pánchezjbeáíáa pk 1 I Torres ik 2 I Barrallo gk 3 I fraola BK 4 ABSTRACT

Átírás

1 Structural nalysis of Historical Constructions Jerzy Jasieńko (ed) 01 DWE, Wrocław, Poland, ISSN , ISN THE HOLE DRILLING TECHNIQUE ON TIMER STRUCTURES: PREVIOUS TESTS pánchezjeáíáa pk 1 I Torres ik I arrallo gk 3 I fraola K 4 STRCT The paper deals with the first works for the application of the Hole-Drilling technique on timber structures in service. The final objective of the research is the deduction of the stresses in structural elements of rchitectural Heritage. The method involves glued strain gages at the surface of a structural element. t least three strain gages are placed in the area under consideration in order to record the strains originated by stress relaxation after eliminating a round of 18 mm diameter and 16 mm depth. The stresses before drill can be deduced with an appropriate mathematical scheme. The Hole Drilling technique can be considered as minor destructive test providing sufficient information to calculate the principal stresses and their directions around the drill. t laboratory four wood specimens (blocks) have been loaded under known stresses in order to compare them with those stresses obtained by the hole drilling procedure. The results show some desviation because the mathematical scheme assumes the blocks as isotropic material. In this sense, the research described has to be improved to take into account the orthotropyc behavior of the wood as structural element. heywords: eole arállángi Támber sírucíuresi píress analysás 1. DESCRIPTION OF THE HOLE DRILLING TECHNIQUE The Hole-Drilling technique is widely applied to obtain the Residual Stresses near the surface of a material by using a three-element strain gage rosette. The application of this technique to the rchitectural Heritage [1-5] is based on the utilization of strain gages to get the structural strains caused by stress relaxation. Over a circumference of 40 mm diameter three strain gages are glued (Fig. 1) and connected to a strain-recording unit measuring the fluctuations around zero until stabilization. fter reaching stability, it is made a drill 18 mm in diameter and 16 mm in depth concentric with the circumference of position of the strain gages. When the drill is completed, the strains are once more measured during 30 minutes at least until their new stabilization. Such strains are originated by the relaxation of the stresses due to the removal of the material and are caused by the existing stresses before the drill operation. This procedure is a particularisation of the Hole-Drilling technique for the residual stresses deduction according to the Standard STM E [6]. This Standard sets one method to obtain the constants, which relate the recorded strains to the stresses that cause them. These constants (called and ) are related with the Poisson coefficient (ν), the Young s Modulus (E) and two parameters a and b that include the geometrical conditions of the test. The values of the constans and are as follows: (1 + n ) - (1) ba - () bb 1 Prof., santiago.sanchez@ehu.es Eng. Grad., ltcardona@yahoo.fr 3 Prof., javier@barrallo.com 4 Master in rch., borjairaola@gmail.com ll authors belong the E.T.S. rquitectura of San Sebastián (UPV/EHU), University of the asque Country 060

2 fter the values of the constants and have been obtained, the stresses near the round before the drill are deduced by means of the expressions: ( e - e ) + ( e + e - ) e1 + e e s max - (3) ( e - e ) + ( e + e - ) e1 + e e s mán + (4) 1 æ e3 + e1 - e ö b arcían ç (5) è e 3 - e1 ø where ε 1, ε and ε 3 are the strains recorded, respectively, at 0º, 5º and 90º with a direction of reference, σ pmax and σ pmin are the maximum and minimum principal stresses whereas β is the angle (measured clockwise) of the ε 1 direction with the direction of σ pmax (or the ε 3 direction with the direction of σ pmin ). Fig. 1 shows the geometry of the analyzed area and the strain gage disposition. On the other hand, the Standard E suggests an experimental procedure (laboratory test) to deduce the constants relating the recorded strains with the stresses that cause them. On a sample of the same material analyzed on a monument, two strain gages are glued at the centre of a vertical face. The strain gages are placed in the horizontal and vertical directions over a circumference of 40 mm diameter. fterwards the sample is loaded to a known compression stress (σ), recording the strains measured in both strain gages which will be called E hb (horizontal strain) and E vb (vertical strain). With the sample unloaded a drill 18 mm diameter and 16 mm depth is carried out. fterwards the sample is loaded identically (σ) and once again the strains measured by the strain gages are recorded. In this case, they will be called E ha (horizontal strain) and E va (vertical strain). The new constants (called m and m ) are obtained according to the expressions: m m ( bha - bhb) + ( bva - bvb) 4 ( (6) s ( bha - bhb) - ( bva - bvb) 4 ( (7) s The principal stresses and their directions can be obtained by using the same expressions mentioned above (3), (4) and (5). σ max Gage 3 (ε 3) β σ min β Gage 1 (ε 1) 1 135º d D 1 135º Fig. 1 Scheme of the geometry of the Hole-Drilling technique. D is the diameter of the circumference where the strain gages are glued and d is the diameter of the drill 061 Gage (ε )

3 . LORTORY TESTS Four blocks (458 cm of section) have been loaded at laboratory under axial (fyber and transversal (normal to the fyber compressions (Fig. and 3) in order to compare the values of the known stresses witih those obtained by means of the hole drilling procedure. locks 1 and are composed of a kind of local pánasíer (named Class I) with a value of Mpa for the Young s Modulus (E) in the fyber direction. oth blocks have been tested in this direction. locks 3 and 4 are composed of another local pánasíer class (named Class II) with a value of 7600 Mpa for the Young s Modulus (E) in the fyber direction and 556 Mpa for the transversal direction. lock 3 was tested in the fyber direction whereas lock 4 was loaded transversally. These values for the Young s Modulus have been obtained at laboratory in previous works. In all the cases a value of ν 0.7 was adopted for the Poisson coeficient (see Discussion). The size of the strain gages, the experimental geometry (Fig. 4) and the size of drill (Fig. 5) agree exactly with the Standard STM E guidelines. Table 1 shows the values of these parameters and the mechanical properties. Likewise it shows the value of the parameters a and b taken from Standard above mentioned. Table shows the comparison between the values of the applied stresses and those obtained by the hole drilling procedure (negative sign means compression stresses). The value of the vertical (direction of the applied load) stresses (σ v ) is easily deduced from the values of σ max, σ min and β. Strain gage length D d Table 1 Geometrical and mechanical parameters Depth of drill E (Mpa) Class I Class II Class II (fyber 7600 (fyber 556 (normal to fyber ν a b 0,7 0,36 0,558 Table Comparison between the stresses applied and those obtained by the Hole Drilling tecnique lock Nº pplied stresses (axial in Mpa Stresses obtained by Hole Drilling (Mpa) σ pmax σ pmin β (º) σ v 1-1,46,3-4, , - 1 8,7-0, ,56 3-3,4 1,1-4, ,54 4 pplied stresses in transversal direction: 0,8 0,4-0, Fig. View of the sample loaded in the fyber direction 06

4 FigK Psáewofíhesampleloadedáníransversaldárecíáon FigK Qdeneralgeomeíryofíheeolearállángprocedure FigK Rarálldeváce OMSP

5 3. DISCUSSION Two remarks have to be mentioned. First is related with the size of drill because the final objetive of the research is the application of the hole drilling procedure to timber structures in architectural heritage. Next step of the research have to be directed to reduce the actual drill size from 18 mm to 10 mm at least. ccording STM E standard there is not problem in reducing the drill diameter but there is a limit because the in situ experimental procedure is based in manual skill (the size of the strain gages, the drill diameter and the diameter of the position of the strain gages are interelated). s a far objective it could be proposed the construction of standard devices for in situ analysis. Results obtained by the hole drilling procedure present desviations in comparison with the known stresses (Table ). This second aspect is originated by the value of the constants relating the recorded strains with the deduced stresses. Standard STM E takes into consideration the isotropic behaviour of the material. s it is well known all timber beams have orthotropic character and consequently it is imperative to include this behaviour in the mathematical scheme for the obtention of real stresses. The only possibility consists on the modification of the expressions (1) and () by means of finite elements analysis around the drill. Even more, it is very important to obtain the mechanical constants of the tested materials by means of tests specially programmed under well known values of temperature and humidity. For the laboratory tests shown here some values for the Young s Modulus obtained from previous tests were implemented and a common value for the Poisson Coeficient (0,7) was taken from the specialized library. t this moment, authors work in this problem in order to include the orthotropic behaviour. CKNOWLEDGEMENTS This research is a part of the project supported by the University of the asque Country (UPV/EHU) entitled nálisis Estructural de Elementos Portantes del Patrimonio Construido, leadered by the first author. REFERENCES [1] Sánchez-eitia S, Schueremans L. (008) The Hole Drilling Technique In site stress measurement on the piers of the Saint Jakobs church in Leuven (elgium). In mrock fník ConfK pcomatfp Eln páíe ssessmení of ConcreíeI Masonry and Támber) Varenna-Lago Como (Italy), Univ. Pol. of Milan and RILEM, [] Sánchez-eitia S, (007) Stress analysís in the ltes Museum (erlín) by means of the Hole Drilling Technique (Donostia Method). Consírucííon and uáldáng Maíeráals 1(8): [3] Sánchez eitia S, (008) Stress analysis of the piers of the Tarazona Cathedral (Zaragoza-Spain) by means of the hole-drilling technique. Consírucíáon and uáldángs Maíeráals (5): [4] Sánchez-eitia S, Schueremans L, Van alen K (009) In site stress measurement on the piers of the Saint Jacobs church in Leuven (elgium). fníernaíáonal gournal án rcháíecíural eeráíage 3(): [5] Sánchez eitia S, Shueremans L (009) The Hole Drilling technique for on site deduction of the stresses states in stone masonry by using eight strain gages. Consírucíáon and uáldángs Maíeráals 3: [6] STM Standard E and 001 (1995 and 001). Standard Test Method for Determinig Residual Stresses by the Hole-Drilling Strain-Gage Method. merican Society for Testing Materials. 064