Load-Carrying Capacity of Timber - Wood Fibre Insulation Board - Joints with Dowel Type Fasteners

Transcription

1 Load-Carrying Capacity o Timber - Wood ibre Insulation Board - Joints with Dowel Type asteners G. Gebhardt, H.J. Blaß Lehrstuhl ür Ingenieurholzbau und Baukonstruktionen Universität Karlsruhe, Germany 1 Introduction So ar wood ibre insulation boards (WIB) are used as thermal and acoustic insulation in timber constructions. As wood-based panels, WIB are suited or the transer o loads caused by wind and earthquakes in timber rame constructions. Until present this task has been undertaken by plywood, particle boards and OSB. This new ield o application o WIB has been analysed in a research project at the Universität Karlsruhe. In this paper a proposal is given to calculate the load-carrying capacity o timber-wib-joints with dowel type asteners. or this purpose the load-carrying capacity o joints in timber and WIB may be determined according to Johansen s yield theory and an extension o this theory. Tests were carried out to estimate the embedding strength o nails in WIB and the crown pull-through resistance o staples in WIB. The test results veriied the calculation model and the stiness properties o timber-wib-joints were evaluated. Characteristics o WIB WIB can be abricated in two dierent manuacturing processes: In both the wood chips are thermo-mechanically pulped. In the wet process the wood ibres are mixed with water and urther aggregate into a suspension. Aterwards the boards are ormed and dried. or the bonding o the wood ibres only the wood s own cohesiveness (predominantly lignin resin) is used. Due to the high use o energy or the drying process, boards with higher thicknesses are manuactured by gluing raw single-layer boards to multilayer boards. At this raw single-layer boards with dierent densities may be combined. In the dry process the wood ibres are dried and sprayed with PUR resin. Aterwards the boards are ormed and the resin hardens. In this process boards with thicknesses up to 4 mm may be manuactured as single-ply boards. WIB may be used in dierent parts o buildings. In roos, rain-tight sub plates (RTSP) are ixed as sub decking. A urther insulation is possible as above-rater or interim-rater insulation with universal insulation boards (UIB). In walls, plaster baseboards (PB) may be used in composite thermal insulation systems. 1

2 11 dierent WIB o three dierent manuacturers were selected to analyse the characteristics needed or the use as sheathing o shear walls. The nominal density range was 11 kg/m up to 7 kg/m. The densities and moisture contents o the tested boards were determined. The measured moisture content range was 7.% to 1.%. Characteristic densities are proposed or the dierent types o WIB and given in table 1. Table 1 Proposed characteristic densities o WIB WIB type Characteristic density in kg/m Rain-tight sub plate (RTSB) Plaster baseboard (PB) 15 Universal insulation board (UIB) 15 1 Embedding strength Several dowel type asteners may be used or the attachment o WIB on the studs or raters. In roos, nails are commonly driven in through the counter battens. In walls, staples are commonly used. An alternative or staples are special screws. or the calculation o timber-wib-connections according to Johansen s yield theory [1] the embedding strength o WIB is required. Apart rom the embedding strengths o the connected parts, the load-carrying capacity depends on the geometry o the connection (thicknesses o the connected parts and diameter o the astener) and the yield moment o the astener. Tests with nails were carried out according to EN 8 [] to determine the embedding strength o mechanical asteners in WIB. In preliminary tests an inluence o the angle between orce direction and production direction had not been identiied. 68 embedment tests with ive diameters (d =.1/.4/.8/4.6/5. mm) were perormed. The embedding strength correlates with the measured density o the test specimen. In igure 1 the embedding strength is plotted vs. the density. The correlation coeicient is r =.88. There is a smaller correlation between the embedding strength and the thickness o the boards. A negative correlation exists between the embedding strength and the diameter o the astener. Due to a correlation between the density and the thickness and the higher correlation between embedding strength and thickness, the parameters diameter and density are considered in the calculation model or the embedding strength. By means o a multiple regression analysis, equation (1) could be derived to estimate the embedding strength h o WIB. h = ρ d in N/mm (1) where ρ is the density o the WIB in kg/m and d the diameter o the astener in mm In igure the tested embedding strength values are plotted vs. the calculated embedding strength values. The correlation coeicient is r =.916. The slope o the regression straight is m = 1. and the y-intercept is b = -.9.

3 Embedding strength in N/mm 11 y =.68x r = n = RTSP PB 1 UIB Density in kg/m igure 1 Embedding strength vs. density Tested embedding strength in N/mm 11 y = 1.x r = n = RTSP PB 1 UIB Calculated embedding strength in N/mm igure Tested embedding strength vs. calculated embedding strength Characteristic values o the embedding strength may be calculated considering the proposed characteristic densities or the dierent types o WIB (equation ()). RTSP ρ k = kg/m.75 h,k = 8.88 d PB ρ k = 15 kg/m.75 h,k = 4.5 d UIB ρ k = 15 kg/m.75 h,k =.5 d in N/mm () UIB ρ k = 1 kg/m.75 h,k = 1.57 d

4 4 Crown pull-through resistance The load-carrying capacity may be calculated according to Johansen s yield theory considering the geometry o the connection (thicknesses o the connected parts and diameter o the astener), the embedding strengths o the members (or WIB presented in chapter ) and the yield moment o the astener. The load-carrying capacity may be increased i the astener can be loaded axially apart rom loading laterally. The increasing value depends on the withdrawal strength o the astener in the irst part and the head/crown pull-through resistance in the second part. In order to calculate the rope eect the crown pull-through resistance o staples in WIB was examined. According to EN 18 [4] about 1 tests with RTSP and PB were carried out. or each o the 15 dierent WIB at least our tests were perormed. The displacement at maximum load correlates with the thickness o the WIB. In igure the displacement at maximum load is plotted vs. the thickness o the WIB. The correlation coeicient is r =.89. The slope o the regression straight is m =.5 and the y-intercept is b =.115. Displacement at maximum load in mm 4 y =.5x r =.89 n = RTSP 5 PB Thickness in mm igure Displacement at maximum load vs. thickness By means o a multiple regression analysis, equation () could be derived to estimate the crown pull-through resistance R ax, o staples in WIB. Rax,.4 ρ = t in N () where ρ is the density o the WIB in kg/m and t the thickness o the WIB in mm In igure 4 the tested crown pull-through resistance values are plotted vs. the calculated crown pull-through resistance values. The correlation coeicient is r =.814. The slope o the regression straight is m = 1.1 and the y-intercept is b = -.1. Test results o one RTSP with relative high thickness and density could not be explained by the regression model but test values are greater than calculated values. To explain thicker WIB with higher densities urther tests are needed. 4

5 Characteristic values o the crown pull-through resistance may be calculated considering the proposed characteristic densities or the dierent types o WIB (equation (4)). R =. ρ t in N (4) ax,,k k where ρ k is the characteristic density o WIB in kg/m Tested crown pull-through resistance in kn,5 y = 1.1x -.1 r =.814, n = 97 1,5 1,,5 RTSP PB,,,5,5,75 1, 1,5 1,5 Calculated crown pull-through resistance in kn igure 4 Tested crown pull-through resistance vs. calculated crown pull-through resistance 5 Calculation model or Timber - Wood ibre Insulation Board Joints The load-carrying capacity may be calculated according to Johansen s yield theory considering the dierent ailure mechanisms. I the astener is driven in through a counter batten, in ailure modes 1a and b an extension o the existing equations is necessary. In ailure modes 1a and b the embedding strength in the counter batten is reached and this eect increases the load-carrying capacity. The load path is rom the stud into the sheathing board. There is no resulting load in the counter batten. orce and moment equilibrium, considering the existing embedding strength distribution and the yield moment, deliver equation (5) or extended ailure mode 1a and equation (6) or extended ailure mode b. The ailure modes activating the inluence o the counter batten are shown in igure 5. h,1 t1 d t t t t t R = β + β β + β β( 1+ β) β 1+ (5) 1+ β t1 t 1 t1 t1 t1 5

6 h,1 t d 4β ( 1+ β) M y t R = β ( 1+ β) + + β β( 1+ β) β 1+ β h 1 d t t (6) where t is the thickness o the counter batten and h, is the embedding strength o the counter batten and h, β = h,1, h, β = h,1 M h,1 h, h, h,1 h, h, igure 5 Extended ailure modes 1a and b according to Johansen s yield theory 6 Tests with Timber-WIB-Joints In order to veriy the results obtained in the tests presented in chapter and 4 and to examine the stiness o joints, urther tests were perormed. Nails and staples were used as asteners. Staples may be driven directly into the WIB or through the counter batten like nails. In igure 6 the test specimens or the tests with nails and staples are shown. The relative deormations between WIB and stud were measured at each o the our asteners. The tests were carried out according to EN 6891 [5], irstly orce-controlled and aterwards displacement-controlled. The maximum load was reached at a displacement o 15 mm. The analysis o the joint stiness requires linear load-displacement behaviour up to 4% o the maximum load. In the tests non-linear load-displacement behaviour was observed. Due to this the analysis o the stiness was calculated or a constant displacement o. mm. 6

7 1 t / t igure 6 Test specimens or tests with nails and staples 6.1 Tests with nails 7 tests with nails and RTSP were perormed. RTSP o three manuacturers in three thicknesses respectively and two nail diameters were used. The load-carrying capacity may be obtained according to Johansen s yield theory considering the results in the presented tests and the extended calculation model. The yield moments o the nails were evaluated according to EN 49 [6]. The embedding strength o WIB was determined in previous tests (see chapter ) and the embedding strength o the timber was calculated according to DIN 15 [7] considering the evaluated densities. Although ailure mode 1b was authoritative, ailure mode was observed in the tests. This may be explained by riction between the joint members. The withdrawal resistance and the pull-through resistance were calculated according to DIN 15. In igure 7 the tested load-carrying capacity is plotted vs. the calculated load-carrying capacity. 18 Tested load-carrying capacity in N n = 7 4 d =.8 d = Calculated load-carrying capacity in N igure 7 Tested load-carrying capacity vs. calculated load-carrying capacity or nails 7

8 6. Tests with staples driven in through counter battens 7 tests with staples and RTSP were perormed. RTSP o three manuacturers in three thicknesses respectively were used. The load-carrying capacity may be obtained according to Johansen s yield theory considering the results in the presented tests and the extended calculation model. The yield moments o the staples were evaluated according to EN 49 [6]. The embedding strength o staples in WIB was calculated according to equation (1) assuming the validity or diameters smaller than the tested ones and the embedding strength o the timber according to DIN 15 [7] considering the evaluated densities. The withdrawal resistance was calculated according to DIN 15. In igure 8 the tested load-carrying capacity is plotted vs. the calculated load-carrying capacity. Tested load-carrying capacity in N n = 7 6 t = 18 mm 4 t = mm t = 5 mm Calculated load-carrying capacity in N igure 8 Tested load-carrying capacity vs. calculated load-carrying capacity or staples driven in through counter battens 6. Tests with staples driven in directly 6 tests with staples and RTSP/PB were perormed. RTSP and PB o three manuacturers were used. The load-carrying capacity may be obtained according to Johansen s yield theory considering the results in the presented tests. The yield moments o the staples were evaluated according to EN 49 [6]. The embedding strength o staples in WIB was calculated according to equation (1) assuming the validity or diameters smaller than the tested ones and the embedding strength o timber according to DIN 15 [7] considering the evaluated densities. The pull-through resistance was determined in previous tests (see chapter 4). In igure 9 the tested load-carrying capacity is plotted vs. the calculated load-carrying capacity. 8

9 1 Tested load-carrying capacity in N n = 6 RTSP, t = 18 mm RTSP, t = mm RTSP, t = 5 mm PB Calculated load-carrying capacity in N igure 9 Tested load-carrying capacity vs. calculated load-carrying capacity or staples driven in directly 6.4 Stiness o timber-wib-joints By means o a multiple regression analysis, equation (7) could be derived to calculate the stiness o a timber-wib-joint. In igure 1 the tested stiness is plotted vs. the calculated stiness. The correlation coeicient is r =.898. The slope o the regression straight is m = 1. and the y-intercept is b = K = 1.5 ρ ρ t d in N/mm (7) ser WIB where ρ WIB is the density o the WIB in kg/m, ρ is the density o the timber in kg/m, t is the thickness o the WIB in mm and d is the diameter o the astener in mm 18 k s,test = 1. k s,cal r =.898 n = 9 Tested stiness in N/mm Staple 1 mm 6 Staple 7 mm 4 Nail.8 mm Nail 4.6 mm Calculated stiness in N/mm igure 1 Tested stiness vs. calculated stiness 9

10 7 Conclusions The embedding strength o nails in WIB was tested. As result o a multiple regression analysis, the embedding strength may be calculated considering the density o the WIB and the diameter o the astener. In urther tests, the pull-through resistance o staples in WIB was examined. The pull-through resistance depends on the thickness and the density o the WIB. To consider the inluence o a counter batten in the ailure modes according to Johansen s yield theory, the equations or two ailure modes were extended. The results o the previous tests were used to calculate predictive values o the load-carrying capacity o timber- WIB-joints tested in urther tests. The stiness o timber-wib-joints may be calculated depending on the densities o the joint members, the thickness o the WIB and the diameter o the astener. With these results the load-carrying and displacement characteristics o timber-wib-joints may be evaluated and urther be used or the calculation o shear walls with WIB sheathing. 8 Reerences [1] Johansen, K. W.: Theory o timber connections. International Association o bridge and structural Engineering, Bern. P [] EN 8:199: Timber structures; Test methods; Determination o embedding strength and oundation values or dowel type asteners [] Blaß, H. J.; Gebhardt, G.: Holzaserdämmplatten Trag- und Verormungsverhalten in aussteienden Holztaeln. Karlsruher Berichte zum Ingenieurholzbau, Band 14, Lehrstuhl ür Ingenieurholzbau und Baukonstruktionen (Ed.), Universität Karlsruhe (TH), 9 [4] EN 18:1999: Timber structures; Test methods; Pull through resistance o timber asteners [5] EN 6891:1999: Timber structures; Joints made with mechanical asteners; General principles or the determination o strength and deormation characteristics [6] EN 49:199: Timber structures Test methods Determination o the yield moment o dowel type asteners [7] DIN 15:4: Design o timber structures General rules and rules or buildings 1