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Experimental Validation of the Seismic Performance of a
Foldable Steel Unit House
Ryosuke MATSUDA
Candidate for the Degree of Master of Engineering
Supervisor: Dr. Taichiro Okazaki
Division of Architectural and Structural Design

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Overview of Unit House

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Commercial unit-house products “SK 5700” and “SK 7500”
produced by FUJI SASH CO., Ltd. [3], were examined. The
products are 5.5 m × 2.2 m and 7.3 m × 2.2 m, respectively,in floor area and 2.7-m high. Each unit is completed with roof
finish, floor finish, furring columns, and electronic wiring. As
shown in Figure 1, the units consist of two short-side frames,
which are identical between SK 5700 and SK 7500, a roof
frame, and a floor frame. In the longer direction, SK 7500 is
32% longer than SK 5700. The unit is folded fortransportability and unfolded on site. Unfolding is done bypulling the roof frame upward while rotating the short-side

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Unit houses are a modular prefabricated steel building system
which can be constructed and disassembled in a short periodof time, can be reused multiple times, and achieves low-priceconstruction. In Japan, the use of unit houses is limited
primarily to temporary, construction-site offices. However, if
combined with appropriate exterior panels, unit houses can beused for ordinary houses or buildings. The rapid hospitalconstruction in Wuhan City, China was a notable use of unit
houses. Wang et al. [1] examined the cyclic-loading
performance of a unit-house framing system assembled into
two stories. Other than this exception, research on the
structural performance of unit houses has focused on theirbeam-to-column connections and vertical joints between
units. The contribution of non-structural components on theperformance of unit houses is not well understood [2].
Therefore, shake-table tests were conducted on a unit houseproduct manufactured by FUJI SASH CO., Ltd. in December
2019 and November 2020. The shake-table test program
along with key findings are reported herein.

frame about a pivot. The units may be connected horizontallyand vertically as needed. A unit-house building is completed
by installing external wall panels along its perimeter. Various
types of external wall panels are available to accommodate awide range of architectural uses.
The columns are cold-formed hollow structural sections
100×100×4.5 (STKR400); the beams are cold formed fromhot-dip galvanized steel sheets SGH400 of thickness 3.2 or
4.5 mm. Figure 2 shows representative beams and beam-tocolumn connections. The beams adopt an open, nonsymmetrical section to support the external wall panels. Rigid
beam-to-column connections are achieved by CJP welds inthe short-side frames and by slip-critical bolted connections
through 12-mm gusset plates in the long-side frame. For thelatter, three and four M16-S10T high-strength, tensioncontrolled bolts are used, respectively, at the top and bottom$[LV

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2273

A

C

5620

SK 5700

Roof frame

2273

Glazing panel
Floor finish
for piping
Door panel

Non-Structure
7420
Floor frame

SK 7500

Figure 1. Structural frame and compositon of the unit house

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透かしを削除する
connections. The faying surface is blast cleaned and coatedwith a thick-film inorganic zinc-rich paint.
Wall panels are fitted into an upper beam and fixed to alower beam using two M10 bolts. Plastic packing is fitted
between the panels and between the end panel and column toensure airtightness and watertightness. The 10-mm gap
between the end panel and column is exhausted when thestory drift angle reaches 0.0042 rad. The contribution of wall
panels and furring columns to lateral-load resistance isneglected in structural design.

Shake-Table Test Plan
Shake-table tests were conducted at the Large-Scale
Earthquake Simulator of the National Research Institute for
Earth Science and Disaster Resilience (NIED). Three
specimens each comprising 4 units connected vertically andhorizontally were constructed and fixed on the shake-tablethrough a steel foundation system. Specimens 1 and 2 used
SK 5700 units and Specimen 3 used SK 7500 units. Steelplates weighting 1,870 kg total were glued to the floor finishof each second-floor unit. The weight equaled 189 and 135%,
respectively, for SK 5700 and SK 7500, of the live load tocompute design earthquake loads for office use (800 N/m2).
As shown in Figure 3, Specimens 1 and 3 were loaded in
the long-frame direction while Specimen 2 was loaded in the
short-frame direction. The fundamental vibration periods ofSpecimens 1, 2, and 3 were computed as 0.41, 0.35, and 0.48
s, in the respective loading directions. The same mix of wall
panels were installed in both floors. For Specimen 1, the south
and north walls had a window panel and four solid panels; theeast wall had a window panel and two solid panels; the west
wall had a window panel and a door panel. Specimen 2 used
the same combination as Specimen 1. For Specimen 3, thesouth and east walls had eight and four glazing panels,
respectively; the north wall had eight solid panels; the west
wall had two solid panels and a door panel.

Specimen 1

Specimen 2
S

E

5520

5520

W

Excitation

N

Specimen 3
N

W
7320
S

Excitation

E

Figure 3. Specimens placed on shake table

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The NS component of the JMA Kobe record from the1995 Kobe earthquake was repeated nine times, amplified to10, 25, 10, 50, 10, 100, 10, 100, and 10%. For the fundamental
vibration periods of the specimens, the acceleration response
spectrum of the 50% motion was equivalent to the design
spectrum specified for Level 2 Design, which is generally
intended for a return period of 500 years.

Shake-Table Test Results
No structural damage was observed up to the 50% excitation.
Modest damage to the external wall panels was noted inSpecimen 1 such as hole elongation of the surface steel sheetsat the bottom bolts.
After the first 100% excitation, structural damage wasobserved at the first story of Specimens 1 and 2. In Specimen
1, bending deformation was observed in the column baseplates, which was exacerbated by eccentricity between theanchor bolts and column. In Specimen 2, weld cracks wereobserved at the connections between the lower beam andcolumn (Figure 4 (a)). No structural damage was observed inSpecimen 3. Damage to the external wall panels was
extensive in Specimens 1 and 2, but very minor in Specimen
3: In Specimen 1, the damage took form of pull-out failure of
the surface steel sheets at the bottom bolts, and deformationnear the upper corner due to contact with the column (Figure
4 (b)); In Specimen 2, the panel had partly come off near theupper corner and around window openings, more badly on thenorth side where the window was larger (Figure 4 (c)). Afterthe second 100% excitation, the same damage developed
further. During dismantling after the test, wide-spreadoccurrence of local buckling deformation was noticed in theupper beams of the short-side frame (Figure 4 (d)).
Figure 5 plots the relationships between shear force anddrift ratio measured for the first story during the first 100%
excitation. Story shear was computed based on the assumed
mass and measured floor acceleration. Indicated in the figure
are the computed elastic stiffness of the system, story shear
Hy1 and Hy2 at the elastic limit determined by column yielding,
based on nominal yield strength and yield strength reported inmill-test reports, respectively, drift ratio where the column
was expected to contact the wall panels, and state points
before and after excitation. During the first 100% excitation,
Specimens 1 and 3 behaved similarly between the north andsouth frames, while Specimen 2 deformed more in the northframe, which had larger window openings, than the south
frame, which had smaller window openings. All specimens
exhibited large loading and unloading stiffness when the drift
ratio exceeded 0.01 rad. Residual drift ratio did not exceed
0.005 rad. Specimen 3 exhibited, whenever the drift ratiochanged signs from positive to negative or vice versa,
stagnation or decrease in story shear followed by sharp
increase.

(a)
(b)
(c)
Figure 4. Damage observed: (a,b,c) after first 100% excitation; (d) after dismantling(d)Wondershare
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BeforeAfter

200 Story Shear [kN]

200 Story Shear [kN]

100

100

Hy1

Hy2

Hy1

-0.04

-0.02

0.04

-0.04

-0.02

100

M2
M1

0.04

-0.04

-0.02

0.04

S.D.R.
[rad]
-Hy1

-100

Before
After
M3

Hy1

Story Drift Ratio
[rad]
-Hy1

200 Story Shear [kN]

10First slip

-0.03 -0.02

0.02
Rotation [rad]S.D.R.
[rad]
-Hy1

-100

20 Bending Moment[kN·m]

-M1
-M2
-M3

Node

R

-100

-Hy2
-200

-200

North Side of Specimen 1

100

100

Hy1

-0.02

0.04

-0.04

-0.02

M3

0.04

-Hy1

-Hy1

-100

-0.04

-0.02

0.04

S.D.R.
[rad]
-Hy1

-100

M2
M1

100

Hy1
S.D.R.
[rad]

Story Drift Ratio
[rad]

-100

-Hy2
-200

-200

South Side of Specimen 1

20 Bending Moment[kN·m]

200 Story Shear [kN]

Hy2

Hy1
-0.04

North Side of Specimen 3

200 Story Shear [kN]

200 Story Shear [kN]

BeforeAfter-20
50% excitation-200

North Side of Specimen 2-0.03 -0.02

0.02
Rotation [rad]

-M1
-M2
-M3
-20
First 100% excitation-200

South Side of Specimen 2

10South Side of Specimen 3

20 Bending Moment[kN·m]

Figure 5. Story shear versus drift ratio of first floor during first 100% excitation
M3
BeforeAfter

200 Sum of C.S.[kN]

100

100

Shear [kN]

Hy1
-0.04

200 Sum of Column

Hy1
0.04

-0.02

-0.04

-0.02

-Hy1

-100

Hy2

-0.04

-0.02

0.04

S.D.R.
[rad]
-Hy1

-100

S.D.R.
[rad]
-Hy1

-100

-Hy2
-200

North Side of Specimen 1-200

North Side of Specimen 2-200

North Side of Specimen 3

Figure 6. Sum of column shear versus drift ratio of first floor during first 100% excitation

Figure 6 shows the relationship between sum of column
shear and drift ratio measured for the first story during thefirst 100% excitation. Column shear was deduced from strain
gauge measurement and equilibrium. Drift ratio wasrepresented by the north frame. The elastic stiffness was 80%,
40%, and 90% of the computed stiffness for Specimens 1, 2,
and 3, respectively. The reason why Specimen 2 exhibitedmuch smaller stiffness than the computed value is believed tobe due to local buckling in the upper beam. Specimens 1 and3 exhibited apparent elastic-plastic behavior, which, as
discussed later, was due to repeated slipping at the slip-criticalbolted connections. Specimen 2 behaved more linearly. The
difference in shear force between Figure 5 and Figure 6 are
attributed primarily to the resistance of external wall panels.
Panels contributed to lateral resistance to the same extent as
structural frame when deformation exceeded 0.02 rad.
Although not reported here, data suggests that the stagnation
or decrease followed by sharp increase of shear force,
observed in Figure 5, was associated with contact andseparation of the wall panels from the columns and slip at
vertical connections between the upper-story and lower-story
modules.
Figure 7 shows the relationship between bending moment
evaluated at the beam-to-column node and slip rotation angleof a slip-critical bolted connection measured during the 50%

M2
M1

10

-0.03 -0.02

100

Hy1
0.04

Story Drift Ratio[rad]200 Sum of C.S.[kN]

0.02
Rotation [rad]

-M1
-M2
-M3
-20
Second 100% excitation

Figure 7. Bending moment
versus slip rotation
angle of slip-criticalbolted connection

and 100% excitations. The sampled location was at the firststory upper beam in the south-west corner of Specimen 3. The
nominal slip resistances M1 and M2, were computed according
to the instantaneous center-of-rotation method [4], andconventional method. The slip resistance of a single bolt wascalculated as the product of nominal bolt pretension and slip
coefficient 0.45. The nominal column yield moment M3, andthe state points before and after excitation, and at first slip, areindicated in the figure. The first positive and negative slips
occurred during the 50% excitation at 70 to 80% of M1. Many
more positive and negative slips occurred during the 100%
excitation, resulting in gradual strength decrease. A steadystate was reached after 5 total slips, when the slip resistance
had reduced to half of the initial resistance.
Similar behavior was observed at all measured locations,
two and eight, respectively, for Specimens 1 and 3. Based on
integration over the respective hysteresis, for the first 100%
excitation, the energy dissipated by the slip-criticalconnections was 82% and 88%, respectively, of the total
dissipated energy for Specimen 1 and 3.
According to literature, slip-critical bolted connections
with inorganic zinc-rich paint applied to the faying surface
tends to exhibit smaller than nominal slip resistance because
the decrease in bolt pretension tends to be larger compared tocases where such paint is not used [5]. It is also reported [6]透かしを削除する
that the slip resistance of such connections tends to decrease
with each slip until a steady value is reached.
Figure 8 shows transition of maximum story shear andstory drift of first story in each specimen. For the 10% to thefirst 100% excitation, story shear was 20% larger in Specimen
3 than in Specimen 1. This difference corresponded to thedifferent in mass of the second floor. For the second 100%
excitation, the story shear were equal between Specimens 1
and 3, presumably because they reached the strength, equal
between the specimens, determined by the slip resistance ofslip-critical bolted connections. The story drifts were similarin all specimens until the 50% excitation. During the second
100% excitation, all 3 specimens deformed to 0.04 rad.

Beam-to-Column Connections Tests
To further understand the performance limits of the system,
isolated beam-to-column connection specimens were tested.
Four types of L-shape specimens were constructed, eachcorresponding to different locations in product SK 5700.
Figure 9 illustrates a specimen which represented the upper
connection in the long-side frame. Specimens were simply
supported, and were subjected to either monotonic tension,
monotonic compression, or cyclic tension and compression.
Cyclic loading was controlled in reference to load Pd that
equaled the design load effect determined by either
earthquake or wind load.
The specimens exhibited excellent performance in therange of cyclic deformation observed in the aforementioned
shake-table tests, ±0.04 rad. All specimens remained elastic
at ±1.0Pd and sustained load nearly ten times Pd at 0.07 rad.
The slip-critical bolted connections in the longer frame
exhibited repeated slip, as in Figure 7, with gradual reduction
in slip resistance at each slip. The welded upper connection in
the shorter frame twisted under tension, and buckled at theunsupported edge of the section. It was confirmed by analysis
that this section inherently develops large warping torsion due
to its geometry and eccentricity between its shear center andline of load application. This behavior agreed withobservation in the shake-table tests where all upper beams had
Story Shear [kN]100

Story Drift Ratio [rad]Spec. 30.04

Spec. 1

Spec. 3

60

Spec. 2
2

4

Excitaion Num.
681First story shear

2

4

Excitaion Num.
68First story drift ratio

Figure 8. Specimens placed on shake table
ColumnGusset PL

400

720
Longer upper beam350

1Spec. 1

0.02

20

buckled at the same location (see Figure 4 (d)). Portal frame
tests are under preparation to complement the findings.

Conclusions
Shake-table tests were conducted to examine the seismic
performance of two unit-house products furbished withexternal wall panels. Three specimens were constructed, eachcomprising four units joined horizontally and vertically toform a two-story building. Different types of wall panels were
installed along the perimeter. The specimens were subjectedto the JMA Kobe-NS record nine times, with different
amplification factors.
After the 50% excitation, which corresponds to Level-2
seismic design requirement, no structural damage and noresidual drift was observed. The 100% excitation caused amaximum drift ratio of 0.04 rad, but resulted in residual drift
less than 0.005 rad. After the 100% excitation, structural
damage was observed in the specimen shaken in the shortside direction: Weld cracks and fracture in the lower beams
and substantial local buckling deformation in the upper beams.
Deformation in the long-side frame concentrated in theslip-critical bolted connections. The slip resistance decreased
after each slip, until reducing to half of the initial resistance.
The energy dissipated by repeated slip at the slip-criticalbolted connections accounted for more than 80% of the totalenergy dissipated by the structural system.
Non-structural external wall panels contributed 20% to
30% to the lateral stiffness of the structural system in theelastic response region, and nearly doubled the lateralresistance of the structural system for a moment when the drift
ratio exceeded 0.02 rad.
Component tests suggested that the beam-to-column
connections of the unit houses can exhibit excellent
performance to double the cyclic deformation range in the
shake-table tests.

Acknowledgements
The shake-table test was done in collaboration with
Nagoya University, Kindai University, Tohoku University,
FUJI SASH CO. Ltd., Iijima structural design office,
INVISIBLE design, and NIED. The contribution of students
from all participating universities are appreciated.

Spec. 20.03

Hy1

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P

Figure 9. Specimens from longer upper connections

References[1] Xiandun Wang et al. Seismic performance of ATLS modular
house based on joint stiffness analysis. Journal of Constructional
Steel Research. Vol. 183. 2021. doi:10.1016/j.tws.2021.107625[2] En-Feng Deng et al. Seismic performance of mid-to-high rise
modular steel construction -A critical review-. Thin-Walled
Structures. Vol. 155. 2020. doi:10.1016/j.tws.2020.106924[3] FUJI SASH Homepage. Unispace series. <http://www.fujisash.
co.jp/hp/product/unit/unispace/sk/>. last accessed 2020/11/7.
(in Japanese).
[4] AISC. Steel construction manual. 13th edition. 2010.
[5] Hwan-Seon Nah et al. Evaluating long term relaxation of highstrength bolts considering coating on slip faying surface. Steeland Composite Structures. Vol. 16. No. 6. pp. 703-718. 2014.
[6] Yasuhiro Ishihara et al. Characteristic investigation and cyclic
slip experiment of HSFG bolted joints which were damaged byearthquake. Journal of Japan Society of Civil Engineers. No. 745.
I-65. pp. 53-64. 2003. (in Japanese).