EXPERIMENTAL TESTING OF REINFORCED END-NOTCHED GLULAM BEAMS

Nosači s redukovanom visinom preseka od monolitnog i lepljeno lameliranog drveta vrlo su zastupljeni u građevinskoj praksi. Mesto nagle promene visine poprečnog preseka nosača predstavlja slabu tačku u konstrukciji, te se preporučuje njeno izbegavanje. Međutim, postoje brojni razlozi za redukciju visine elementa. Najčešći razlog za to jeste ograničenje visine iznad oslonaca, ali postoje i drugi, poput poboljšanja bočne stabilnosti nosača, ostvarivanja veze elemenata itd. [1]. U svim ovim slučajevima, neophodan je adekvatan proračun nosača s redukovanom visinom preseka. Kapacitet nosivosti nosača znatno je umanjen, kao rezultat koncentracije napona na mestu redukcije visine preseka. Redukcija visine preseka na zategnutoj strani elementa izaziva pojavu napona zatezanja upravno na vlakna koji, zajedno sa smičućim naponima, može da izazove pojavu pukotina, tipično od mesta nagle promene visine (slika 1). Pukotine su nepoželjna pojava sa estetske tačke gledišta, ali su takođe veoma opasne po konstrukciju, jer propagacija pukotine s porastom nivoa opterećenja može dovesti do loma. Ojačanje nosača s redukovanom visinom preseka je ekonomično rešenje kako bi se povećala nosivost.


INTRODUCTION
Notched solid timber and glued laminated timber (glulam) beams are very common in structural engineering practice.Notches represent a weak spot in structure, and it is advisable to avoid them.However, there are various reasons for beam notching.The most common one being limitation in construction height at the supports, and others such as: stabilization of structural elements against lateral buckling, intersection of members and joint details etc. [1].Adequate design of notched beams is necessary in these cases.
The load carrying capacity of timber beams is considerably reduced as a result of stress concentration around the notch.Notches made on the tension side induce tensile stresses perpendicular to grain which, accompanied by shear stresses, can cause longitudinal splitting typically starting at the notch corner (Figure 1).Cracks are unattractive appearance from aesthetic point of view, but are also very dangerous from structural perspective because crack propagation as the load level increases can lead to a failure of a beam.Reinforcement of such members is a cost-effective alternative for enhancing the load carrying capacity of structures in service.
U proteklim decenijama, mnogi istraživači bavili su se drvenim nosačima s redukovanom visinom preseka i očigledno je da ojačanje i sanacija ovakvih elemenata predstavljaju veoma važnu temu u oblasti drvenih konstrukcija.U svojoj doktorskoj disertaciji, Jockwer [1] je dao detaljnu analizu različitih proračunskih pristupa neojačanih, ali i ojačanih greda s redukovanom visinom preseka.Franke, Franke i Harte [9] bavili su se metodama povećanja nosivosti drvenih greda, Notched ends of beams should be reinforced so as to avoid brittle failure and increase load carrying capacity of the beams.There are various types of strengthening techniques which are mainly based on preventing the expected cracks.Parameters such as ease of installation, invisibility of reinforcement, simplicity of design approach and cost are all important for determining the strengthening method.Different types of elements (rods [3], screws [4,5], plates and sheets [6]) and materials (wood-based materials, steel, advanced composite materials like carbon or glass fibre based polymers [7,8]) have been successfully used as reinforcement of notched timber beams.
In the past decades, many researchers have dealt with notched timber beams and it is obvious that notched beam strengthening and repair represents very important topic in the field of timber structural design.In the PhD thesis Jockwer [1] gave a thorough analysis of different design approaches of both unreinforced and reinforced notched beams.Franke, Franke and Harte [9]dealt with methods for repair of structural performance uključujući i one s redukovanom visinom preseka.U svom radu, Oudjene i grupa autora [10] predstavljaju numerički pristup za modeliranje neojačanih i ojačanih greda s redukovanom visinom preseka.Dietsch [11] je govorio o neophodnosti novih proračunskih metoda za ojačanje drvenih greda i o njihovoj implementaciji u okviru Evrokoda 5 [12], naglašavajući važnost adekvatnog analitičkog proračuna.
This paper presents experimental results of endnotched glulam beams that were reinforced with screws.Screws are economic and time-efficient solution for reinforcement and they can be easily applied [13], which is the reason why they were chosen in this study.Five unreinforced and ten reinforced end-notched glulam beams were tested in bending to the point of failure, with two different reinforcement schemes considered.The results in terms of load-deformation relationship, failure mode, ultimate load carrying capacity and stiffness were compared between tested beam series.The conclusions on effectiveness of screws as a reinforcement method were made.

EXPERIMENTAL TESTING
The experimental research was conducted at the Laboratory of Structures, Faculty of Civil Engineering, University of Belgrade.Five unreinforced (Series U) and ten reinforced (Series R) notched glulam beams were tested in bending to the point of failure.Unreinforced beams were used as a control series.Reinforcing was performed with screws.Five reinforced beams had screws installed perpendicular to beam axis (Series R-s90) and five had screws positioned at an angle of 45° to beam axis(Series R-s45).
The glulam beams were made from spruce timber classified in the strength class C22 according to EN 338 [14], making the beams class GL22h [15].Before the tests were performed, the beams were conditioned at a temperature of T = 20 ± 2 ºC and a relative humidity of RH = 65 ± 5%.After testing, moisture content was measured in each beam using a digital hygrometer at different locations.The moisture content in tested beams varied between 11.0% and 11.9%.
The overall length of the beams was 4000 mm and the cross section was 100 x 220 mm.Each beam was composed of seven 32 mm thick laminations.At the notched ends, the height of the beams was reduced to 110 mm (by half) and the length of notches was 250 mm.The reinforcement selected in this study was traditional wood screws with a diameter 10 mm and length of 200 mm for Series R-s90 and 250 mm for Series R-s45(Figure 3).Threaded part of screws was 125 mm and 160 mm, respectively.According to the manufacturer the steel grade of screws was 5.6.Two screws in one row were positioned near the both notched ends of the beams.The requirements for minimum screw edge distances and spacing were satisfied while keeping the reinforcement as close as possible to the notch corners.
All beams were subjected to bending test in accordance with EN 408 [16].The beams were tested to failure under monotonic load in four-point bending configuration over a simply supported span of 3750 mm.The distance between two loading points was 1350 mm, while the distance from the loading points to the supports was 1200 mm.The specimens were supported on roller bearings at the ends.Roller bearings were also used at the load application points.The effects of local indentation at load application and support positions were minimized by placing steel plates.
A schematic illustration of the bending test configuration for Series U, Series R-s90 and Series R-s45 is shown in Figure 4, while Figure 5 shows experimental test set-up.
When considering the specimens that were going to be reinforced special attention was put on inserting the screws.The preparation of Series R specimens is shown in Figure 6.The holes for screws were pre-drilled very carefully to a diameter of 8 mm, with approximately drilling length of 200 mm and 250 mm.The screws were inserted using a moment wrench.Elektronski ugibomeri (LVDT) korišćeni su za merenje ugiba u sredini raspona nosača, kao i za merenje otvaranja pukotina na mestima nagle promene visine preseka.Podaci o ugibima sa LVDT-ova i odgovarajući podaci o opterećenju sa dozne zabeleženi su pomoću akvizacionog sistema.Sopstvena težina hidrauličke prese i čeličnog profila uzeti su u obzir.Ovo dodatno opterećenje iznosilo je 1,3 kN.
Efekti redukcije visine preseka na mehaničke karakteristike lepljeno lameliranih drvenih nosača su značajni.Svi ispitani neojačani nosači (Serija U) pokazali su linearno ponašanje do loma.Lom je nastupio na mestima nagle promene visine usled prekoračenja napona na zatezanje upravno na vlakna, kao što je prikazano na slici 9. Otvaranje pukotina (oblik loma 1) na mestu nagle promene visine očigledan je mehanizam loma neojačanih nosača.Međutim, smicanje u ravni pukotine (oblik loma 2) takođe je imalo značajan uticaj.Zbog krte prirode ponašanja drveta pri zatezanju i The load was applied monotonically using a hydraulic jack until the failure occurred and recorded with a compression load cell.The load was transformed from one point to two points with a steel beam.Monotonic static load was applied in a stroke-controlled rate of 4 kN per minute, so as to cause the failure of the unreinforced beams in approximately 5 minutes.The reinforced beams were tested with the same load rate in order to ensure a fair comparison of test results.The failure of the reinforced beams was achieved in about 10 minutes.
Linear variable differential transducers (LVDTs) were used for the measurement of mid-span deflection of the beams as well as the measurement of crack opening in notch details.The deformation data from LVDTs and corresponding load data from a loading cell were recorded by a computerized data acquisition system.Self-weight of hydraulic jack and steel beam were added to the recorded load.This additional load was 1.3 kN.

Load-deflection behaviour and failure modes
The load-deflection behaviour to the failure of tested beams is shown in Figures 7 and 8.
The effects of the notches on the mechanical properties of glulam beams are significant.All tested beams with unreinforced notches (Series U) exhibited linear load-deflection behaviour until the point of failure.Beams failed at the notch details due to excessive tensile stress perpendicular to grain, as it is shown in Figure 9. Crack opening (Mode 1 fracture) at the notch corner was the obvious failure mechanism of unreinforced notched beams.However, crack shearing (Mode 2 fracture) also had a considerable influence.Due to brittle nature of wood behaviour in tension and in smicanju, lom nosača Serije U bio je iznenadan i bez znakova upozorenja.Pre dostizanja graničnog opterećenja, primećeno je vrlo malo otvaranje pukotina.Nakon razvijanja inicijalne pukotine na mestu redukcije visine preseka, došlo je do nekontrolisanog rasta pukotina.To je uzrokovalo razdvajanje preseka na dva dela (gornji i donji).Putanja pukotine bila je uglavnom pravolinijska, a površine pukotine bile su ravne.shear, failure of Series U beams was sudden without warning signs.Prior to ultimate load, only very little crack opening was observed.After the development of initial crack at the notch corner, uncontrollable crack growth occurred.This led to a separation of the cross-section in two parts (upper and lower).The crack path was generally clear and straight.Ojačani nosači s redukovanom visinom preseka (Serija R) u suštini su imali linearno ponašanje do loma.Devet od deset nosača doživelo je krti lom.Jedan od nosača imao je pad nosivosti, ali je nastavio da nosi opterećenje do loma.Iako je granično opterećenje povećano, ojačanje nije bilo dovoljno da se promeni oblik loma iz kombinovanog usled zatezanja upravno na vlakna i smicanja u lom usled savijanja.Slika 10 prikazuje tipičan lom ojačanih nosača.
Despite the reinforcement intervention, initial cracking of the notch corner cannot be prevented.It can be seen that crack initiation started at relatively low loads.This can be explained by the very small deformation capacity of wood before the tensile strength perpendicular to grain is exceeded.Excessive crack opening was limited by the reinforcement.With further loading the stable crack growth was accompanied by sharing of the crack.At failure, unstable crack growth occurred and crack shearing increased considerably.It can be assumed that the shear failure was dominant failure mechanism.In most cases, failure was accompanied by withdrawal of the screws.
At the notch corner vertical reinforcement screws were subjected to combined loading parallel and perpendicular to the shear plane.There were clear plastic deformations in the reinforcement indicating that plastic hinge was formed in the fracture region in the case of these beams.The idea of inclined screws was to load the reinforcement axially (in tension), the direction in which they demonstrate the highest stiffness.Therefore, Series R-s45 beams were expected to have much higher load carrying capacity, but due to insufficient anchorage length of the screws, they failed even earlier than the beams from Series R-s90.Since conventional screws require pre-drilled holes for installation, better results could be achieved with selftapping screws for reinforcing and strengthening procedures of timber structures.

Load carrying capacity, deformability and stiffness
The results of experimental tests in terms of load carrying capacity, deformability and stiffness for the three series of beams are given in Table 1.The ultimate load was taken as a maximum force, which caused the failure of the beams.The mid-span deflection was as the value that corresponded to the ultimate load.The bending stiffness was calculated from liner part of the load-deflection curve of each beam, using the mid-span deflection equation for four-point bending: -nagib krive opterećenje-ugib između 10% i 40% graničnog opterećenja; l -raspon grede; c -rastojanje između oslonca i koncentrisane sile.U tabeli 1 prikazana su poređenja graničnog opterećenja, ugiba u sredini i krutosti na savijanje za ojačane nosače (Serija R-s90 i Serija R-s45) i neojačane nosače (Serija U). where: E -modulus of elasticity, I -moment of inertia, -slope of load-deflection curve between 10% and 40% of ultimate load, l -beam span, c -distance between support and load application point.
The unreinforced notched beams (Series U) had an average ultimate load of 12.8 kN.The load carrying capacity of the beams was considerably reduced due to presence of notches.High coefficient of variation (25.2%) in ultimate load for Series U beams can be explained by natural variability in timber properties.Introduction of reinforcement at the notched ends of the beams resulted in improvement in load carrying capacity.The reinforced beams obtained an average ultimate load of 35.9 kN and 34.1 kN, for screws positioned at the angles of 90° and 45°, respectively.All reinforced beams showed an increase in ultimate load when compared with the loads recorded for the beams without reinforcement.This increase was 180.5% and 166.4%.Unreinforced notched beams completely lost their load carrying capacities after the first crack developed.On the other hand, reinforced beams continued to carry the load after initial cracking.However, insufficient anchorage length did not allow for the reinforced beams to fail in bending, since the withdrawal of the screws occurred before the beams reached the load carrying capacity of beams without notches.
The reinforced beams underwent large deformations before the failure when compared with the unreinforced ones.Average measured mid-span deflection at ultimate load was 52.9 mm, 42.3 mm and 11.5 mm for beams of Series R-s90, Series R-s45 and Series U, respectively.At failure, the reinforced beams exhibited 3.6 -4.6 times larger mid-span deflections.Hence, screws helped improve the deformability of the beams.
All the beams had similar bending stiffness values.This was expected since the applied reinforcement was not meant to improve the bending stiffness.Series R-s45 had a bit lower value, which can be explained by the variability in timber properties that generally exists when this material is in question.
Proračun nosača s redukovanom visinom preseka kod oslonca, dat u Evrokodu 5, ne predviđa proračun mogućih ojačanja.Kao dodatak Evrokodu 5, Nemački nacionalni aneks [17] predlaže metod proračuna ojačanja (slika 12).Ovaj pristup baziran je na ideji da se napon koji se javlja na mestu nagle promene visine nosača raspodeljuje između ojačanja i drveta putem faktora α k .notches according to Eurocode 5 from Eq. 2 is: When compared with the average experimental value for Series U, which was 12.8 kN, it can be seen that the suggested design method overestimated the ultimate load by 7.0%.It can be noted that this design approach ignores the in-plane shear, which appears at the notch, taking into account only the vertical component that causes tensile stress perpendicular to grain of timber.
The design of end-notched shear stress given in the Eurocode 5 does not provide the calculation of possible reinforcement.In addition to Eurocode 5, the German National Annex [17] proposes reinforcement design method at the notch (Figure 12).This design approach is based on the idea that the stress which apperas around the notch is divided between reinforcement and timber using factor Vertikalno ojačanje grede sa redukovanom visinom kod oslonca [18] Figure 12.Vertical reinforcement of end-notched beam [18] Maksimalna smičuća sila kod oslonca nosača s redukovanom visinom: Maximum shear force at the support of end-notched beam: U ovom radu zavrtnji su usvojeni kao ojačanje i deo sile koji oni preuzimaju može se sračunati prema izrazu: In this paper screws are selected as reinforcement and and part of the load carried by screws can be calculated as follows: Za ojačane nosače, karakteristična vrednost kapaciteta zavrtnja na čupanje usvaja se prema [18]: For reinforced beams the screw withdrawal capacity was adopted according to [18]: , with the assumption that the screws carry the entire load the ultimate load for vertically reinforced notches according to [18] is: The made assumption is valid, because the tests were carried out to the point of failure and not for the service loads.Timber part of the cross-section in this case loses its load-carrying capacity with crack initiation at the notch, and screws continue to carry the load to failure.The difference from experimentally obtained average value, which was 35.9kN, is 1.7%.
For screws positioned at an angle of 45° to beam axis, the withdrawal strength is taken as ,45,90 0.86 =⋅ axax Ff according to [18].The value for inclined screw withdrawal capacity for screw diameter d = 10 mm and anchorage length l e f = 94 mm is:

FVnFkN
The difference from experimentally obtained average value, which was 34.1 kN, is 7.0%.
There is a good agreement between analytical and experimental results for reinforced beams load-carrying capacity.Since the design method is based on the concept that reinforcement carries only the vertical force component that causes tensile stress perpendicular to grain of timber, analytical values underestimate the ultimate load.

CONCLUSIONS
The experimental procedure performed in this research included bending tests of 15 notched glulam beams to the point of failure (five unreinforced beams and ten reinforced ones).The screws were selected as reinforcement.The effects of reinforcing were evaluated in terms of load-deflection behaviour, failure mode, ultimate load carrying capacity and stiffness of reinforced beams, which were compared with the unreinforced notched beams.In addition, comparison with analytical design method was made.The following conclusions were drawn: − Notched glulam beams when subjected to bending failed due to stress concentration at the notch corner.The load carrying capacity of these beams is defined by excessive crack opening.Brittle failure mechanism is typical for unreinforced notched beams.
− Initial cracking of the notch corner cannot be prevented by the reinforcement.However, by using reinforcement at the notches failure is delayed significantly.
− The reinforcement prevents excessive crack opening at the notch corner.Nevertheless, final failure of the reinforced notches due to excessive crack growth is caused by crack shearing.
− Reinforcing intervention of notched glulam beams with screws increases the load carrying capacity and deformability.
− At the notch corner, reinforcement screws perpendicular to beam axis are subjected to combined parallel and perpendicular to the crack surface loading.Hence, reinforcement with high strength and stiffness in both directions is required to achieve the best reinforcing effect.
− Reinforcement screws inclined at an angle of 45° to beam axis are dominantly loaded in the axial direction.Special attention should be paid to the anchorage length of these screws.
− The suggested design method in Eurocode 5 overestimated the ultimate load of unreinforced beams 7.0%.
− As for reinforced beams, if the assumption that the entire load is carried by screws is made, there is a good agreement with the proposed design method in the German National Annex to Eurocode 5.This research gives an insight into reinforcing possibilities of notched timber members.It can be a good basis for further investigation of the effectiveness of other types of reinforcement like carbon or glass fibre based polymer bars.In addition, results obtained from tests can be useful in developing appropriate analytical design models for notched timber beams.Existing design methods should be revised, as they overlook the complex stress state around the notch area

Marija TODOROVIC Bosko STEVANOVIC Ivan GLISOVIC Tijana STEVANOVIC
Notches made at the ends of timber beams significantly decrease load carrying capacity of a structural member.This paper presents an experimental research on bending behaviour of end-notched glulam beams, with and without reinforcement.Screws for timber were used as reinforcement, positioned at angles of 90° and 45° to the longitudinal beam axis.The unreinforced beams failed due to crack opening and its propagation in a brittle manner.Cracks that appeared in the notch details resulted from combined excessive tensile stresses perpendicular to grain and shear stresses.This study shows that reinforcing the beams at the notched ends can improve their load carrying capacity and deformability.However, applied screws did not help the beams achieve ductile failure in bending.The shear failure was dominant failure mechanism for reinforced beams.In addition, analytical calculations were performed in accordance with Eurocode 5 so as to compare the results with experimental research.Key words: notch, glulam, screws, reinforcement, experimental investigation