D11_1_holedrilling.pdf

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DELIVERABLE D11.1

Technical guidelines for an appropriate use of the suggested equipmentHOLE DRILLING

Issued by: GEOCISA/ October 2004

WORK PACKAGE 11 (WP11 Leader: ZAG): Guidelines and recommendations

CONTRACT N: EVK4-CT-2001-00060

PROJECT N: EVK4-2001-00091

ACRONYM: ONSITEFORMASONRY

On-site investigation techniques for the structural evaluation of historic masonry buildings

PROJECT CO- Bundesanstalt fr Materialforschung und -prfung

ORDINATOR: (Germany)

Project funded by the European Community under the

Environment and Sustainable Development Programme(1998-2002)

Guideline Hole Drilling D11.1 Page 1


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TABLE OF CONTENTS

1 INTRODUCTION 32 BASIC PRINCIPLES 33 APPLICATION TO MASONRY INVESTIGATIONS 4

3.1 POSSIBLE TARGETS 43.2 CONDITIONS FOR APPLICABILITY 43.3 CALIBRATION TESTS 4

4 REQUIREMENTS 54.1 EQUIPMENT 54.2 PERSONNEL 5

5 INVESTIGATION DESIGN 56 ACQUISITIONS 5

6.1 PREPARATION 56.2 SET-UP 66.3 DATA ACQUISITION 66.4 DATA STORAGE 6

7 MEASUREMENT REPORT 68 ANALYSIS 7

8.1 DATA-PROCESSING 78.2 DATA-VISUALISATION 78.3 DATA-INTERPRETATION 7

9 FINAL REPORT 810 SAFETY INSTRUCTIONS / ENVIRONMENTAL CARE 811 GUIDELINES / STANDARDS 812 LITERATURE / REFERENCES 8



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1 INTRODUCTION

This guideline is addressed to the engineer or technical personnel responsible ofperforming the evaluation of existing stresses in the surface of stone masonry elements by

the hole-drilling method.

Hole drilling method is a strain release based technique for the estimation of the stressfield in the surface of stone masonry elements. The method proposed in these guidelinesconstitutes an extension of the Standard test method for determining residual stresses bythe Hole-drilling strain gage methodregulated by ASTM E 837-95 Standard.

2 BASIC PRINCIPLES

The hole drilling is a method for evaluating the real existing stresses in the surface of

stone masonry elements according to the following steps:

Three electrical resistance strain gages (numbered in a clockwise convention) are

placed in the area under consideration.

Strains are recorded before drilling in order to obtain a reference zero reading.

A hole is drilled at the geometric centre of the strain circumference.

Strains are again recorded after drilling the hole.

Strains induced by the stress release are obtained by the difference between the

values recorded after reading and those recorded before reading.

Existing stresses are estimated from those strains and two constants A and B, asindicated in ASTM E 837-95 (as described on chapter 3.3).

Figure 1.- Typical 3-element Clockwise Strain Gage Rosette (ASTM E 837-95)



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Constants A and B above mentioned take into account the material characteristics

(Young modulus and Poisson ratio), the general geometry of the problem and the

finite dimensions of the strain gauges.

3 APPLICATION TO MASONRY INVESTIGATIONS

3.1 POSSIBLE TARGETS

Estimation of stress field in those cases in which a complex stress situation of an specificelement is suspected.

This method can be applied essentially in regular masonries, provided that:

the three gauges are placed in the same stone block or ashlar

the diameters of the strain gauge circle and of the drilled hole as well as the depth of

the latter fulfil the limits indicated in point 3.2 of these guidelines.

The method could also be applied in irregular masonry, provided that the above conditionsare fulfilled, but interpretation of results can be much more difficult and must take intoaccount that significant variations in the magnitude and direction of stresses can exist dueto the irregularity and inhomogeinity of the masonry.

3.2 CONDITIONS FOR APPLICABILITY

The diameter of the drilled hole (d) should be related to the diameter D by:

0,3 < d/D < 0,5.

The depth of the hole should be at least 0,4 D.

Accuracy in the stresses estimation could be significantly limited if the temperaturechanges significantly during the performance of the test.

3.3 CALIBRATION TESTS

Estimation of stresses by this method requires the previous obtaining of constants A andB. They must be determined experimentally by means of a calibration test in which aprismatic block sample is subjected to a uniform uni-axial compression stress. Strains arerecorded in the direction of that principal compression and in the perpendicular one. Testsare repeated twice, a first one before drilling a hole and the second one after drilling it, andA and B are obtained applying the following relationships (from ASTM E 837-95):

( ) ( ) ( )beforeiafterical

=i

( ) ( )

cal

calcalA

2

13 +=



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( ) ( )

cal

calcalB

2

13

=

Alternatively A and B could be determined from the dimensionless and almost materialindependent coefficients a and b, given by tables included in ASTM E 837-95 standard,and the values of Young modulus and Poisson ratio of the material tested, applyingexpressions like (5) and (6) included in the mentioned ASTM standard. The application ofthese values of a and b requires checking previously that the geometric parameters

defining the test (GW, R1, R2, 1 and 2 Figure 1) fulfil the characteristics assumed forderiving those dimensionless values of a and b used in that ASTM standard.

4 REQUIREMENTS

4.1 EQUIPMENT

Electrical resistance strain gauges (at least 3 per measuring point, preferably 6) andmaterial needed for their installation and connection (glue, cables, protections, etc.).

Automatic equipment for recording the readings of the strain gauges used.

Drilling machine for stone

4.2 PERSONNEL

Technicians performing this test must be experienced on the installation and use ofelectrical resistance strain gauges and on handling drilling equipment on stone.

5 INVESTIGATION DESIGN

Selection of points for application of the method should be carefully done taking intoaccount that this is a minor destructive technique in which a minor but perceptible damageis produced in the stone masonry surface and based on previous theoretical analysis ofthe structure or element under study.

Selection of measuring points should be based on this analysis together with an on-siteinspection, so that results obtained could be useful to reach conclusions about the problemstudied.

6 ACQUISITIONS

6.1 PREPARATION

Stone surface must be prepared before gluing the strain gauges according to normalprocedures with this type of sensors. Normally surface preparation is made by a millstonemachine until a smooth surface is achieved.

In order to avoid experimental problems or anomalous results it is advised to use 6 straingages instead of the minimum 3. In this case strain in each direction should be obtained bythe average of the strains measured by the two gages placed in the same direction.



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Each strain gage is connected by means of a Wheatstones half bridge circuit including anon loaded gage for compensating temperature effects.

6.2 SET-UP

Strains should be recorded during a certain period of time before drilling, in order toregister the fluctuations around the zero reading. This period of time should be aminimum of 60 minutes, but longer periods are advisable if significant temperaturechanges are foreseeable during the duration of the test.

6.3 DATA ACQUISITION

The strain recording procedure should be as follows:

a) Measurement of the zero reading of strains (see 6.2).b) After the hole has been drilled, strains are measured again corresponding to therelieved stress situation. They should be recorded during at least 120 minutes, in order tocheck that any surface heating produced by the drilling process has dissipated and strainreadings are stabilised.

The effective strains corresponding to the stresses released by drilling are the differencebetween the strains measured in b) and the strains measured in a).

6.4 DATA STORAGE

It is advisable to use an equipment that records the strain readings during all this processso that the complete test could be reproduced if necessary. Fluctuations of the zeroreading in phase a), gives an estimation of the uncertainty of the strain measurementsand, consequently, of the real accuracy of the estimated stresses.

It is also advisable to record the temperature on the stone surface near the measuringpoint along the whole test process.

7 MEASUREMENT REPORT

The field report should contain the following information:

- Date, time, location and climatic conditions.

- Positions of points selected for applying the method.

- Complete information on readings evolution both before and after drilling, intables, digital files, etc. Inclusion of graphics to illustrate such evolution isstrongly recommended.

- Graphic of evolution of the stone surface temperature near the measuring pointsalong the test process.

- Values selected for each strain gauge as representative of zero and afterreleasing stresses readings.

- Effective released strains in each direction proposed for the calculation ofprincipal stresses as indicated in 8.1



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Unless otherwise stated by the end-user requirements, this measurement report and theraw data will become a sub-part of the final report.

8 ANALYSIS

8.1 DATA-PROCESSING

The principal stresses are obtained, from the effective strains corresponding to thestresses released by drilling, by means of the following expressions (from ASTM E 837-95):

( ) ( ) ( )( )

+++

+=B

EEEEE

A

EETp

4

221313

4

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max

( ) ( ) ( )( )

++

+=B

EEEEE

A

EETp

4

221313

4

312/122

min

+

=13

22135,0

EE

EEEarctg

where Tpmax is the Maximum Principal Stress, Tpmin is the Minimum Principal stress, isthe angle between E1 and Tpmax in a clockwise convention.

Once the principal stresses and the angle between them and the strain gauges axes havebeen obtained, stresses in any other direction could be obtained by application of commonexpressions of the theory of elasticity. In particular, the vertical stress (Tvertical) can beobtained from:

( )2cos5,05,0 minmaxminmax ++= ppppvertical TTTTT

where is the angle between the maximum principal stress Tpmax and the verticaldirection.

A and B for use in the above expressions should be obtained as indicated in 3.2.

8.2 DATA-VISUALISATION

Evolution of the strain readings during the test should be visualized in order to check thatthey are stable enough and that heating due to the drilling process has dissipated.

8.3 DATA-INTERPRETATION

Measured stresses should be compared with those expected from theoretical structuralanalysis. Interpretation of any discrepancy should be made together with the structuralexpert in charge of this structural analysis



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9 FINAL REPORT

The final report will include:

description of object and measurement positions (including photos)

relevant information about the structure and condition of the test object (if available)

or a hint where these information can be found

measurement results, specifically effective strains released in each direction by the

drilling process. Expected accuracy in the estimate of such effective released

strains.

principal stresses and vertical stress estimated at each measuring point

conclusions and further required investigations if any.

annexes, including the measurement report and the report on the calibration tests

performed to obtain constants A and B or on the procedure followed to estimate

them (see 3.3).

10 SAFETY INSTRUCTIONS / ENVIRONMENTAL CARE

The general employment protection guidelines have to be applied.

Safety requirements on location have to be kept in accordance with the instructions givenby the instructing party. In particular the general safety precautions and rules on buildingsites have to be followed (helmet, safety shoes).

For outside measurements the strain gauge measuring equipment should be kept in the

vehicle to protect it from influences of the weather.

11 GUIDELINES / STANDARDS

ASTM E 837-95 Standard test method for determining residual stresses by the Hole-drilling strain gage method.

ASTM E 1237-93 (2003) Standard guide for installing bonded resistance strain gages.

12 LITERATURE / REFERENCES

Barrallo, J., Zulueta, A. and Sanchez Beitia, S., Stress Measurements on AncientStructures by the Hole Drilling Method, Experimental Techniques Rev.,Vol.19 ,N 3,Society for Experimental Mechanics, Bethel (USA), (1994),pp. 9-13.

Zulueta,A., Barrallo, J., and Sanchez Beitia, S., The Donostia Method for StressMeasurements in Architectural Heritage, Strain (Journal of the British Society for StrainMeasurements), vol. 35, n 3, august 1999, pp.117-112. The University of Edinburg Ed.

Zulueta,A., Barrallo, J., and Sanchez Beitia, S.,El Mtodo Donostia para la Medida deEsfuerzos en Servicio en Patrimonio Arquitectnico, Ingeniera Civil, n 114/1999, pp. 37-43. Ed. CEDEX, Ministerio de Fomento, Madrid (Spain), (1999).



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Zulueta, A., Barrallo, J. and Snchez Beitia, S., .El Mtodo Donostia en torno a los 20Kg/cm2 . Ingeniera Civil, n 1/2002, pp. 20-36.Ed. CEDEX, Ministerio de Fomento, Madrid(Spain), (2002).


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