Anchor Load Cells Installation - considerations

Transcription

1 Anchor Load Cells Installation - considerations This article outlines the main points to check during installation of load cells to control tensions while installing and service life, applied on bar and cable ground anchors as well as prestressing and post tensioning tendons. It is only a practical guide based on experience and is not intended to replace any Engineers design. However simple they may appear, load cell installations pose unexpected questions. These challenges include load range, alignment, positioning and diameter consistency, as well as temperature effects, all of which will have significant impacts on the outcome. No one load cell installation is the same and careful planning of each project should be carried to ensure a successful installation. Load cells are the preferred system to monitor anchor behaviour. The more complex the engineering design laying behind the more meticulous and precise installation must be. The main considerations are: Type of structure to anchor Expected anchor loads Anchor head: dimensions and terminology Anchor: bar and cable dimensions Unexpected readings NOTE: There are outstanding issues to be considered when installing load cells.

2 TYPE OF STRUCTURE TO ANCHOR Item Considerations Possible solutions Structural concrete elements Compressive forces can be applied to concrete elements either before concrete casting, i.e. pre tensioning at the precast concrete plant, or once the concrete is casted, i.e. post tensioning usually onsite. Pre and post tensioning multi strand anchor blocks are designed specifically to the number of tendons / strands. Load cells are used regularly to control tensions applied to the tendons at the precast concrete plant. In this use, the difference between Outer (OD) and Inner Diameters (ID) is usually small so the available circular crown section is thin. Ensure the dimensions of the load cell will effectively fit to the anchor block. Frequency of load cell calibration must be kept to once every six months or higher when used in production at the precast concrete plant. Ground Engineering structures Tension load is used to stabilize and to retain ground deformation using bar or cable / strand anchors, i.e. support and retaining walls, slopes or embankments, bridge abutments, slabs subjected to under pressure, etc. It is a priority to learn: type of anchor that will be in place, i.e. bar or cable anchor, passive or active, above or below groundwater level; loads the anchoring element will bear. Temporary ground anchors are those with a service life of up to two years. Permanent ground anchors have a service life longer than two years. Chosen load cell type and dimensions are a function of the type of anchor and its service load. Protection at the anchor head is mandatory for service lifespans in excess of two years. Automatic data acquisition is of great help for those spots difficult to access or located in remote sites. Strut monitoring Loads in (excavation) struts can be monitored using a load cell, measuring the full load on the full section of the strut. It takes a lot of expertise to install a load cell to monitor a strut. Placing a load cell in a strut may alter the loading conditions both in the strut and in the cell. Monitoring outcome from load cells installed in a strut easily correlates to measurements obtained using strain gauges. Strain gauges properly installed on a strut give an idea about moments in the element flexion, compression, torsion-, additionally to load. NOTE: The kind of structure to anchor and the kind of anchoring element determine the definitive design for the load cells. 2

3 TYPE OF STRUCTURE TO ANCHOR Item Considerations Type of anchor load cells There are three types of load cells provided by Geosense: Hydraulic load cells; Strain gauge load cells; Vibrating Wire (VW) load cells. The factors to take into account when deciding which load cell type to use are: Duration of monitoring; Accuracy required; Data logging requirement; Robustness. Possible solutions Hydraulic load cells These are low cost but they are the least accurate as they are significantly affected by temperature. Hydraulic load cells are useful for monitoring anchors on long term projects where any changes in load are likely to be significant. They can be upgraded to be data logged by attaching a Vibrating Wire (VW) or analogue pressure sensors. When fitted with a Bourdon gauge they offer quick and simple readings. Standard output from hydraulic load cells: manometer for pressure indication. Strain gauge load cells Should be used if dynamic monitoring is required. This is because they give a constant output and do not have to be polled for information. Strain gauge load cells are very accurate over the short term but may be prone to drift in the long term. This is due to glue drift of the attached sensors but also down to electrical drift of the sensors and other electronic components. They can be easily and cheaply data logged and are, arguably, the easiest and cheapest to use for wireless monitoring applications. Output signal from strain gauge load cells: mv/v; Ω for thermistor if in place. Vibrating wire (VW) load cells These are the highest in cost. VW load cells are also the most robust and suitable for civil engineering applications. Bearing this in mind, VW load cells certainly are our choice for loads of 1000 kn and bigger. Sensors used are electromechanical, mechanically attached in the sensor, so they are very stable over the long term. Due to the fact that each load cell has either three or six gauges, individually read, eccentric loading is also easily spotted. In terms of accuracy, VW load cells are the equal of strain gauge load cells. VW load cells cannot be dynamically monitored. Output signal from VW load cells: Hz or Digits; Ω for thermistor if in place. NOTE: Service duration, precision needed, type of readings output, and robustness are factors to consider when choosing a type of load cell. 3

4 TYPE OF STRUCTURE TO ANCHOR load cell ground load bearing plates anchor head retaining structure An anchor is an installation capable of transmitting an applied tensile load to a load bearing structure or to the ground. NOTE: In case a load cell is installed, the anchor becomes a measurement anchor. 4

5 EXPECTED ANCHOR LOADS Item Considerations Possible solutions Capacity of the load cell It is crucial to know the service and maximum expected loads at the anchoring element. Base readings In addition to the base reading showing on the calibration certificate the unloaded, onsite base values must be known and taken as base reference. Test loads Investigation test: a load test to establish the ultimate load resistance of an anchor. Suitability test: a load test to confirm that a particular anchor design will be adequate in particular ground conditions. Acceptance test: a load test to confirm that each anchor conforms with the acceptance criteria. It is recommended that the chosen load cell should work within 50% to 80% of the calibrated range of the load cell. Onsite unloaded reading is the reference value any future reading will be subtracted to in order to get the actual load. Both load and temperature must be recorded. During all testing the load shall be applied and released smoothly. Loading velocity is a key factor to control during stressing and destressing and will certainly affect anchors behaviour. Over range capacity for Geosense load cells is +20% Full Scale F.S. In case it is found that the percentage of the calibrated load to the design load is over 100%, the load cell with the next highest load range must be chosen. Take several unloaded base readings (load, temperature) right before installation. With no significant environment and temperature changes, these base readings should all be within ±0.25% Full Scale F.S. Always follow onsite written test procedures approved by Client s Technical Representative. Load cells should not be over loaded by more than +50% F.S. Anchor load cells readings load and temperature- and their times, strands displacements and any significant data must be carefully recorded during tensioning operations. Linear versus polynomial calculation Depending on the kind of output signal obtained from the anchor load cell, values can be calculated using a linear or a polynomial conversion. Due to environmental factors that may affect the reading of a load cell, site base readings should always be used as the reference starting value, including load (Hz/Digits, mv/v) and temperature. Readings from the readout unit directly display the average reading for the vibrating wire or strain gauges active in the load cell. At the lower range of loading 1% 10% F.S., some anchor load cells appear to be less precise compared to the same load cell working in its mid load range. Whenever beginning a new project, it is interesting to use both linear and polynomial calculations to compare the values obtained, if possible. If some extra degree of accuracy is required, polynomial conversion shall be used. Conversion formulae and factors are available in the calibration certificates and in the corresponding manuals. NOTE: Ensure expected load range is fully understood and proper unloaded base readings are taken right before installation. 5

6 ANCHOR HEAD: DIMENSIONS Item Considerations Possible solutions Sizing It is essential to choose the right diameters for the load cells before putting forward a Purchase Order. The important information for deciding your load cell size is the outer diameter OD of the bolt or cable anchor being measured and the expected load at the anchoring element. Different manufacturers may have different cell sizes for equivalent loads. If the diameter of the bolt or cable anchor is x, it is recommended that the smallest load cell inner diameter ID be ID min = x + 5 mm. Load distribution plates To ensure load is applied equally over the annular loading surface of the cell. Uneven or deformed load bearing plates will affect the readings in the load cell. For loads up to 4500kN, load bearing plates should be 1 ½ 2 thick. Different types of load cells may need one or two load bearing plates. Hydraulic jack load cell reading Anchors are loaded using a hydraulic jack. It is common practice to make straight comparisons between load values showing on the jack to loads measured by the load cell. Outside diameters for multi strand hydraulic jack and load cell should be the same. Ø jack < Ø load cell higher load values Ø jack = Ø load cell true load values Ø jack > Ø load cell lower load values A fully comprehensive document is needed onsite showing all the dimensions and technical features for the hydraulic jack. Before used onsite, both the hydraulic jack and the load cell should come with their up-to-date Calibration Certificates. Eccentric loading Load is not applied at the axis of the anchor + load cell setting. Eccentric loading will affect the readings in the load cell as the load cell is under higher loads in one portion with respect to the other. Effects are minimised by having several sensors within the load cell and averaging, the use of load bearing plates and proper installation procedures. NOTE: Check that design of anchor loading and anchor tests together with dimensions for the hydraulic jack, load cell and load bearing plates are correct. 6

7 ANCHOR HEAD: TERMINOLOGY strands wedge plate wedges load cell load distribution plates anchor load bearing plate / abutment plate Anchor load cell and load bearing plates. NOTE: Load cells are the stethoscopes for anchors. 7

8 KNOWLEDGE ARTICLE ANCHOR HEAD: TERMINOLOGY anchor load bearing / abutment plate load distribution plates anchor nut Terminology for a ground bar anchor. thread bar bearing plate bar anchor VW load cell Ground bar anchor terminology. NOTE: Expected loads and anchor dimensions must be understood to choose the right load cell. 8

9 ANCHOR HEAD: TERMINOLOGY anchor load bearing / abutment plate wedge plate anchor nut wedges cables / strands load distribution plates bearing plate cable anchor VW load cell Cable / strand ground anchor terminology. NOTE: To install a load cell on a cable anchor may take a little bit more than to install it on a bar anchor. 9

10 ANCHOR: BAR AND STRAND DIMENSIONS Item Considerations Possible solutions Different sizes for bar and cable anchors Different bar and cable anchor manufacturers have different sizes for their anchoring elements. Anchoring elements are manufactured using different steels and overall designs. It is advisable to double check anchor bar, cable and head dimensions before deciding for any load cell. All technical documents must be at hand when ordering anchor load cells. The figures below are for information purposes only, not to be taken for granted or for calculation. Please check manufacturer s specifications and Project Technical requirements when going through design of anchors and / or choosing a load cell. BAR Ø DSI Prestressing Steel GEWI Steel bar GEWI Plus Steel bar Ø OD Yield Force Ø OD Yield Force Ø OD cables Yield / Force mm kn mm kn mm kn strands load distribution plates NOTE: It is advisable to double check any technical documents before ordering a load cell. 10

11 ANCHOR: BAR AND STRAND DIMENSIONS The figures below are for information purposes only, not to be taken for granted or for calculation. Please check manufacturer s specifications and Project Technical requirements when going through design of anchors and / or choosing a load cell. BAR Ø Ischebeck hollow CTS/Titan Williams Form Eng. 150 KSI Williams Form Eng. B7X Ø OD Yield Force Ø OD Min. Ultimate Str. Ø OD Yield Force mm kn mm kn mm kn cables / strands load distribution plates NOTE: It is advisable to double check technical specifications before ordering a load cell. 11

12 ANCHOR: BAR AND STRAND DIMENSIONS The figures below are for information purposes only, not to be taken for granted or for calculation. Please check manufacturer s specifications and Project Technical requirements when going through design of anchors and / or choosing a load cell. STRAND Ø 0.5" (13 mm) diam. - 7 wires per strand 0.6" (15 mm) diam. - 7 wires per strand DSI Williams Form Eng. # Strands Ultimate Str. Ø Wedge Plate # Strands Ultimate Load Ø Wedge Plate kn mm kn mm cables 127 / strands NOTE: It is advisable to make sure load cell match Project Technical Requirements. 12

13 UNEXPECTED READINGS Item Considerations Possible solutions Unsynchronized readings The reading on the jack is taken at a different time from that taken on the load cell. The strands are tensioned using the jack, the wedges are put in place and the load locked. The load applied by the jack diminishes during the load wedge locking process. In case the load is applied too fast, some accommodation time is needed for all the load to be distributed along the anchoring element. It is recommended to carry out Lift Off Testing for load cell reading versus hydraulic jack reading to identify any loss of load transfer. Hydraulic jack longer than the load cell setting This is prone to inaccurate load measurement due to eccentric loading, resulting in bending and causing some friction between the ram and the seal, i.e. binding of the seal of the jack see Anchor Head Dimensions-. The load is over registered by the jack, often taken as under registering in the load cell. Load cells come calibrated from the factory before use onsite. The hydraulic jack ram has been used to install many anchors after its calibration, so it must be assumed that the calibration certificate corresponding to the load cell is more up-to-date than the calibration certificate for the hydraulic jack. Hydraulic jack ram different from load cell diameter The ram size varies from the load cell size see Anchor Head Dimensions-. This causes bending of the distribution plate which in turn causes over or under measurement of load depending on the ratio load cell / jack areas. Match the load cell and the hydraulic jack ram diameters. Usually, this is not easy to achieve as we will have to bear with the means available onsite. Suitable, thick load distribution plates must be used to distribute the load as even as possible to the load cell Uniformly Distributed Load UDL-. Temperature effects Changes in environment temperature will have several effects on the readings registered by the load cell. Thermal influences are complex to discern because it is not only the load cell that is affected but any structural elements surrounding the load cell, i.e. brick, concrete or steel structures. One way to understand the effects of temperature changes is to record the installed load cell readings together with both ambient and cell temperatures when no other changes are taking place. An option is to take the readings always at the same time of the day, preferably early morning right before sunrise. NOTE: Check points have to be allowed for onsite to make sure the system anchor + load cell works properly. 13

14 UNEXPECTED READINGS Centred load cell in an anchor installation. Avoid eccentric loading! Eccentric load cell in an anchor installation. NOTE: Eccentric loading must be avoided when installing load cells in anchors. 14

15 UNEXPECTED READINGS Item Considerations Possible solutions Load cell is giving unstable readings Are the readings alright with a different readout? Are the load readings outside the range of the load cell? Is there a source of electrical noise nearby, such as a generator or a motor? If yes then suspect low battery. Make sure the expected loads and the load cell range match. Have any electrical noise source removed away. VW load cell: unstable readings Strain gauge load cell: unstable readings Is there any significant temperature effect? Is the correct swept frequency being used? Is the load cell part of an unexpected electrical loop? See Temperature effects in this document. Check swept frequency coming from readout and / or datalogger. Check insulation resistance between the cell body and any cable. The reading should be >500 MΩ. Hydraulic load cell: unstable readings Manometer Were manometer fitted check for damage? Check the needle return to zero with no load. Check for leaks on the cell. Vibrating Wire VW Is the correct swept frequency being used? Check swept frequency coming from readout and / or datalogger. NOTE: Changes in environment conditions will have several effects on the readings registered by the load cell. 15

16 KNOWLEDGE ARTICLE LOAD CELLS APPLICATIONS Single strand hydraulic jacks tensioning an active, multi-stage ground anchor. Multi strand hydraulic jack tensioning an active ground anchor with a load cell. NOTE: It is crucial to foresee convenient onsite conditions before installing load cells. 16

17 KNOWLEDGE ARTICLE LOAD CELLS APPLICATIONS Vibrating wire VW anchor load cell installation in an active, multi-stage strand ground anchor (1/2). Vibrating wire VW load cell installed to control an active, multi-stage strand ground anchor (2/2). Vibrating wire VW bar anchor load cell load to monitor a ground bar anchor. Strain gauge anchor load cell to monitor a passive ground bar anchor. NOTE: Clean and tidy installations favour reliable readings. 17

18 SUMMARY TYPE OF STRUCTURE ANCHOR LOADS Structure Type of load cell Base reading Ensure design for the anchor block and the load cell elements are the same -OD & ID, load-. Chose your load cell bearing in mind service load, kind of anchor and lifespan. Take a series of unloaded base readings right before onsite installation. Load Follow approved onsite testing and loading procedures. Loads below 10% F.S. may result in imprecise readings. Load cells should not be overloaded by more than +50% F.S. Linear polynomial calculation Agree with the Client which one to use. ANCHOR HEAD DIMEN- SIONS Jack load cell reading ODs for jack and load cell should be the same. Load bearing plates Bearing plate thickness should be 1 ½ 2. Eccentric loading Bar and strand anchor Always avoid eccentric loading. Having several sensors fitted in the load cell and averaging minimises the effects of eccentric loading. Bar and cable anchor manufacturers produce anchoring elements in different sizes. UNEXPECTED READINGS Dimensions Anchor, load cell and hydraulic jack diameters must be consistent. Temperature effects Types of sensors Troubleshooting Take readings from anchor load cell always at the same time of the day, preferably right before sunrise. Load cells including a thermistor allow for more accurate temperature compensations. Check for easy to understand subjects when a load cell is giving unstable readings. NOTE: Positive installations need planning and good care onsite. 18