Anchor Load Cells Installation considerations...while installing and service life, applied on bar and cable ground anchors as well as prestressing and post tensioning tendons. It is

KNOWLEDGE ARTICLE MC, GC, TL 31/05/2017 V0.0

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.

Anchor Load Cells Installation - considerations

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.


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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.

TYPE OF STRUCTURE TO ANCHOR

NOTE: The kind of structure to anchor and the kind of anchoring element determine the

definitive design for the load cells.


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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.


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NOTE: In case a load cell is installed, the anchor becomes a measurement anchor.

An anchor is an installation capable of transmitting an applied tensile load to a load

bearing structure or to the ground.

ground

retaining

structure

anchor head

load cell

load bearing

plates



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NOTE: Ensure expected load range is fully understood and proper unloaded base readings

are taken right before installation.


Capacity of the load cell

It is crucial to know the service

and maximum expected loads

at the anchoring element.

It is recommended that the chosen load

cell should work within 50% to 80% of

the calibrated range of the load cell.

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.

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.

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.

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.

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.

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.

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.

EXPECTED ANCHOR LOADS


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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.


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

IDmin = 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.

ANCHOR HEAD: DIMENSIONS


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NOTE: Load cells are the stethoscopes for anchors.

ANCHOR HEAD: TERMINOLOGY

Anchor load cell and load bearing plates.

load cell

load distribution plates

anchor load bearing

plate / abutment plate

wedge plate

wedges

strands


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NOTE: Expected loads and anchor dimensions must be understood to choose the right load

cell.

Terminology for a ground bar anchor.

Ground bar anchor terminology.


bar anchor VW load cell

thread bar

anchor load bearing / abutment plate

load distribution plates

bearing plate

anchor nut


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NOTE: To install a load cell on a cable anchor may take a little bit more than to install it on

a bar anchor.

Cable / strand ground anchor terminology.


anchor nut

anchor load bearing / abutment plate

load distribution

plates

cables /

strands

bearing plate

cable anchor VW load cell

wedge plate

wedges


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NOTE: It is advisable to double check any technical documents before ordering a load cell.

ANCHOR: BAR AND STRAND DIMENSIONS

load distribution

plates

cables /

strands


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 Yield Force

mm kN mm kN mm kN

15 159 16 100 18 170

20 283 20 157 22 255

26.5 523 25 245 25 329

28 308 28 413

32 764 32 402 30 474

36 967 35 645

40 1194 40 630 43 973

47 1648 50 980 57.5 1740

63.5 1758 63.5 2122


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NOTE: It is advisable to double check technical specifications before ordering a load cell.


load distribution

plates

cables /

strands




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

30 471 - 583 26 567 32 210

40 592 - 597 32 834 32 294

52 546 36 1054 38 404

73 500 - 594 46 1734 51 677

103 500 - 572 57 2727

127 603 65 3457

130 550 75 4568 76 1466


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NOTE: It is advisable to make sure load cell match Project Technical Requirements.


cables /

strands




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

1 261 120 1 261 108

2 521 120 2 522 108

3 782 120 3 783 108

4 1043 120 4 1044 127

5 1303 143 5 1305 127

6 1564 143 6 1566 127

7 1825 143 7 1827 127

8 2085 141 8 2088 187

9 2346 161 9 2349 187

10 2610 187

11 2871 187

12 3128 161 12 3132 187


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NOTE: Check points have to be allowed for onsite to make sure the system “anchor + load

cell” works properly.


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.

UNEXPECTED READINGS


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NOTE: Eccentric loading must be avoided when installing load cells in anchors.

UNEXPECTED READINGS

Centred load cell in an anchor installation.

Eccentric load cell in an anchor installation.

Avoid eccentric loading!


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NOTE: Changes in environment conditions will have several effects on the readings

registered by the load cell.


Load cell is giving unstable

readings

Are the readings alright with a different

readout?

If yes then suspect low battery.

Are the load readings outside the range

of the load cell?

Make sure the expected loads and

the load cell range match.

Is there a source of electrical noise

nearby, such as a generator or a motor?

Have any electrical noise source

removed away.

Is there any significant temperature

effect?

See “Temperature effects” in this

document.

VW load cell: unstable readings Is the correct swept frequency being

used?

Check swept frequency coming from

readout and / or datalogger.

Strain gauge load cell: unstable

readings

Is the load cell part of an unexpected

electrical loop?

Check insulation resistance between

the cell body and any cable. The

reading should be >500 MΩ.

Manometer

Were manometer fitted check for

damage?

Check the needle return to zero with

no load.

Check for leaks on the cell.

Hydraulic load cell: unstable

readings

Vibrating Wire VW

Is the correct swept frequency being

used?

Check swept frequency coming from

readout and / or datalogger.

UNEXPECTED READINGS


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NOTE: It is crucial to foresee convenient onsite conditions before installing load cells.

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.


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NOTE: Clean and tidy installations favour reliable readings.

LOAD CELLS’ APPLICATIONS

Strain gauge anchor load cell to monitor

a passive ground bar anchor.

Vibrating wire VW load cell installed to control an

active, multi-stage strand ground anchor (2/2).

Vibrating wire VW anchor load cell installation in

an active, multi-stage strand ground anchor (1/2).

Vibrating wire VW bar anchor load cell load to

monitor a ground bar anchor.


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TYPE OF STRUCTURE Structure Ensure design for the anchor block and the load cell elements are

the same -OD & ID, load-.

Type of load cell Chose your load cell bearing in mind service load, kind of anchor

and lifespan.

ANCHOR LOADS Base reading Take a series of unloaded base readings right before onsite instal-

lation.

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 cal-

culation 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 Always avoid eccentric loading. Having several sensors fitted in the

load cell and averaging minimises the effects of eccentric loading.

Bar and strand anchor 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 Take readings from anchor load cell always at the same time of the

day, preferably right before sunrise.

Types of sensors Load cells including a thermistor allow for more accurate tempera-

ture compensations.

Troubleshooting Check for easy to understand subjects when a load cell is giving

unstable readings.

NOTE: Positive installations need planning and good care onsite.

SUMMARY

Documents

Anchor Load Cells Installation considerations...while installing and service life, applied on bar and cable ground anchors as well as prestressing and post tensioning tendons. It is