Frictional Coefficients between Timber and Other Structural

Materials

Quin-jung MENG, Takuro HIRAI and Akio KOIZUMI

Laboratory of Timber EngineeringHokkaido University, Japan

Background

In most of the design standards of timber constructions,

frictional resistance is not counted as a mechanical

element of structural resistance.

This is because of the conservative considerations:

(1)Reduction of initial friction due to stress relaxation,

(2)Difference in mechanical characteristic between friction and other mechanical elements, and/or

(3)Uncertain effective vertical loads reduced by vertical components of earthquake forces.

However, in wooden light frame constructions;

Earthquake Wind

Floor

Wall

Shear force

Shear forces are transmitted from the bottoms of walls to the floors by both frictional resistance due to vertical loads and lateral resistance of nailed joints.

Similarly, in timber log constructions;

Earthquake Wind

Shear force

Shear forces are transmitted through the interlayers between piled up logs by frictional resistance due to vertical loads and lateral resistance of dowels and/ or notches.

Frictional resistance between main and side members due to secondary axial forces increase the maximum lateral resistance and ductility of nailed/ bolted joints.

Focussing on details of mechanical joints;

Shear force

Secondary axial force

If we consider these actual mechanical behavior linked with the frictional resistance between structural members, it seems more reasonable to count properly the effect of frictional resistance in structural design of timber constructions.

However, we have few information about frictional resistance between structural materials commonly used in timber constructions and their effects on mechanical behavior of timber constructions.

What should we conduct for taking the frictional resistance into

consideration?The first step: Practical evaluation of frictional coefficients

The next step: Analyses of effects of frictional resistance on mechanical behavior of timber constructions

The final step: Proposal of structural design considering the effects of frictional resistance

Target of this study

On the first step, we conducted experimental evaluation of frictional coefficients between timber and several kinds of structural materials commonly used in timber constructions in this study.

Test materialsWe selected:

Structural softwood timber (Mixture of Larix, Abies and Cryptomeria) Softwood plywood Hardwood plywood

Oriented strand board (OSB)Medium density fiberboard (MDF)Volcanic silicate ｓ fiber reinforced multi-layer board (VS) Steel (SS400: Ra 3.6-6.3)

Material Specific gravity

Moisture content

Timber 0.30-0.56 12.0-13.4 %Softwood plywood

0.54-0.59 10.0-10.4 %

Hardwood plywood

0.55-0.62 10.7-11.7 %

OSB 0.69-0.74 8.7-9.5 %

MDF 0.78-0.80 7.5-8.2 %

VS 0.72-0.77

Basic properties of test materials.

Mechanical characteristics of test materials

The test materials are categorized as:

Orthotropic: Timber

Hardwood plywood

Softwood plywood

Oriented strand board (OSB)

Isotropic: Medium density fiberboard

(MDF)

Volcanic silicate ｓ fiber

reinforced multi-layer board (VS)

Steel (SS400: Ra 3.6-6.3)

Combinations of test materials

We tested every combination of slip

directions:

For example, friction between timber and plywood were measured for four combinations of slip directions.

Timber Surface veneer of plywood Parallel to the grain Parallel to the grain

Perpendicular to the grain Parallel to the grain Parallel to the grain Perpendicular to the grain

Perpendicular to the grain Perpendicular to the grain

Measurement of Frictional Coefficients

Dead load

Displacement transducer

Load cell

Hydraulic cylinder

Structural sheet material

Timber

0

20

40

60

80

100

120

0 5 10 15 20Displacement (mm)

Fri

ctio

nal

for

ce (

N)

Determination of Frictional Coefficients

Static frictional coefficient

Dynamic frictional coefficient

Friction between timber and SS400 steel

Test results

Timber: Parallel

0.15

0.20

0.25

0.30

0.35

0.40

0.3 0.4 0.5 0.6

Specific gravity

Fri

ctio

nal c

oeff

icie

nt

0.15

0.20

0.25

0.30

0.35

0.40

0.3 0.4 0.5 0.6

Specific gravity

Fri

ctio

nal

coe

ffic

ien

t

Static

Dynamic

Static

Dynamic

Timber: Perpendicular

Results of frictional tests between timber and

SS400 steel were summarized as:

(1) Frictional coefficients had negative correlations

with specific gravity of timber.

(2) Frictional coefficients perpendicular to the timber

grain were larger than those parallel to the grain.

＞

＜

Friction between timber and VS board

Timber: Parallel Timber: Perpendicular

0.15

0.20

0.25

0.30

0.35

0.40

0.3 0.4 0.5 0.6

Specific gravity

Fri

ctio

nal

coe

ffic

ien

t

0.15

0.20

0.25

0.30

0.35

0.40

0.3 0.4 0.5 0.6

Specific gravity

Fri

ctio

nal

coe

ffic

ien

t

Static

Dynamic

Static

Dynamic

Results of frictional tests between timber and

VS board showed the similar tendencies:

(1) Frictional coefficients had negative correlations

with specific gravity of timber.

(2) Frictional coefficients perpendicular to the timber grain were larger than those parallel to the grain.

＜

＞

Friction between timber and MDF

Timber: Parallel Timber: Perpendicular

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.3 0.4 0.5 0.6Specific gravity

Fri

ctio

nal c

oeff

icie

nt

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.3 0.4 0.5 0.6Specific gravity

Fri

ctio

nal c

oeff

icie

nt Static

Dynamic

Static

Dynamic

Results of frictional tests between timber and

MDF also showed the similar tendencies :

(1) Frictional coefficients had negative correlations

with specific gravity of timber.

(2) Frictional coefficients perpendicular to the timber grain were larger than those parallel to the grain.

＞

＜

Friction between timber and OSB

Timber: ParallelOSB: Parallel

Timber: PerpendicularOSB: Parallel

0.10

0.15

0.20

0.25

0.30

0.35

0.3 0.4 0.5 0.6

Specific gravity

Fri

ctio

nal

coe

ffic

ien

t

0.10

0.15

0.20

0.25

0.30

0.35

0.3 0.4 0.5 0.6Specific gravity

Fri

ctio

nal

coe

ffic

ien

t

Static

Dynamic

Static

Dynamic

Friction between timber and OSB

Timber: ParallelOSB: Perpendicular

Timber: PerpendicularOSB: Perpendicular

0.10

0.15

0.20

0.25

0.30

0.35

0.3 0.4 0.5 0.6

Specific gravity

Fri

ctio

nal

coe

ffic

ien

t

0.10

0.15

0.20

0.25

0.30

0.35

0.3 0.4 0.5 0.6

Specific gravity

Fri

ctio

nal

coe

ffic

ien

t

Static

Dynamic

Static

Dynamic

Results of frictional tests between timber and OSB were summarized as:

(1)Frictional coefficients had negative correlations with specific gravity of timber.

(2) On the other hand, we found little difference among all combinations of slip directions.

＞

～～ ～～ ～～＜

Friction between timber and hardwood plywood

Timber: ParallelPlywood: Parallel

Timber: PerpendicularPlywood: Parallel

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.3 0.4 0.5 0.6

Specific gravity

Fri

ctio

nal

coe

ffic

ien

t

0.10

0.150.20

0.25

0.30

0.350.40

0.45

0.3 0.4 0.5 0.6

Specific gravity

Fri

ctio

nal

coe

ffic

ien

t

Static

Dynamic

Static

Dynamic

Timber: ParallelPlywood: Perpendicular

Timber: PerpendicularPlywood: Perpendicular

Friction between timber and hardwood plywood

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.3 0.4 0.5 0.6Specific gravity

Fri

ctio

nal

coe

ffic

ien

t

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.3 0.4 0.5 0.6Specific gravity

Fri

ctio

nal

coe

ffic

ien

t

Static

Dynamic

Static

Dynamic

Results of frictional tests between timber and hardwood plywood were summarized as:

(1) Frictional coefficients had negative correlations

with specific gravity of timber.

(2) The observed order of frictional coefficients among four combinations of slip directions was:

＞

＜～～＜ ～～

＜

Friction between timber and softwood plywood

Timber: ParallelPlywood: Parallel

Timber: PerpendicularPlywood: Parallel

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.3 0.4 0.5 0.6

Specific gravity

Fri

ctio

nal c

oeff

icie

nt

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.3 0.4 0.5 0.6

Specific gravity

Fri

ctio

nal c

oeff

icie

ntStatic

Dynamic

Static

Dynamic

Timber: ParallelPlywood: Perpendicular

Friction between timber and softwood plywood

Timber: PerpendicularPlywood: Perpendicular

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.3 0.4 0.5 0.6Specific gravity

Fri

ctio

nal

coe

ffic

ien

t

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.3 0.4 0.5 0.6Specific gravity

Fri

ctio

nal

coe

ffic

ien

t

Static

Dynamic

Static

Dynamic

Results of frictional tests between timber and softwood plywood showed the similar tendencies:

(1) Frictional coefficients had negative correlations with specific gravity of timber.

(2) The observed order of frictional coefficients among four combinations of slip directions was:

＞

＜～～＜ ～～

＜

(1) Frictional coefficients had negative correlations with specific gravity of timber for every combination of materials and slip directions.

(2) Frictional coefficients were qualitatively affected by the combination of slip directions. The differences among the combinations of slip directions, however, was quantitatively a little for the friction between timber and plywood and was not clear for the friction between timber and OSB.

Summing-up of test results

Evaluation of frictional coefficients for practical

designArchitectural Institute Japan classifies typical softwood species for structural use into the following three groups:

Specific gravity Group

Minimum Average J 1 0.42 0.47 J 2 0.37 0.42 J 3 0.32 0.37

Timber direction

J1(0.47)

J2(0.42) J3(0.37)

Parallel StaticDynam

ic

0.2560.212

0.2720.223

0.2880.235

Perpendicular StaticDynam

ic

0.2870.225

0.3060.240

0.3250.255

From the negative correlations with specific gravity of timber, we can roughly estimate the frictional coefficients from the average specific gravity of each group of structural softwood timber.

For example, frictional coefficients between timber and SS400 steel are estimated as:

Timber

Plywood

J1(0.47)

J2(0.42)

J3(0.37)

Par. Par. StaticDynam

ic

0.2960.200

0.3150.222

0.3350.243

Par. Per. StaticDynam

ic

0.2930.205

0.3120.230

0.3300.250

Per. Par. StaticDynam

ic

0.3140.231

0.3320.255

0.3500.279

Per. Per. StaticDynam

ic

0.3640.272

0.3750.292

0.3860.312

Frictional coefficients between timber and softwood plywood are estimated for four combinations of slip directions as:

Earthquake force

Floor

Wall

Shear force

An example of structural calculation considering

frictional resistance

The shear force is transmitted from the bottoms of walls to the floor sheathings by friction and lateral resistance of nailed joints.

Consider a wooden light frame construction subjected to an earthquake force.

Here we assume: Shear force beard by the shear walls = base shear factor (α)×mass (m)×g = base shear factor (α)×vertical load (W )

Floor

W

α×W

Wall

Earthquake force

Shear walls that resist the earthquake force

We also assume:

Vertical load distributed to the shear walls that causes frictional force = 0.5×W

Earthquake force

Shear walls parallel to the earthquake force

Shear walls perpendicular to the earthquake force

W = 0.5W + 0.5W

Ve

He

Ve =0.5H

e

There is the risk at the earthquake that the vertical component of the earthquake force may reduce the effective vertical load, which causes frictional force.

Considering this risk, we assume: Ratio of vertical component Ve to horizontal component He of the earthquake force = 0.5

The ratio rn of required lateral resistance of nailed joints to the total shear force results in:

rn = 1-0.5μ(1-0.5α)/α, where μ= frictional coefficient.We temporarily adopt both static and dynamic frictional coefficients because of poor information about detailed dynamic response of timber constructions.

If we use spruce (J3) as the bottom plates of walls and softwood plywood as the floor sheathing panels, we can conservatively estimate:

Static frictional coefficient: 0.3Dynamic frictional coefficient: 0.2

Rati

o o

f re

qu

ired late

ral

resi

stan

ce o

f naile

d

join

ts

Base shear factor

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Dynamic Static

Estimated result

Remained Problems

What characteristics affect the frictional

coefficients?

Surface roughness? Surface hardness?

Is effect of moisture content of timber practically negligible or not?

How to estimate the effective vertical load or stress relaxation?