Development of Wooden Portal Frame Structures with Improved ...

Development of Wooden Portal Frame Structures with Improved Columns by Dr. Masahiro Noguchi Post Doctoral Fellow Tokyo Institute of Technology, Yokohama, Kanagawa, Japan Prof. dr. Kohei Komatsu Professor Research Institute for Sustainable Humanoshere, Kyoto University, Uji, Kyoto, Japan

Summury This paper proposes two semi-rigid timber frames with a more effective structural performance are compared and a structural design method is derived. Usually the joints are located at the intersection of the beam and the column. The first frame type applies two types of joints. A high strength capacity rigid glued joint is used to replace the traditional beam-to-column joint while a second ductile semi-rigid joint is positioned at the area with low bending moments.. The beam pieces that run between the column and the semi-rigd joints are so well fixed to the column that they form one integral part. In the second frame type the horizontal beam between the columns is extended beyond the location of the semi-rigid joints of the first frame. This creates a large overlapping area where mechanical fasteners such as bolts are generously spaced. Due to the large fastener spacing the stiffness is enhanced as well as the strength.

Introduction In Japan, the lifetime cycles of most housing are around 20-30 years. This might be considered as wastes of resources and energies from the global environment perspective. Most governing factor is durability due to old fashioned use of the house. As a solution of this problem, it is thought to build houses with skeleton structures which allow free partition of spaces by future owners. To develop the skeleton structure effectively, multi-story frame with span of 6 to 10m are required. For this reason, we pay our attentions on the multi-story wooden portal frame structures. The approaches of many researches in the past 1-8 were that the structural performances were improved by improving only the moment transmitting connections. However, it can be thought that more parameter influences: as example, location of the moment connection, member, and so on. In this article, two types of wooden portal frame structures were proposed. Both having vertical columns added a short horizontal member with glue joints as shown the shadow area in Fig. 1 (a) and (b). These vertical members were defined as “improved columns”. The aim of this article was to show the structural advantages of this type of improved columns.

Material and methods Concept First type is the structures changed the location of moment transmitting ductile connection with improved columns (Type E), in Fig. 1 (a). There is no ideal rigid beam to column joint having stronger than member, regarded as rigid, ductile. Thus, in order to improving the structural performance of semi-rigid structure, it could be thought better to make the position move to where bending moment is small. Second type is the structure whose panel zones were extended with the improved column (Type S), in Fig 1 (b). The panel zone, in this article, was defined as the overlapping area where column and beam met. The structural performances of moment resisting joints were always limited in the height of both members geometrically. If the height at joint part is increased, the larger moment resisting joint can be made. In bolted cross lapped joint, mechanical properties were governed by the mechanical property of single bolted joint and the adjacent bolt space. This result in that beam to column moment resisting joint having higher capacity on stiffness and strength could be expected, extending the panel zone. Finally, the portal frame with traditional bolted cross lapped joints is shown in Fig. 1 (c) as the control type (Type C).

Test materials Nine portal frame specimens were built, three types × three replications. Each column member was 3000 × 200 × 120 mm, pairs of beam members were 3000 × 200 × 60 mm. All specimens were made of Sugi (Japanese Cedar, Cryptomeria japonica) glulam having JAS (Japanese Agricultural Standard) strength grade of E65 – f 220 (MOE = 6500 MPa and MOR= 22 MPa). The average moisture content was 11 %. All specimens were two story miniature semi-rigid frame structures. Each leg joint was shown in Fig. 2 (a).

(a)

Type E

(b) Type S Fig 1

Specimen

(c) Type C

(a) Leg joint

(b) Knee joint of Type C Fig 2

(c)

Knee joint of Type S

Joint detail

Preparation of the improved columns Two rectangle holes were made in each column and drilled eight circular holes as shown in Fig. 3 (a). Each rectangle hole was cross-section of 200 ×30 mm, depth of 160 mm. Each circular hole have diameter of 18 mm, length of 100 mm. Tenon member was also made, as shown in Fig. 3 (b). Central tenon was the width of 29.5 mm, the depth of 200 mm, and the tenon length of 155 mm. Similar to the mortise, each tenon have eight circular hole having diameter of 18 mm, length of 100 mm were drilled in longitudinal direction. The tenon and the slender steel rods of diameter 16 mm were driven into the rectangle holes and circular holes respectively, and they were fixed with epoxy resin adhesive using sledgehammer. They finally they were formed a F-shaped assemblage were completed. We confirmed the adhesive injection by observing overflow of adhesive from holes. The insert length of the steel rod in each member was set to 100 mm. The time to cure was set at least two weeks.

Fig 3

(a) Mortise (b) Tenon Construction of T-shaped member for the improved column

Assemble of portal frame specimens Three types of portal frame specimens were assembled with improved columns, pairs of beams and short bases using bolts as shown in Fig. 1. The clearance between bolts and pre-drilled holes were 1.5 mm, hole diameter was 12 mm and bolt diameter was 10.5 mm. Figures. 2 (b) and (c) show the bolt arrangement. Bolt arrangement of Type E was geometrically the same as that of Type C. Measurements and test procedure The portal frame specimens were subjected to cyclic loading by applying a horizontal lateral

force at the top of the specimens, as illustrated in Fig. 1, (c). Cyclic loading tests were carried out based on the protocol shown in Table 1. Story drift θdrift was calculated by the Eq. (1).

θ drift =

δa − δb h

･･･ (1)

where δa : Displacement at roof beam (mm) δb : Displacement at column base (mm), h : Distance between device for δa and that forδb. Table 1 Load protocol Cycle no. 1 2 3 4 5 6 7 Last

Displacement Angle R (rad) ±1/240 ±1/170 ±1/120 ±1/85 ±1/60 ±1/42.5 ±1/30 Until failure

The number of cycles 3 3 3 3 3 3 3

Results and Discussions Failure mode In Type C, failure did not occurred up to the end of the stroke length of hydraulic actuater. Bolted moment transmitting joints were yielded and then worked as the plastic hinges, which made collapse mechanism of the structures. Similarly, Type E specimens gave no failure up to the end of stroke. However collapse mechanism was different from Type C. Fig. 4 shows the typical failure mode of Type E specimens. As can be seen in Fig. 4, the failure was occurred not at the regions where vertical and horizontal member met, but the regions where the bolted moment transmitting joints were located. Keep this failure mode in mind to the latter discussions. In Type S, the sprit failure occurred at the outer bolt hole in moment joint as illustrated in Fig. 2 (c). But final fatal reduction of load was due to the sprit of bolt hole in timber beam (see Fig. 2 (c)).

Fig. 4

Failure mode of Type E

Shear force - story drift curve Typical shear force - story drift curves for the three different types of portal frames specimens are shown in Fig. 5. From Fig. 5, it is obvious that type E and S specimens have remarkable advantages on structural performance to the control type, especially stiffness. Therefore, the portal frames proposed in this article obviously have high possibility for rational wooden portal frame structures. In latter chapters, the detail features were discussed.

30

Type S Shear force (kN)

20

Type E 10

Type C 0 -0.05

0

0.05

0.1

-10 -20 Story drift (rad) Fig 5

Shear force-story drift relationship

0.15

Stiffness Table 2 shows the test results with respect to the initial stiffness determined by both visual readings (visual method) and method proposed by Japan Housing and Wood Technology Center (HOWTEC) 9 for the three different types of portal frames. The main difference between visual and HOWTEC methods are whether initial slip value was contain or not. The stiffness determined by visual method did not contain initial slip, while those by HOWTEC method contain initial slip. As can be seen in Table 2, the stiffness of Type E and S are around 1.7 and 3.5 times as large as that of Type C, respectively. The differences on stiffness between HOWTEC method and visually method were small. As the shear stresses at panel zone are concentrated in moment transmitting joints, the deformations of panel zone must occur as long as we use elastic material, not rigid body. The deformations of panel zone make the rotation of joint unavoidable. Therefore, for making rigid joints, the improvements of panel zone are effective solution. As the elasticity of timber is low, to avoid the concentrated shear stress at panel zone with elongating the panel zone using improved column, such as Type S, can be thought as the effective solution in the timber moment transmitting joints.

Off-set value (rad)

0.030 0.020 0.010 0.000 Type E Type C Fig 8

Type S

Definition and values of off-set

Next, we will discuss about the effects of the clearance between bolt and predrilled hole. In practice, the clearances are always required to erect the structures on the sites. However, the clearances tend to bring undesired initial slag in bolted cross lapped joints. Fig. 8 (a) shows the initial slag. Fig. 8 (a) shows definition of the off-set value due to the initial slag. As shown in this Fig., the ratio of the average off-set value of type E and type S to type C were half and quarter, respectively. More information can be obtained from Fig. 8 (b), which shows the dispersions of the off-set values in both Type E and S types were much smaller than that of type C. The reason was discuss as follows separately. In Type E, it was thought that the rotational angle was not same as the story drift angle. While, in the type S, the distance between each bolt and shaft was much lager than that of Type C, Type C of 73.5 mm, Type S of 237 mm. As the initial slip was roughly in inverse proportion to the distance at the same rotational angle of joint, the slip of type S was around one-forth times as large as that of Type C.

Generally, the secondary stresses due to shrinking of the member caused by thermal and moisture changes were, in statically undetermined structures, occurred, i.e.: the portal frame structures using knee joints with adhesive and no clearance moment joints like expanded tube joints, While, in case of the portal frame structure with bolted knee joint having some clearances, i.e.: the structure proposed in this article, it was thought that the secondary stresses can be avoided to release using the clearances. Because there are poor information associated with this problem in timber engineering. Therefore, it is thought that both proposed portal frames have also advantage for secondary stress.

Strength and Ductility Average yield, ultimate and maximum strengths and standard deviations for the three different types of portal frames are shown in Table 3. As can be seen in Table 4, the strength of Type E and S are around 1.25 and 1.45 times as large as that of Type C, respectively. The difference on strength ratio among the types of specimen was small. As both Type E and C made collapse mechanisms with bolted cross lapped joints in the same manner, we tried to explain the difference between the strength of Type C and Type E using classic yield collapse model, as shown in Fig. 9.

(a) Type C

(b) Type E Fig 9

Collapse mechanism

Conclusions In this article, two types of wooden portal frame structures were proposed. Both structures have improved columns. First type was the structures changed the location of moment transmitting ductile connection with improved columns. The second type of structure whose panel zone was extended using improved column. From the test results, the stiffness were improved around 1.7 and 3.5 times as large as that of control, the strength were improved around 1.25 and 1.45 times, respectively. Therefore, the portal frame structures with improved columns have structural advantages, especially stiffness.

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