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Comparison of Displacement Measurements and Simulation on Fillet Weld of Steel Column Base

Jun 25, 2021 Excavator blog

Abstract: The Hyogoken Nanbu earthquake (Kobe earthquake) that occurred on January 17. 1995, caused extensive and

severe damages to a large number of buildings in Kobe city area. After the earthquake many steel structures were constructed

using frame welded joint of welded construction and welded base. However, the capacity of these weld joints to absorb energy

during earthquakes is small. For that reason, it is believed that in the design of steel structures that use welded joints, strong

earthquake resistant characteristics must be provided in special for those joints of the steel welded bases. Moreover, these weld

joints have little capacity to absorb energy during earthquakes. Therefore, for designing steel structures incorporating welded

joints, strong earthquake-resistance characteristics must be specially provided for those joints of steel welded bases.

Furthermore, structural monitoring will be necessary. Using simple dynamic measurements and simulations, this report

evaluates the resistance and displacement characteristics of fillet welded construction by piezoelectric joint sensors.

Keywords: Anchor Bolt, Deformed Bar, Health Monitoring, Piezo Electric Limit Sensor, Steel Weld Joint

1. Introduction

Japan’s social capital stock was accumulated and

concentrated during its era of high economic growth. Its

future deterioration is a mounting concern. Over the next 20

years, facilities 50 years old or older will become

increasingly common. Therefore, the urgent need exists to

maintain and renew such aging infrastructure. Unfortunately

many steel structures were constructed using frame-welded

joints of fillet welded construction and a welded base [1].

Many steel-framed buildings in Japan use welding or bolting

as a joining method. For bolt fastening, when a dynamic

external force such as an impact, vibration or thermal load

(expansion) affects the bolted joint, the bolt often loses its

fastening force because of nut loosening. By contrast, few

accidents occur with welded joints [2].

However, because of heat effects during welding,

brittleness develops around the joint as it hardens. In fact, a

relation exists between heat treatment of quenching and

annealing of the steel material [3]. Therefore, achieving

structural soundness might be difficult. Even if one strives to

analyze the results of measurements at the initial stage of

joining and the results of aging over 10 years using finite

element method (FEM), one cannot assess crack growth or

perform defect location realistically. The problem is regarded

as extremely difficult.

In Japan, which has experienced the extremely powerful

Great East Japan Earthquake, architectural design standards

are necessary to prevent buildings from collapsing when

absorbing seismic energy capable of plasticizing an entire

building when a huge earthquake with seismic intensity of 6

or greater occurs. However, at present, no report describes

monitoring of structural integrity by long-term precise

measurements concentrated only on the joint part [4]. This

is true also for other economically developed countries that

have experienced strong earthquakes. For this study, we 112 Nobuhiro Shimoi et al.: Comparison of Displacement Measurements and Simulation on Fillet Weld of Steel Column Base

constructed a monitoring system able to measure structural

soundness “easily,” “inexpensively,” and “over a

long-term” through autonomous damage inspection of

welded joints of steel structures, and by using sensor output.

We investigated the design and measurement technology of

a piezoelectric joint sensor that enables displacement

prediction [5].

2. Destructive Testing of Fillet Welds

2.1. Comparison with Conventional Technology

Various methods are used as measurement technologies

for quantitative evaluation of soundness for disaster

prevention and reduction of structures. Assuming a sensor

system used for displacement and vibration measurements

with static loading, displacement is measured using a laser

displacement meter or a contact displacement meter; natural

vibrations are always measured using a fine vibration meter.

A method exists of identifying the location of fracture and

stress concentration using FEM analysis [6, 7]. Moreover,

X-ray analysis using FEM is useful for nondestructively and

quantitatively evaluating the residual stress of structures.

Nevertheless, it is difficult to analyze crack growth using

this method. Among these methods, for microwave tremor

measurement, the natural period of the structure is obtained

using the Fourier spectrum ratio of the vertical component

and the horizontal component. The amplification

characteristics and natural period are obtained by finding

the H/V spectrum ratio and by normalizing the horizontal

vibration to vertical vibration. The measurement system

comprises a microwave tremor generator, a data logger, and

a PC. It costs about 1.5–2.5 million yen per measurement

unit. In the method using a laser Doppler velocity meter

(LDV), the laser light is irradiated onto the measurement

target. The speed is detected from the phase difference

between the irradiation light and the reflected light because

of the Doppler effects. This measuring system consists of

two LDV devices, a data logger, a PC, and a digital

displacement meter. The cost per measuring unit is about

45–60 million yen. The X-ray non-destructive device can be

installed for monitoring limited places, but it is not practical

for long-term measurement because it requires a power

source. Also, the equipment cost is about 8–10 million yen.

Long-term monitoring of more than 20 years is necessary to

achieve safety and soundness of joints of structures.

However, no measurement device currently guarantees the

required monitoring period or a method or sensor system

related to smart sensing that enables danger prediction

[8-10].

2.2. Overview of Installation Test

Figure 1 presents the test specimen shape and dimensions.

This test piece is intended for exposed column bases of a

low-rise steel frame. Plate 9 mm thick base is welded to a 100

× 100 × 6 mm square steel pipe. It is fixed to the pedestal

using 12 M27 anchor bolts. The joint between the base plate

and the square tubular column is fixed by melting with a

three-layer fillet weld.

Table 1. Load pattern characteristics.

Load

Maximum displacement (mm)

Drift angle (rad)

Load direction

Load 1

5

1/100

+ -

Load 2

10

1/50

+ -

Load 3

15

1/25

+ -

Figure 1. Test specimen layout.

Figure 2. Load test devices. International Journal of Mechanical Engineering and Applications 2020; 8(5): 111-117

113

2.3. Test Method

Figure 2 presents the measurement apparatus of (1) the

loading device, (2) the displacement meter, and (3) the piezo

electric joint sensor.

Figure 3 presents details of the piezoelectric joint sensor

shape and dimensions. The piezoelectric joint sensor base

plate is a 40 × 190 mm × 2 mm general rolled steel plate after

drilling two 12.3 mm drill holes and an 8 mm hole for cable

ducts, and after bending of both ends of about 40 mm at 135

deg. This angle is designed so that the piezoelectric joint

sensor (Piezoelectric Film: DT-2-028 K/L [11]) can be

mounted at a 45-degree angle to the weld surface when a

square steel tubular column is welded in a T-shape. The sensor

output has a structure in which maximum voltage of about 1 V

is generated depending on the weld joint breakage state [12].

Figure 4 shows the anchor plate portion of the test specimen as

fixed to the base using high tension bolts. A 500kN hydraulic jack

was connected to the load section provided on the top of the test

specimen. Then horizontal force was simulated during an

earthquake. The horizontal force is based on the top displacement.

The angle relation between the force and the inclination is

presented in Table 1 [13]. Load 1 shown in the table is the limit of

the safety standard in the Building Standards Law. Load 2 is the

positive load corresponding to the deformation limit value during

a strong earthquake. At applied force 3, deformation

(displacement amount) of three times the applied force 1 is used,

but the displacement value is equivalent to the numerical

calculated value indicating complete failure of the test piece.

Figure 3. Piezoelectric limit sensor characteristics.

3. Welding Joint Relation Between

Displacement Measurement and

Sensor Output

3.1. Relation Between Welding Force Applied to the Welded

Joint and Piezoelectric Joint Sensor Output

Figure 5 (a) portrays the + direction force and the output

results for the piezoelectric joint sensor on the sensor B side.

Measurement results indicate that the applied force became

about 12kN when about 11 min and 45 s had passed. High

output of about +530 mV and-70 mV was recorded from the

sensor. Furthermore, measurement became difficult after

output of about + 300 mV and-50 mV from the sensor when

the applied force was about 13kN. The force was stopped after

about 2 min to prevent danger. In all cases, the sensor output

was shown clearly in front of the complete fracture region and

near the limit region.

Figure 5 (b) presents a positive direction force and

output results for the piezoelectric joint sensor on the

sensor A installation side. Measurement results show that

the applied force became about 12kN when about 11 min

and 45 s had passed on the time axis. Output of about +52

mV and-120 mV from the sensor were recorded.

Measurement became difficult after recording +20 mV

output from the sensor when the applied force was about

13kN. Furthermore, after recording +20 mV output from

the sensor when the applied force was about 13kN, it

became difficult to measure. The sensor output on side A

was confirmed immediately before the complete

destruction region, but the sensor output on the B side was

small: the value is about one-fifth of that of that A side. In

addition, the output judgment near the limit area showed

that the value was small and difficult to judge.

Figure 4. Setting of the piezoelectric limit sensor.

Figure 5 (c) presents the relation between the negative force

and the output of the piezo limit sensor on the Sensor A

installation side. About 34 min and 20 s after the sensor, when

the applied force was in the negative direction and the applied

force was about 10kN, outputs of +580 mV and-100 mV were

obtained from the sensor. Furthermore, after about 38 min and

30 s, outputs of +100 mV and-150 mV were recorded at 13kN

when the applied force was 13kN. The sensor response was

lost. After 2 min had elapsed, the applied force level.

On the minus side (pulling force) compared to the + side

(pulling force), output was recognized earlier in the complete

destruction area.

Figure 5 (d) presents the negative force and the output 114 Nobuhiro Shimoi et al.: Comparison of Displacement Measurements and Simulation on Fillet Weld of Steel Column Base

results for the piezo limit sensor on the Sensor B

installation side. At approximately 34 min and 20 s, the

output from the sensor in the negative direction was about

100 mV at about 10kN. At about 38 min and 30 s, the output

was about 80 mV at 13kN when the force was about 13kN.

Later, the loading was stopped for safety. Compared to the

positive side compression, the negative side tensile force

applied was 10kN for the first recording and 13kN for the

second, similarly to that for Sensor a, with a low level

output early in the complete destruction area. Similarly,

force was stopped to prevent danger. The sensor A

installation side output was confirmed immediately before

complete destruction of the region, but the sensor B

installation side output was small. The value is about

one-fifth of that of that sensor A side. The output judgment

near the limit area demonstrated that the value was small

and difficult to judge.

Figure 5. Relation between piezoelectric limit sensor output and loading.

3.2. Relation Between Sensor Joint Displacement and

Sensor Output

A maximum force of 15kN was applied in the + direction

for about 20 min. Then force was also applied to the-side

under the same conditions. The relation between each

displacement and the sensor output was measured.

Figure 6 (a) shows the displacement by the + direction

force and the output result of the B side piezo electric joint

sensor. When the displacement became about 8 mm after

about 11 minutes and 45 s, the sensor output showed high

outputs of about +530 mV and-70 mV. Furthermore, for

displacement of about 10 mm, the sensor output was about

+300 mV and the output was about-50 mV. The sensor

response ceased after recording. Therefore, the loading was

stopped after 2 min for safety. In each case, results show that

the sensor output increased immediately before the complete

destruction area and near the limit area.

Figure 6 (b) presents displacement attributable to the

positive force and the output result of the piezo electric joint

sensor. Based on the measurement results, the applied

displacement 8mm at about 11 min and 45 s. From A side

sensor outputs of about +52 mV and-120 mV were recorded.

In addition, at approximately displacement 10mm, the

sensor output was about +20mV. Just stopped for safety after

measuring from output the sensor. The sensor output can be

confirmed immediately before the complete destruction area,

but the sensor A value is about one-fifth less than the output

of B. In addition, the output judgment near the limit area was

small and difficult to judge.

Figure 4 (c) shows the displacement applied in

the-direction and the output result of the A side piezoelectric International Journal of Mechanical Engineering and Applications 2020; 8(5): 111-117

115

limit sensor. In addition, about 34 minutes and 20 seconds,

when the displacement became 8 mm the sensor output was

+580 mV and-100mV were recoded.When the displacement

become about-10mm after 4 minutes and 50s, sensor output

showed level of ±100 mV, After that, no output was recorded

and the application was stopped after 2 minutes to prevent

danger. Destruction progresses in about 2 times longer than

the force in the + direction.

Figure 6 (d) presents the output result for sensor B side in

the-direction displacement.

When the displacement became about-5 mm after about 34

minutes and 45 s, the piezo electric sensor output showed low

level outputs of about +100 mV and-20 mV.

Furthermore, for displacement of about 10 mm, the sensor

output showed +20mV and-70mV were recoded.

3.3. Load and Displacement Measurement Results

Compared to Simulation Results at Welded Joints

Using analysis software (FORUM8’s subscription Ver.7

original specification), we analyzed the relation between

force and displacement with spring coefficient added to a

simplified welded structure model [7, 14, 15].

Figure 7 portrays the test specimen. According to the

analysis using a simplified model of the welded structure, the

displacement in the critical area before fracture was 8 mm.

The displacement at complete fracture was 10 mm.

Figure 6. Relation between piezoelectric limit sensor output and displacement.

Figure 7. Three-dimensional finite element analysis of the load and displacement of the specimen layout. 116 Nobuhiro Shimoi et al.: Comparison of Displacement Measurements and Simulation on Fillet Weld of Steel Column Base

Simulation results show that the maximum displacements

of 8 mm and 10 mm respectively occurred with 12kN and

15kN. The output of this sensor also indicates a maximum

value of about 0.6 V. These results suggest that the piezo

joint sensor reliability is high.

Figure 8 presents results of a comparison between the

measured value and the analytically obtained value. The output

result of the piezo limit sensor is displayed on the loop curve

of the load and displacement relation obtained by numerical

analysis. Values obtained through numerical analyses are

shown as the broken line. The results of actual measurement

are shown as the solid line. By comparison and verification,

displacement proportional to the magnitude of the load was

recognized. The piezo limit sensor output was also measured at

the maximum displacement value, which was almost equal to

the applied force. This result proves that it shows the same

characteristics as the measurement results of the test specimen.

Furthermore, regarding the relation between the applied force

and displacement, results show large deformation on the

tensile side, but small deformation on the compression side.

Reliability was also demonstrated: similar results were

obtained from numerical analysis of the welded structure.

Figure 8. Relation between the load and displacement loop by simulation and

sensor output point. As a feature, it has been found that the tensile side is

deformed greatly; the compression side is reduced.

4. Conclusion

Considering results obtained for the applied force and

sensor output 3, the figure and displacement and sensor output

Figure 4 by the mounting test, based on the maximum output

result of the sensor, the displacement was measured from 8

mm to 10 mm, respectively, when the applied force was 12 kN

to 15 kN. The sensor effectiveness is shown also because the

analysis result for displacement and the actual measured

displacement are both 8–10 mm and because the piezo limit

sensor measures the maximum output at the same point.

Furthermore, regarding the relation between applied force and

displacement, results showed that deformation on the tensile

side increased and deformation on the compression side

decreased rapidly. We obtained important reference values for

future numerical analysis of welded structures. Moreover, we

were able to obtain data that are expected to be helpful for

structural design using welding. The piezoelectric joint sensor

measurements used for this test had the characteristic of

showing a critical value in the welded structure from the

output results obtained under each condition, before the

critical region where structural soundness is maintained. For

this reason, a good possibility exists of conducting

measurements for structural risk prediction. The possibility

exists of long-term risk prediction measurement at joints of

welded structures, where real-time monitoring of structural

integrity is difficult using conventional methods. The wider

use of this method is expected to contribute to construction

and maintenance of a safe and secure society.

Acknowledgements

This research was partially supported by JSPS

KAKENHI Grant No. 20H00290, for which we express our

appreciation.