be still unstable or wildly fluctuating it must be assumed that the integrity of the circuit is good A faulty instrument must be suspected and Soil Instruments should be consulted TIP If the location on site of cable damage is found the cable can be spliced in accordance with recommended procedure with suitably qualified personnel 12 User Manual Appendix A Vibrating Wire Data Frequency Units f The tension of a wire can be measured by registering the frequency note at which it naturally vibrates If the wire is plucked electronically the frequency at which it vibrates can be measured The most common units used to express frequency are Hertz Hz or Kilohertz KHz The disadvantage of these units is that there is no linear conversion from Hertz to change in wire tension Linear Units L In order to overcome the problem of a linear conversion described above the frequency value can be squared thereby rendering it linear but quite large To reduce its size it is often divided by 1000 or multiplied by 10 3 The expression f 1000 or f x 103 is the most commonly adopted as a linear digital output Period Units P Electronic devices and d
4. 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 in situ materials User Manual 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 Where E units N m Example calculation Steel pipe outside diameter 1 016m Steel pipe inside diameter 0 984m Calculated change from the strain gauges 54 688 1 Young Modulus of Elasticity of the steel pipe 200 000 000 000 N m Stress Ex 200 000 000 000 x 0 000054688 10937600 N m2 Area Tt r JT x outside diameter 2 2 TE x inside diameter 2 JE x 1 016 2 TE x 0 984 2 JE x 0 508 TE x 0 492 TE x 0 258064 TE x 0 242064 0 810732 m 0 760466 m 0 050266 m SXxA 10937600 x 0 050266 549789 4N Force 10 User Manual Section 8 Temperature Effects It is best practice 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 Temperature induced expansions and contractions can cause real changes in stress in the concrete if the
5. Manual Section 6 Protection of the Installation All efforts must be taken to protect the gauge Where the gauge is installed in a vulnerable position good practice is to spray the area with marker paint as a warning and or cover the gauge with a protective cover These can be ordered separately from Soil Instruments 6 01 Cable Routing and Protection Having established and marked the preferred route of the cables and decided upon the type of cabling arrangement to be adopted begin running the cables from the gauges furthest from the readout location Where cables are not to be ducted they should be routed in a position where they are least likely to be damaged using strong tape or cable ties Cable should be supported every 400 500mm and care should be taken to avoid over stretching the cables especially where movement loading could take place Where significant movement could take place the cable ties should be left a little slack and sufficient cable left free and positioned so that it cannot be damaged Once the cables have been fixed a full set of readings should be recorded for each instrument Since the gauges are used to record changes in data and not absolute data the stage when the Base or Zero reading is recorded is flexible Where loading tests are to be performed the Base reading for the test data should be recorded just prior to the beginning of the test Data from Vibrating Wire instruments can be recorded in 3
6. concrete is restrained these are superimposed on other load related stresses Differences between the coefficient of expansions of the concrete and that of the steel in the strain gauge itself give rise to an apparent change in strain in the concrete This apparent change can be corrected for using the equation below Aus corrected Aus UL TC x Temp Tempo Where Awe is the change in strain T C is the thermal coefficient of concrete in ue C I C is the thermal coefficient of the gauge 12 2 we C T emp is the current temperature Tempo is the initial temperature User Manual 11 Section 9 Troubleshooting Guide If a failure of any vibrating wire instrument or its electrical cable is suspected the following steps can be followed The instrument themselves are sealed and cannot be opened for inspection The Troubleshooting Flowchart should also be followed if any instrument failures are suspected The steps below and the Troubleshooting Flowchart are applicable generally to any vibrating wire instrument Step 1 Before any of the following steps are followed the readout unit should be used to verify the stability of the reading and the audio signal from the portable logger should be heard The period reading from the instrument should not vary by more than 2 units and the audio signal should be crisp and of a consistent tone and duration An unstable wildly fluctuating reading from an instrument or an unst
7. d to the steel surface being studied Deformations i e Strain changes of the surface will cause the two mounting blocks to move relative to each other thus altering the tension in the steel wire The tension is measured by plucking the wire and measuring its resonant frequency of vibration The wire is plucked and its resonant frequency measured by means of an electromagnetic coil positioned next to the wire Portable readouts or fixed data acquisition systems are available from Soil Instruments will provide the necessary excitation to pluck the wire and will convert the measured frequency to display the reading directly in microstrain Set Screw Set Screw queer O 80 Pickup coil Note actual cable length dependent on order and coil is secured with a cable tie after positioning gauge User Manual Section 3 Equipment Supplied For each installation you should have been supplied with e Two groutable anchors mounting blocks e Sensor unit gauge e Each gauge is supplied with a pickup coil fitted with a user specified length of cable The cable can be spliced to other cables for routing to a terminal location Cables can be routed over distances in excess of 1000 metres without degradation of signal Together with these a setting bar and drill template should be used to accurately locate the groutable anchors mounting blocks such bars and templates are available from Soil Instruments Although gauges are checked p
8. eady audio signal are both indications of possible problems with instruments or their related electrical cables Important If a portable data logger is giving faulty readings or audio signals from all instruments a faulty readout unit must be suspected Another readout unit should be used to check the readings from the transducers and Soil Instruments should be consulted about the faulty readout unit Before proceeding to Steps 2 and 3 if possible the continuity should be checked between conductors and earthing screen of the electrical cable If a continuity exists a damaged cable is confirmed Step 2 The resistance across the two conductors of the electrical cable should be checked This can be done using a multimeter device across the two exposed conductors if the cable has not been connected to a terminal cabinet or can be done just as easily across the two conductors if the instrument has been connected to such a terminal or Dataloggers The resistance across the two conductors should be approximately of the order of 800 to 1809 The majority of these resistances arise from the instrument and the remainder from the electrical cable connected to the instrument Step 3 If the resistance across the two conductors is much higher than the values quoted in Step 2 or is infinite a damaged cable must be suspected Step 4 If the resistance across the two conductors is much lower than the values quoted in Step 1 say 800 or less it
9. formats Period Linear or Engineering Units The required format should be established prior to any test beginning and remain consistent for the duration of the monitoring program A monitoring schedule should be established by the engineers responsible for the structure so that the monitoring personnel are aware of the data gathering requirements User Manual Section 7 Data Interpretation Data from strain gauges is generally presented in micro strain ue where strain is the ratio of the change in length per unit length AL Practical K factor 36150 Gauge calibration constant Conversion of Period and Linear Units to micro strain is carried out using either of the formulae detailed below Period Units la d HE TE l T Ty Where HE 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 107 B Batch Factor supplied with each gauge 10 10 is positive the resultant strain is tensile T m Please note when Linear Units ug K F4 Fg x 1074 x B Where HE Change in micro strain K Gauge Calibration Constant Fy Base reading in f 1000 units F Current reading in f 7 1000 units B Batch Factor supplied with each gauge Please note when F Fy is positive the resultant strain is tensile The calculation of Load in a member using data from strain gauges
10. igital technology often utilise the counter function available in some common circuits Period Units represent the time taken for the wire to vibrate over one full oscillation expressed in seconds Due to the very small size of the number generated most equipment manufacturers display the unit multiplied by 10000000 or 10 The relationship between Period Units and frequency units is expressed as Pul frequency Period units are therefore convenient to measure but do not have a linear relationship to change in wire tension Calibration Constants Fach instrument is supplied with a Calibration Constant value to convert the raw data into engineering units The value of the calibration constant will vary depending upon the engineering units into which the data is to be converted and the readout units For example the data from piezometers may convert into Kg cm mH20 Bar Psi etc and therefore the Calibration Constant for each will be different Some instruments have Generic Calibration Constants and others are calibrated to generate the Constant The constant is generated by using the following calculation Constant K Range Reading Full Range Reading Range Zero x 10 13 User Manual Appendix B Trouble Shooting Flowchart v Is reading from portable logger stable sensible and audio signal steady Does a continuity exist There is no reason to Yes sus
11. of the two mounting holes Drill 2 holes of 20mm diameter to a depth to suit the rebar stems on each mounting block Ensure all dust and debris has been removed from the holes prior to grouting the instrument in place Once grout has set refer to manufacturers guide remove setting jig taking care not to lose the three set screws 5 01 Setting the Strain Gauge When the mounting blocks are grouted in place the strain gauge is now slid into the holes of the mounting blocks being careful to see that the V grooved end of the gauge lies inside the mounting block without the slot The cone point setscrew should be tightened hard down into the V groove in this end of the gauge Clip the plucking coil over the gauge secure with the cable ties and connect it to the readout box Check that readings can be recorded Reading frequency can be made to increase by gripping the coil assembly and pushing in the direction of the free end Readings can be made to decrease by pushing directly on the free end When the desired reading is obtained midrange 2500us compressive readings approximately 3500use and tensile strains approximately 1500us the free end is secured inside the mounting block by tightening down hard on the oval point setscrews The reading may alter slightly during this operation which is normal The final position of the gauge should be accurately recorded since this detail will be required for interpretation of its readings User
12. pect a faulty instrument A damaged cable or H is very high A severed cable is suspected causing very high or infinite resistance It must be suspected that the portable logger used first is faulty Contact Soil Instruments Ltd gt Yes User Manual between earthing screen Yes damaged cable joint and conductor are suspected No v Check magnitude of resistance R between conductors R lt 80 Ohm H is between 80 amp 180 Ohm Vv Vv A faulty readout is A damaged cable is suspected Check suspected causing a reading of instrument short with another unit See step 4 l A faulty instrument is i reet Coi nli No gt possible Contact alternative logger Soil Instruments Ltd 14 DECLARATION OF CONFORMITY WITH COUNCIL DIRECTIVE 3 23 EEC as Amended by 93 68 EEC Date of Issue 12 October 2008 Dot Ref call CE VvDANAIOS reete 2 AS EEC Low Voltage Directie as amended by 936o EEC Conforming Abopnaratus All Vibrating Wire Instruments Manufactured by Soil Instruments Ltd see pages 3 10 af Technical File Manufacturer Soil Instruments Ltd Responsible Person Chris Rasmussen Sail Instruments Ltd Bell Lane Ucktield East SUSSEX TN22 TOL standards Referenced Annex of 1 3 1 Dh coed da 35b amp 3c Directive Technical Fie oll Instruments Ltd ELI Low Voltage Directive 37 3 EEC
13. rior to leaving Soil Instruments damage can occur during transit It is suggested that the gauges are visually checked immediately upon receipt Additionally it is prudent to check the operation using a Vibrating Wire readout device to ensure steady readings if an audio signal is available on the readout device this can give a good indication of the quality of the signal Before installation a note should be made of the batch factors for the Strain Gauges for future data interpretation User Manual Section 4 Preparation of the Gauges Where a number of gauges are to be installed on a structural element it is essential that each gauge and its associated cables are accurately and effectively identified A permanent marking system should be adopted to ensure gauges can be identified throughout their working lifetime and this information safely stored for future reference Check the resistance between the two lead wires It should be around 150 ohms If the gauge contains a thermistor check its resistance Check the reading against that which should be obtained at the existing ambient temperature Any faulty units should be returned to the factory Gauges should not be opened in the field The choice of gauge installation location is a critical factor in the design of an instrumentation scheme Prior to installation of the gauges it is essential to plan the cable routing from the gauge location to the terminal position The cables should be ma
14. rked and where possible extended before the gauge is installed Two approaches to cable routing are commonly adopted each with their own merits a Each gauge sensor coil is factory fitted with a length of cable to reach from the gauge location to the terminal This removes the need for jointing cables on site in often unsatisfactory conditions It is however the slightly more expensive option and involves the handling and tying of often long lengths of cable b The gauge sensor coils are factory fitted with a short say 3m length of cable and the cable from each gauge in an array is routed to a high quality cable joint where it is connected to a multicore cable The multicore cable is then routed to the terminal location With the cable routing planned and the gauges marked with their appropriate location description mounting the gauges can begin User Manual Section 5 Gauge Mounting The Soil Instruments Concrete Surface Mount Vibrating Wire Strain Gauge is attached to mounting blocks which must first be grouted into the surface to be studied A setting bar is used to correctly space apart the two blocks Firstly the two mounting blocks are fitted over the ends of the setting bar and the jig is used to position them correctly Then the setscrews in the mounting blocks are tightened down onto the setting bar Avoid excessive tightening as this only damages the setting bar unduly Use the drill template to mark the position
15. scil INSTRUMENTS Concrete Surface Mount Vibrating Wire Strain Gauge User Manual Man221 3 0 2 11 08 14 T Millward P Day C Rasmussen Manual No Revision Date Originator Checked Authorised for Issue User Manual Contents Section 1 Section 2 Section 3 Section 4 Section 5 5 01 Section 6 6 01 Section 7 Section 8 Section 9 Appendix A Appendix B User Manual FOFeGWOLFIG 12 sivixcousuuxesdes exa Re ac Ra sus EE aE REMPLI SRM RI ERN SVR V MF IMKU M SM MIN PIE ERIS ME EINE MM UE 3 EMEP OCU CUIONN eese 4 Equipment SUD DINGG sssini aa consduueensuees cae seubsussaunicrnesureuensuabbesmeuonNan 5 Preparation of the Gauges sssssss2s222 2222 22222202022000u2000u20u0usuuuununnnnnnnnnnnnnnnnnn unus 6 Gauge MOUNTING ssxikkk asuxikE Ead d ACQICROROEA QOL RE ER RCRCWCRCH Re CFL AAN 7 Seng trie Strain Gage esos neidsetaia ad vii dotada wis accident nara oan wa Ep dc Coma t rp Dre If 7 Protection of the Installa tiori iiiiiiioisati ses esa au PuF i n P SvcsE PPM S sEU aiaa 8 Cable Routingand Protection iss ordina gu vaa vi soie raa ru vaa Ye Ehe ege rx Ra PEU Ere re eu da abusu YE REOS uia 8 Data Interpretation 2ucxdsexduvu s ida vo CURIE VusEUEUS CRINE NEM E NE GUN E VENUS SVKENM UE VENUE VEU NM KE VE ER D UE 9 Temperat re El CCES iovvieu xevuvasesuenw svvvevuvssv veVxa EE VeVVvewessosw veresvu ev sskxa E VeVuveweswesFdverWbw Es 11 Troubleshooting GUIdS cias vices sa RR CR d AEn GR RR RR
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