Takuro Hirai

Evaluation of drag coefficients of poplar-tree crowns by a field test method

To estimate the wind force that causes windthrow damage to a tree, the drag coefficients of actual-sized trees were evaluated by a field test method. In this method, wind velocity and stem deflection were monitored simultaneously. The wind force acting on a tree crown was calculated from stem deflection; stem stiffness was evaluated by conducting tree-bending tests. The results of tests conducted on three poplar trees showed that drag coefficient decreased with an increase in wind velocity. Although the variation in the drag coefficient was large at low wind velocity because of the vibrating behavior of the stem subjected to variable wind force, the variation at wind velocities above 10 m/s was small. The average drag coefficient at a wind velocity of 30 m/s was estimated by the curve-fitting of a power function to the wind velocity-drag coefficient relationship to be 0.102, which was smaller than that of actual-sized conifers studied in previous wind tunnel experiments. The drag coefficients of these crown areas in the defoliation season were smaller than those measured in the leafy season.

Load-carrying capacity of steel-to-timber joints with a pretensioned bolt

Previous experimental studies reported that bolt pretensioning greatly increases the initial stiffness and load-carrying capacity of bolted joints. It is also a matter of great importance to structural designers to understand the effect of pretension on the load-carrying capacities of bolted joints, and this study presents an extended yield model that considers the fastener’s pretension force. In the extended yield model, the load-carrying capacity was defined as the load at a slip of 15 mm. The ultimate fastener bending angle at the yielded cross section equivalent to this joint slip, which was affected by the fastener’s axial force, was iteratively evaluated in numerical analyses. The introduction of bolt pretensioning largely increased the joint slip resistance at initial loading, but it decreased the ultimate fastener bending angle. This decrease of fastener bending angle resulted in a relatively low stiffness hardening (or secondary stiffness), which is caused by secondary axial forces associated with embedment of steel plates into the wood member. Prediction was verified by the tests of 36 steel-to-timber joints under three different pretension forces and two loading directions relative to the grain. Some of the observed load-carrying capacities of the joints, particularly in loading perpendicular to the grain, however, were not as high as those expected by the numerical analyses considering the given pretension forces.

Bearing properties of Shorea obtusa beneath a laterally loaded bolt

Empirical equations to determine the bearing strength have been proposed by many researchers and design standards. Because these equations have been developed mainly based on test results of softwood species, it is a matter of great importance (to ASEAN structural engineers) to verify the applicability of these equations for tropical hardwood species, which are commonly used in many ASEAN countries. In this study, wood specimens of Shorea obtusa (a tropical hardwood species) were used and the bearing test under full-hole confi guration was carried out for fi ve different loading angles to the grain. The bearing stress-embedment curve obtained from the test was approximated by a linear elastic-plastic diagram indicating the initial and fi nal stiffness of the curve. Testing showed that the average bearing strength parallel to the grain was 7.25% lower than the prediction given in Eurocode 5. The bearing strength perpendicular to the grain evaluated based on bearing load at initial cracking was substantially different from any predictions given by previous studies or design standards. It was also found that the bearing strength and initial stiffness from the bearing stress-embedment curve for loading at intermediate angles to the grain could be satisfactorily predicted with Hankinson’s formula.

Effects of pretension in bolts on hysteretic responses of moment-carrying timber joints

The adoption of a concept similar to the prestressing technique used in laminated wood decks of bridge structures might increase the initial stiffness or ultimate resistance of dowel-type timber joints by applying pretension to their bolts. This study investigated the effect of pretension in bolts on hysteretic responses and ultimate properties of moment-carrying timber joints with steel side plates. A pretension of 20 kN that yielded a prestress level of 1600 kPa or about 90% of the allowable long-term end-bearing strength of spruce species was applied to the bolts of prestressed joints. The superiority of the prestressed joint over the non-pre-stressed joint was proved by very high hysteretic damping, equivalent viscous damping ratio, and cyclic stiffness. At any given rotation level, hysteretic damping reduction and moment resistance decrement due to continuously reversed loads were found to be small because bolt pretensioning minimized the pinching effect. This study showed that the hysteresis loop of the prestressed joint can be obtained by adding the frictional hysteresis loop due to pretension force into the hysteresis loop of the non-pre-stressed joint. Despite a great increase of initial stiffness, only slight increments in ductility coefficient and ultimate moment resistance were found in the prestressed joint.

Mechanical Effect of Acetic Acid Lignin Adsorption on Honeycomb-Patterned Cellulosic Films

Abstract We have already fabricated honeycomb-patterned cellulosic films with cellulose I and II polymorphisms as a basal framework in order to create an artificial woody cell wall.[ 1 , 2 ] The adsorption of an isolated lignin, acetic acid lignin (AL), was attempted onto the honeycomb films not only to develop materials further mimicking the cell wall but also to elucidate the mechanical effect of isolated lignin on the tensile strength of the cellulosic architecture. The tensile strengths of honeycomb-patterned cellulosic films were improved by the AL adsorption. Although the cellulosic films without lignin weakened under high moisture content conditions as compared with those under the low content conditions, the lignin-adsorbed cellulosic film maintained significant tensile strength even under the high content conditions. This result suggests that lignin contributes to reinforce the mechanical strength of cellulose framework, in particular high moisture conditions.

Preparation of board-like moldings from composites of isolated lignins and waste paper II : effect of inorganic salt addition on board performance and evaluation of practical use of MDF

Board-shaped composites with medium density (MDF) were prepared from isolated lignins and waste newspaper, in addition to preparation of the composites with high density (HB). The board properties of both composites concerning bending strength and water resistance were improved by the addition of hardwood acetic acid lignin (HAL). The internal bond strength and water resistance of MDF, in particular the degree of thickness swelling (TS), were also improved by prolonged molding time. Adding inorganic salts contributed to the improvement of TS. The effect depended on the charge of the cation. Considering practical utilization of lignin-based MDF as a structural material, its performance was evaluated by combination of the single-shear test of nailed joints and the modulus of rigidity. As a result, this MDF had sufficient strength to be utilized as an internal shear wall material. Therefore, lignin can be considered as an alternative to conventional adhesives for the production of boards such as HB and MDF.

Evaluation of the section modulus for tree-stem cross sections of irregular shape

The effect of section modulus, as estimated for tree stems of irregular cross section with hollow trunks, on windthrow resistance is discussed. The sample trees were 12 aged poplar trees growing along the roadside in Sapporo. Binarized bitmap images of photographs of the crosscut surface of the sample tree stems were used to calculate numerical solutions for section moduli. The error for image resolution was simulated to less than 1% under the condition in which the image was divided into more than 400 pixels. The coefficient of variation in section modulus concerning six neutral axis directions in a horizontal plane was 11%–14%. The reduction in section modulus caused by decay or hollow trunk was 36%–56% of the full cross section. The effect of section modulus on critical wind velocity was found to be considerable. The critical wind velocity calculated for some of the sample trees was less than 40m/s, and these trees were considered to be in danger of bending failure of the stems.

One-year stress relaxation of timber joints assembled with pretensioned bolts

In our previous study, great increases of hysteretic damping and initial slip resistance of timber joints were attained by applying axial pretension to the steel fasteners. To evaluate the effectiveness of this method, 1-year stress-relaxation measurement was carried out. Nine prestressed joints were prepared and three of them were restressed after 3 and then 6 months after the initial prestressing. All joints were exposed to indoor conditions, and relaxation of the pretension was regularly measured from time-dependent decreases of axial strain of the bolts. After measurement, the joints were subjected to cyclic and monotonic loading tests until failure. The average ratio of residual stress to the initial prestress after 1 year was about 0.23 and 0.66, respectively, for joints without restressing and those with restressing. A simulated stress-relaxation curve developed from the four-element relaxation model predicted 3% of the initial stress after 5 years. Without a regular restressing program, the initial prestressing effect therefore must be considered negligible. However, about 20% of the pre-stress level can be reasonably assumed if restressing is carried out annually. This small residual stress was found to introduce suffi cient frictional damping to signifi cantly increase the equivalent viscous damping ratio of the joints.

Lateral resistance of anchor-bolt joints between timber sills and foundations : II. Effective lateral resistance of multiple anchor-bolt joints

Monte Carlo simulations were conducted to estimate the effective lateral resistance of multiple anchor-bolt joints with ordinary specifications of Japanese post and beam constructions. Basic lateral load-slip curves of single anchor-bolt joints required in the simulations were determined from the test results of our earlier report. The effective lateral resistance of multiple anchor-bolt joints was estimated for some combinations of loading direction, length/diameter ratio of anchor bolts, lead-hole clearance, and number of anchor bolts. The principal results of the simulations are: (1) anchor-bolt joints loaded perpendicular to lateral forces are not recommended to be counted as supplementary resisting elements because their supplementary shares are far less than those expected from their allowable lateral resistance; (2) multiple anchor-bolt joints with small length/diameter ratios have comparatively lower effective resistance ratios than multiple anchor-bolt joints with large length/diameter ratios; (3) the effective resistance of multiple anchor-bolt joints is affected not only by lead-hole clearance or number of bolts but also variance of load-slip characteristics of single anchor-bolt joints.

Lateral resistance of anchor-bolt joints between timber sills and foundations III: numerical simulations of the effect of sill thickness on the effective lateral resistance of multiple anchor-bolt joints

Effective lateral resistance of multiple anchorbolt joints was estimated by considering sill thickness or length/diameter ratios of anchor bolts. Load-slip relationships of single anchor bolt joints were analyzed by the stepwise linear approximation based on the generalized theory of a beam on an elastic foundation and the criterion of “fracture bearing displacement” for several sill thicknesses or length/diameter ratios of anchor bolts. Monte Carlo simulations of the effective lateral resistance of multiple anchor-bolt joints were conducted using the analyzed load-slip curves of single anchor-bolt joints. Effective resistance ratios of multiple anchor-bolt joints were simulated for some combinations of length/diameter ratios of anchor bolts, lead-hole clearances, and number of anchor bolts. The simulated results are: (1) the influence of lead-hole clearance becomes more apparent as length/diameter ratios of single anchor-bolt joints decrease; (2) there is no obvious effect of number of anchor-bolts over the range of 5 to 15; (3) average effective resistance ratios can be adopted for allowable stress design; and (4) reduction of effective resistance ratios should be considered particularly for small length/diameter ratios of anchor-bolt joints.