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Experimental tests and numerical modelling of wall sandwich panels

Fabrizio Gara ⇑, Laura Ragni, Davide Roia, Luigino Dezi

Dept. of Civil and Construction Engineering and Architecture, Università Politecnica delle Marche, via Brecce Bianche, 60131 Ancona, Italy

a r t i c l e i n f o

Article history:

Received 23 July 2011

Revised 15 November 2011

Accepted 2 December 2011Available online 4 February 2012

Keywords:

Sandwich panels

Bearing wall panels

Full scale compression test

Diagonal compression test

Finite element analysis

a b s t r a c t

This paper presents the first part of an experimental investigation carried out on a construction system

based on completed in situ sandwich panels with non-shear connectors, concerning the study of vertical

panels used as structural walls. Compression tests with axial and eccentric loads were carried out on sev-eral full scale panel specimens with different slenderness ratios in order to study the behaviour of panels

under vertical in-plane forces. Additionally, diagonal compression tests were performed on square spec-

imens in different configurations in order to study the behaviour of panels under horizontal in-plane

forces. The most significant load–displacement diagrams for increasing load are illustrated and the failure

modalities are discussed. The semi-composite behaviour of the panels, guaranteed by the internal layer of

polystyrene and the reinforced concrete beams at the panel ends, is highlighted. Finally, some numerical

simulations are performed with non-linear finite element models and some useful design indications are

given.

2011 Elsevier Ltd. All rights reserved.

1. Introduction

Construction systems based on sandwich panels are commonly

used worldwide for intensive building production. Sandwich pan-

els are typically constituted by two concrete layers which are sep-

arated by an internal insulation layer of various materials (i.e.

expanded and extruded polystyrene, rigid polyurethane foam)

and are usually joined with ‘‘shear connectors’’ (i.e. truss connec-

tors) able to transfer the longitudinal interface shear between the

layers so as to ensure a fully-composite or a semi-composite

behaviour of the sandwich panel.

This paper deals with a construction system that utilises

sandwich panels, both for structural walls and floors, which are

obtained by self-supporting reinforced insulation layers completed

in situ with spritz-beton. The prefabricated modular elements are

made of an undulated (corrugated) layer of expanded polystyrene,

with suitable density, reinforced by two metallic meshes con-

nected by means of orthogonal steel wires welded to the meshes(steel connectors). Thanks to the easy and fast mounting proce-

dures, this construction system presents some technical advanta-

ges that make it often competitive in comparison with traditional

methods or precast systems. From a structural point of view these

panels are characterised by orthogonal connectors (‘‘no-shear con-

nectors’’) so that their semi-composite behaviour depends on the

shear stiffness of the expanded polystyrene layer and, above all,

on construction details, such as reinforced concrete regions at

the panel ends.

The structural behaviour of such a sandwich panel can be theo-

retically studied by means of analytical models which are usually

referred to in the literature as models for multilayered beams

[1,2] or two-layers composite beams [3,4] with deformable inter-

layer connection. As this paper deals with experimental investiga-

tion, a literature review of theoretical models is beyond the scope

of this paper and only some of the most recent works are cited as

example from which a comprehensive list of references describing

these models may be founded. However the behaviour of single

sandwich panels is obviously much easier to predict than the

behaviour of panels constituting real building walls. To evaluate

the structural performances of buildings constructed with this kind

of construction system, in addition to specific modelling taking

into account the semi-composite behaviour of the sandwich pan-

els, other aspects need to be considered in the numerical evalua-

tions, like (i) the restrain degree of wall-floor node depending onthe connection details and (ii) the bi-dimensional behaviour of

the panels which are each-other connected in real buildings. For

this purpose, experimental results of full scale tests, are an essen-

tial instrument to calibrate both theoretical methods and numeri-

cal models.

In the technical literature several experimental campaigns on

precast sandwich panels with shear connectors can be found

[5–13]. On the contrary, very few experimental tests have been

performed on sandwich panels with in situ sprayed concrete and

no-shear connectors [14,15] and on 3D full scale mock-up [16] in

order to study the behaviour of the panels in real structures. Con-

sequently, general conclusions on the structural behaviour of this

0141-0296/$ - see front matter 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.engstruct.2011.12.027

⇑ Corresponding author. Tel.: +39 071 2204550; fax: +39 071 2204576.

E-mail addresses: [email protected] (F. Gara), [email protected] (L. Ragni),

[email protected] (D. Roia), [email protected] (L. Dezi).

Engineering Structures 37 (2012) 193–204

Contents lists available at SciVerse ScienceDirect

Engineering Structures

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e n g s t r u c t

http://dx.doi.org/10.1016/j.engstruct.2011.12.027mailto:[email protected]:[email protected]:[email protected]:[email protected]://dx.doi.org/10.1016/j.engstruct.2011.12.027http://www.sciencedirect.com/science/journal/01410296http://www.elsevier.com/locate/engstructhttp://www.elsevier.com/locate/engstructhttp://www.sciencedirect.com/science/journal/01410296http://dx.doi.org/10.1016/j.engstruct.2011.12.027mailto:[email protected]:[email protected]:[email protected]:[email protected]://dx.doi.org/10.1016/j.engstruct.2011.12.027

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construction system cannot be drawn and further experimental

investigations are needed. For this reason, an extensive experimen-

tal campaign, including a large number of tests on floor and wall

panels, cyclic tests on wall-floor connections and a load test on

the floor of a full scale 3D mock-up, has been carried out.

This first paper refers only to tests performed on wall panels. In

particular, the results of compression tests with axial and eccentric

loads carried out on panels with an internal layer of different thick-ness, are presented. After that, the experimental results obtained on

panels with non-undulated polystyrene sheet and on panels with

undulated polystyrene sheet and half the number of connectors

are discussed. Additionally, the results of diagonal compression

tests carried out on square specimens of different configurations

in order to study the behaviour of panels under horizontal in-plane

forces are illustrated: wall standard panels, as well as wall panels

externally prestressed to simulate the effects of vertical loads and

panels stiffened with four orthogonal walls to simulate the behav-

iour of the wall in a real building are considered. For each test,

the load–displacement diagram and the failure modalities are

examined. Finally some numerical simulations, performed with

non-linear finite element models, are also reportedand some useful

design indications are given.

2. Experimental campaign

The sandwich panels considered in this study are made of a

sheet of polystyrene reinforced by two 80 mm 75 mm metallic

meshes assembled by means of steel connectors. The sheet of poly-

styrene has an undulated profile and density of about 15–25 kg/

m3. The galvanised welded wire meshes and the connectors

welded orthogonally to the meshes, are made with U3 wires of

high yield steel. Wall panels (WP) were completed simply by

spraying concrete onto the external surfaces of the sheet, first up

to the metallic mesh and then up to the final thickness of the con-

crete layer, using manual tools or pumps (Fig. 1). A ready-mixed

concrete, with sand no greater than 3 mm and specific additivesto improve adhesion and workability, was used.

2.1. Mechanical properties of materials

In order to evaluate the mechanical properties of the used

materials, several tests were carried out on concrete, metallic

meshes and internal layer consisting of a polystyrene sheet and

metallic connectors.

The concrete was characterised by means of tests on

40 40 160 mm specimens sampled during the cast, and tests

on cored specimens with a diameter of 94 mm and length of

250 mm sampled from the reinforced concrete beams at the ends

of the panel after the concrete curing. In accordance with EN ISO

12504-1 [17], bending tests were first carried out to evaluate the

flexural strength of the rectangular specimens, then the two resul-

tant parts of the specimens were used for compression tests. A to-

tal number of eight specimens were prepared so that eight flexural

tensile tests and sixteen compression tests were performed. Table 1

reports the mean values of the compression ( f cu) and flexural ten-

sile concrete strength ( f cfm). The cored specimens were divided into

two sets, one of which was subjected to the compression test and

the other to the indirect tensile splitting test, in accordance withEN 12390-6 [18]. Four cored specimens were sampled and, conse-

quently, four compression tests and four tensile splitting tests

were performed. The average values of compression strength ( f cu)

and tensile concrete strength ( f ct ) are reported in Table 1. From

these specimens, a mean value of 10500 MPa for the concrete

elastic modulus (E c ) was also estimated.

Tensile tests and weld shear strength tests were carried out on

six samples of metallic meshes following EN ISO 15630-2 [19].

Four samples reached the yield stress showing a very low ductility

(less than 2). The mean values of tensile strength ( f m) and percent-

age elongation at failure ( A gt ) are reported in Table 1.

The other two samples showed a brittle fracture and a strength

value which was about 20% lower than the yielding strength. It is

worth noticing that the failure of all the mesh samples occurredat a welded joint as a consequence of the welding disturbs. Finally,

the weld shear strength tests gave an average shear force of

2.64 kN, which is 1.25 times greater than the wire yielding force,

as prescribed by the code. The mechanical properties of the inter-

nal layer were obtained by means of shear tests according to the

Standard Test Method for Shear Properties of Sandwich Core Mate-

rials [20]. Samples with dimensions of 400 445 mm made of

three concrete layers and two internal layers were tested. In

polystyrene

steel connectors

concrete layerswire meshes

(a) (b) (c)

Fig. 1. ‘‘Concrewall’’ wall sandwich panels: (a) schematic sketch of the components; (b) concrete spraying onto the external surfaces up to the metallic mesh and (c) up to thefinal thickness.

Table 1

Mean values of material properties.

Prismatic specimens

(MPa)

Cored specimens

(MPa)

Metallic meshes

(MPa)

f cu = 21.95 f cu = 25.10 f m = 769.00

f cfm = 5.52 f ct = 2.40 A gt = 7.62

194 F. Gara et al. / Engineering Structures 37 (2012) 193–204

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particular, eight samples were considered with internal layers of

different thickness and with a different number of connectors.

Subsequently, the polystyrene sheets of four samples were dis-

solved in order to obtain samples with metallic connectors only.

Load and unload cycles were performed up to failure (Fig. 2a). Slip-

ping between the concrete layers was measured by means of two

LVDTs and the applied load was determined by means a pressure

transducer. In Fig. 2b load–displacement graphs are illustrated

with reference to samples 1S and 3S, with polystyrene sheet thick-

ness of 80 mm and 160 mm respectively, and sample 5S which issimilar to sample 1S but without the polystyrene sheets. By com-

paring the results of the tests on 1S and 3S samples it is evident

that the stiffness decreases considerably when the thickness of

the internal layer increases, whereas by comparing the results of

1S and 5S samples it can be observed that the contribution of con-

nectors is negligible with respect to the contribution of the

polystyrene.

For each test the initial shear modulus was calculated by means

of

Gi ¼ K i2 Ai

hi ð1Þ

where K i is the initial stiffness of the sample, and hi and Ai the thick-

ness and the area of each internal layer. Table 2 shows all theobtained results.

2.2. Panel geometry

A total of twenty two panels were built: sixteen for compres-

sion tests with axial and eccentric load and six for diagonal com-

pression tests.

The panels for compression tests had a total height of 2940 mm,

a width of 1120 mm and concrete layer thickness of 35 mm. To

avoid stress concentrations and to facilitate the handling

operations two reinforced concrete beams were built at the ends

of the panels by dissolving a portion of the polystyrene sheet

in order to obtain a proper anchorage of the meshes (Fig. 3). Forthe compression tests, wall panels (WP) with three different

(a)

30

5Displacement [mm]

L o a d [ k N ]

30

0

1S (80 mm) 3S (160 mm)

30

5Displacement [mm]

L o a d [ k N ]

30

0

1S (80 mm) 5S (160 mm)

(b)

Fig. 2. Shear tests: (a) test configuration and (b) load–displacement cycles.

Table 2

Shear tests: samples and results.

Sample hi (mm) Polystyrene Connectors K i (kN/mm) Gi (N/mm2)

1S 80 Yes Double 15.70 3.45

2S 120 Yes Double 9.20 3.03

3S 160 Yes Double 7.30 3.20

4S 80 Yes Single 12.80 2.81

5S 80 No Double 0.47 0.10

6S 120 No Double 0.15 0.04

7S 160 No Double 0.07 0.03

8S 80 No Single 0.20 0.04

F. Gara et al. / Engineering Structures 37 (2012) 193–204 195

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thicknesses of the internal layer, 80 mm (WP08), 120 mm (WP12)

and 160 mm (WP16), were built. For each thickness, two panels

were tested under axial load and two under eccentric load. In addi-

tion, two different kinds of wall panel were prepared for the com-

pression tests: the WPN panel with a non-undulated polystyrene

layer and the WPH panel with half the number of connectors

(Fig. 4). In these cases only one compression test with axial load

and one with eccentric load were performed for each kind of panel.

The list of all the panels with the overall thickness ( h), the internal

layer thickness (c ), and the kind of test performed are reported in

Table 3.For the diagonal compression tests, only 1120 mm 1120 mm

WP08 panels were considered since the thickness of the internal

layer does not influence the panel behaviour under in-plane forces.

In addition, prestressed and transversally stiffened panels were

considered in order to simulate the effects of vertical load and

the stiffening contribution of walls and floors orthogonal to the

panels, respectively. To better distribute the compression load,

two triangular reinforced concrete regions at two opposite corners

of the standard wall panels (Fig. 5a) and two reinforced concrete

L-shaped beams in the stiffened panels (Fig. 5b) were built.

In the prestressed panels two steel threaded bars are applied

with prestressing loads of 30 kN and 90 kN. The list of all the

specimens with overall thickness (h), internal layer thickness (c ),

prestressing forces and loading type are reported in Table 4.

2 9 4 0 m m

1120 mm

h

2 7 0 0 m m

A A

c

35 mm

1 2 0

1

2 0

4φ8

φ6/200mm

35

Fig. 3. Panel for compression tests.

8040

Section B-B

WP08

Section A-A

WP08

WPN08

AA

B

B11201245

40

1 5 0

1 5 0

8 0

8 0

2 0

80 15

φ3

1 5 0

2 0

7 5

concrete polystyrene wire meshes φ3

WPH08

1 5 0

Fig. 4. Details of WP, WPN and WPH panels.

Table 3

Geometric characteristics of panels for compression tests.

S pe cimen Panel Compression loa ding c (mm) h (mm)

2a.1 WP08 Axial 80 150

2a.2 WP08 Axial 80 150

3a.1 WP12 Axial 120 190

3a.2 WP12 Axial 120 190

4a.1 WP16 Axial 160 230

4a.2 WP16 Axial 160 230X.2 WPN08 Axial 80 150

Y.2 WPH8 Axial 80 150

2b.1 WP08 Eccentric 80 150






X.1 WPN08 Eccentric 80 150

Y.1 WPH08 Eccentric 80 150


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2.3. Test configuration and instrumentation

For the compression tests the configuration of Fig. 6 was

adopted: panels were placed vertically with the bottom end pinned

(cylindrical pin) and the top end restrained so as to prevent lateral

displacement. This static scheme simulates the restraint condition

of panels in real multi-storey buildings when connections between

floor and wall panels produce negligible bending moments.

In the axial compression tests the load is applied at the panel

axis, while in the eccentric compression test the load is applied

A

A

1 1 2 0 m m

1120 mm 150

Section A-A

A

A

1 1 2 0 m m

1 4 2 0 m m

1120 mm

Section A-A4φ8

φ6/200mm

(a) (b)

Fig. 5. Panels for diagonal compression tests: (a) wall panel and (b) transversally stiffened panel.

Table 4

Geometric characteristics of panels for diagonal compression tests.

Specimen Panel Compression loading Prestressing load (kN) c (mm) h (mm)5.1 WP08 Diagonal – 80 150

5.2 WP08 Diagonal – 80 150

5.3 WP08 Diagonal 30 80 150

5.4 WP08 Diagonal 90 80 150

C.1 WP08a Diagonal – 80 150

C.2 WP08a Diagonal – 80 150

a With traversal stiffening walls.

axial load eccentric load

S4

S5

hydraulic

jacks

cylindrical pin

Sf

reaction frame

S1

S2

S3

Svf Svb

Fig. 6. Compression tests with axial and eccentric load: test configuration and instrumentation.


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at the axis of an external concrete layer. In both cases the load is

applied by means of four hydraulic jacks of 500 kN fixed to a reac-

tion frame (Figs. 6 and 7). The hydraulic jacks are managed by

means of a hydraulic control unit equipped with a pressure trans-

ducer to measure the applied load. A steel plate is placed between

the panel and the actuators in order to distribute the load uni-

formly. Furthermore, metallic profiles are used to confine the rein-

forced concrete beams at the panel ends.

Each panel is instrumented with two LVDTs (Svf and Svb), work-

ing in a range of ±50 mm, applied in an extensometric configura-

tion to measure strain over a base length of 0.9 m. Three other

transducers (S1, S2, S3) are placed horizontally at 1/4, 1/2 and 3/4

of the panel height to measure horizontal displacement. Finally,

two transducers (S4, S5) are placed within the panel thickness, atan angle of 45 with respect to the vertical direction, at 3/4 of

the panel height, in order to measure the relative displacement be-

tween the concrete layers (Fig. 6).

Diagonal compression tests are carried out by means of a slide

pushed by six hydraulic jacks. The panels, rotated 45, are placed

between the slide and the reaction frame (Figs. 8 and 9). To avoid

any stress concentration, metallic L-shaped profiles are used to dis-

tribute the load at the panel corners. Also in this case the applied

load is measured using a pressure transducer. Each panel is instru-

mented with four LVDTs, working in a range of ±50 mm, placed

vertically (Svf and Svb) and horizontally (Shf and Shb) on the front

and back panel surfaces, in an extensometric configuration to mea-

sure strain over a base length of 500 mm.

2.4. Test results

In this section the main results of the tests carried out on the

wall panels are illustrated: first the results of the axial and eccen-

tric compression tests and then the results of the diagonal

compression tests are reported and discussed. As regards the com-

pression tests, Fig. 10 shows the lateral deflections recorded by the

LVDT at mid-height of the panels (S2) on the sixteen panel speci-

mens under axial (continuous lines) and eccentric (dashed lines)

increasing load. The firsts three graphs report the results of four

Fig. 7. Compression tests: overview and details of the top and bottom restraints.

Fig. 8. Diagonal compression tests of panels without and with prestressing load.

Svf

Shf

Svf

Shf

Fig. 9. Diagonal compression test without and with prestressing load: test

configuration and instrumentation.


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compression tests, two with axial and two with eccentric load,

carried out on specimens of the same typology (standard panels)WP08, WP12, and WP16, respectively. The fourth graph shows

the results of tests on panels with non-undulated polystyrene

sheet (WPN08) and half a number of connectors (WPH08), one

under axial load and the other under eccentric load.

The maximum load (Ultimate Load) achieved in each test is

reported in Table 5, together with the mean value of the ultimate

load (Mean U.L.) reached by two specimens of the same typology,

and the mean ultimate uniformly distributed load (Mean U.U.D.L.),

i.e. the ultimate load divided by the panel width. In both the cases,

with axial and eccentric load, the ultimate loads decrease by

increasing the panel slenderness ratios, defined as L/h, where L is

the total height and h the overall thickness of the panel, as shown

in Fig. 11. It is worth noticing that in the case of the axial compres-

sion test the ultimate load of the panels is strongly influenced byany small undesired eccentricity due to imperfections in the spec-

imen and test set-up (load, restraints, etc.). However, due to the

large difference between the values of undesired and imposed

eccentricity, the behaviour of the axially and eccentrically loaded

panels are significantly dissimilar.

In particular, as regards axially loaded specimens, the lateral

deflection of the panel remains generally small under increasing

loading (Figs. 10 and 12a) up to load values close to the ultimate

load. Failure occurs as a result of overall buckling of the specimen,

due to compression, followed by the crushing of the concrete layer

in compression and the rupture of the metallic mesh inside theother concrete layer, subjected to tension (Fig. 13a). As regards

the eccentrically loaded specimens, Fig. 10 shows that the load

vs lateral deflection plots are nearly linear at the earlier stages of

loading; later, after the first crack has appeared, the panels exhibit

a non-linear behaviour. The failure of the panel occurs, in this case,

because of the rupture of the metallic mesh in the concrete layer in

tension. However, a not very ductile behaviour was observed, since

mesh failure occurs at the joints where the effective cross-section

of the metallic wires may be reduced and the steel strength and

ductility are lower due to welding (Fig. 13b and c).

The different behaviour and different failure modes between

wall panels under axial and eccentric loading are also shown in

Fig. 14a which reports the displacement measured by the trans-

ducers Svf and Svb placed in a vertical position on the front and backfaces of specimens 2a.1 and 2b.2. Continuous lines refer to tests

with axial load (2a.1), dashed lines to tests with eccentric load

(2b.2). In the case of axially loaded specimens, the two concrete

layers initially behave in the same way, both characterised by

shortening deformations; only during a second phase does the

behaviour of the two concrete layers become different, with one

concrete layer characterised by shortening deformation and the

other by elongation. In the case of eccentrically loaded specimens

a different behaviour is observed from the beginning of the test.

In fact, with a low load level, the two concrete layers behave

differently, one with shortening deformation and the other with

elongation, and this denotes a predominant flexural behaviour of

the panel.

In Fig. 14b the longitudinal (slip) and transversal (separation)components of the relative displacement between the two

500

1000

2a.12a.2

2b.12b.2

L o a d [ k N ]

0

WP08

500

1000

3a.13a.2

3b.13b.2

L o a d [ k N ]

0

WP12

500

5 10 15 20Lateral deflection [mm]

1000

25

4a.14a.2

4b.14b.2

L o a d [ k N ]

0

WP16

500

5 10 15 20

1000

25

Y.2X.2

Y.1X.1

L o a d [ k N ]

0

Lateral deflection [mm]

WPN08WPH08

Fig. 10. Axial and eccentric compression tests: load-lateral deflection diagrams at mid-height of the panel.

Table 5

Compression tests with axial and eccentric loading: ultimate loads.

Specimen Paneltype

Loading UltimateLoad (kN)

Mean U.L.(kN)

Mean U.U.D.L.(kN/m)

2a.1 WP08 Axial 701 742 662.5

2a.2 WP08 Axial 783

3a.1 WP12 Axial 806 825 736.6

3a.2 WP12 Axial 844

4a.1 WP16 Axial 855 881 786.6

4a.2 WP16 Axial 907

X.2 WPN08 Axial 736 657.1

Y.2 WPH08 Axial 765 683.0

2b.1 WP08 Eccentric 375 388 346.4

2b.2 WP08 Eccentric 401

3b.1 WP12 Eccentric 460 503 448.7


4b.1 WP16 Eccentric 524 577 515.2


X.1 WPN08 Eccentric 461 411.6

Y.1 WPH08 Eccentric 591 527.7

300

650

1000

U l t i m a t e L o a d [ k N ]

Slenderness ratio L / h

1612 14 18 20

axial loadeccentric load

Fig. 11. Influence of panel slenderness ratio on axial and eccentric ultimate loads.


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concrete layers of the above mentioned specimens 2a.1 and 2b.2

are plotted, for increasing axial (continuous line) and eccentric

(dashed line) loadings.

These components are calculated from the vectorial decomposi-

tion of the displacement recorded by transducers S4 and S5. The

longitudinal slip exhibits an initial nearly linear behaviour fol-

lowed by a non-linear behaviour until ultimate values of about

1.5 mm. Compared to the slip, the separation is characterised by

much lower values, which are practically negligible, meaning that

the two concrete layers deflect together.

0 2020

3.0

0


W a l l h e i g h t [ m m ]

150 kN

300 kN

450 kN

600 kN

4a.2

0 2020

3.0

0


W a l l h e i

g h t [ m m ]

150kN300kN450kN

4b.2

600kN

(a) (b)

Fig. 12. Lateral deflection at different load stages: (a) axial load and (b) eccentric load.

Fig. 13. Specimens after failure (a) axially and (b) eccentrically loaded; (c) mesh failure.

400

0-3

Displacement [mm]

L o a d [ k N ]

800

3

2a.12b.2

0

Svf

Svf Svb

Svb400

0-0.5

800

2

2a.12b.2

0Displacement [mm]

L o a d [ k N ]

(a) (b)

Fig. 14. (a) Vertical deformation of the two concrete layers and (b) slip and separation between the concrete layers in axially and eccentrically loaded specimens.


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The high values of the ultimate loads obtained, the influence of

the slenderness ratio on the ultimate loads as well as the low val-

ues of relative displacement between the concrete layers confirm

that these wall panels behave as semi-composite elements. How-

ever, some aspects deserve to be discussed. First of all, it is worth

noticing that the slip between the two concrete layers is restricted

not only by the shear deformable internal layer but also by the

solid reinforced concrete beams at the bottom and top ends of

the panels. Consequently, the results of the tests presented in this

paper may be considered as representative only for real buildings

in which the connections between floor and wall panels are built

with solid reinforced concrete regions. Furthermore, it is important

to underline that the reinforced concrete beams at the panel ends

also cause a higher degree of flexural restraint between the floor

and wall panels, which may lead to high values of vertical load

eccentricity and, thus, to ultimate loads significantly lower than

the values obtained in the tests.

With regard to the tests on WPN08 and WPH08 wall panels,

under both axial and eccentric load, the values of ultimate loads

are similar to or higher than those obtained by standard panels

WP08. These panels can therefore be considered as valid alterna-

tives for standard panels even if a greater number of specimens

should be tested to arrive at some general conclusions.

As regards the diagonal compression tests, Table 6 reports the

first cracking load, the failure load and the failure modes. It isworth noticing that specimens 5.1 and C2 were characterised by

premature failure of one of the two concrete layers of the panels,

due to a small undesired eccentricity of the axial load. For this rea-

son, these results are not taken into consideration in the following

comments. High first cracking loads were observed for all the

specimens, while concerning the ultimate load the only significant

result is that provided by specimen C1, which is the only one

reaching a diagonal tensile failure. In fact, for the other specimens

concrete crushing occurred at the load application point. Specimen

C1, which is similar to specimen 5.2 but with transversal stiffening

walls, nearly simulates a pure shear test thanks to the diffusion of

the vertical load along the panel perimeter guarantee by the trans-

versal walls. The highest stress values are reached in the central

part of panel C1, where tensile and compression stress values are

similar. Diagonal tensile failure occurred with a load of 341 kN.In Fig. 15a the largest crack is marked with a thick dashed line.

On the contrary, for specimen 5.2, a concentration of the compres-

sion stresses occurred around the load application point, causing

the crushing of the concrete at a load value of 342 kN, as shown

in Fig. 15b.

The influence of transversal walls can also clearly be seen in

Fig. 16 where both the vertical shortening deformation (Svf and

Svb) and the horizontal elongation (Sof and Sob) measured on the

two sides of specimen C1 (with transversal walls) (Fig. 16a) and

specimen 5.2 (without transversal walls) (Fig. 16b) are reported.

In the panel with transversal walls, at the first stage of loading,

the average horizontal elongation is nearly equal to the vertical

shortening deformation; later, after the cracking of the concrete

layer, it becomes larger. On the contrary, in panels without trans-

versal walls, at the first stage of loading, the average horizontal

elongation is lower than the vertical shortening and becomes sim-

ilar after concrete cracking.

The effects of prestressing can be observed in the results of tests

on specimens 5.3 and 5.4, prestressed with forces of 30 kN and

90 kN respectively. The first concrete cracking appeared at a

slightly higher load for specimen 5.4 than for specimen 5.3. How-

ever, a lower failure load was achieved by specimen 5.4 than by

specimen 5.3, since the prestressing force incremented the

compression stresses around the load application point where

the failure occurred.

Finally, it may be noticed that in all the tests performed, failure

did not occur suddenly but was always preceded by extensive

diffuse concrete cracking. All the specimens were in fact already

micro-cracked before the tests, due to concrete shrinkage andspecimen handling. The micro-cracks constitute weak zones in

the concrete where cracks may preferentially occur. Nevertheless,

the specimens revealed a high capacity for stress redistribution

thanks to the metallic mesh inside the concrete layers. However,

it should be underlined that the results considered here are

relevant to tests on symmetrically loaded panels, with the two

concrete layers equally loaded; in reality this is an ideal condition

Table 6

Diagonal compression tests: cracking load, ultimate load, and failure modes.

Specimen First cracking load (kN) Failure load (kN) Failure modes

5.1 144 302 a*

5.2 129 342 a

5.3 118 332 a

5.4 168 306 a

C1 103 341 b

C2 137 225 b*

a – localised concrete crushing; b – failure due to diagonal tension.* Failure of one of the two concrete layers.

Fig. 15. Diagonal compression test: crack pattern at failure for panels (a) with and (b) without transversal stiffening walls.


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that can only occur when concrete layers are connected by rein-

forced concrete beams.

3. Numerical simulation

Compression tests were numerically simulated with a displace-

ment based non-linear static analysis taking into account bothgeometrical and material non-linearities, performed with the

structural analysis programme Seismostruct [21]. Specimens were

modelled with non-linear finite element models. In particular 20

beam elements were used for each concrete layer. The nodes at

each end of the two concrete layers were joined with two rigid

elements to simulate the reinforced concrete beams while the

internal nodes were joined with shear elastic links. The links are

axially rigid and shear deformable with the shear stiffness pro-

vided by the internal layer (Fig. 17).

The specimen is restrained with a cylindrical pin at the base and

a horizontal support at the top. In simulating axial compression

tests a small eccentricity of the vertical was considered in order

to simulate geometrical imperfections of panels and uncertainties

of the load position. To simulate eccentric compression tests theload was applied at the axis of a concrete layer ( Fig. 17). In both

cases the vertical load was applied incrementally until failure of

the sample was reached.

A non-linear constitutive law [22] was considered for the con-

crete and a symmetric elasto-perfectly-plastic bilinear model for

the steel. The values of the mechanical parameters were deter-

mined from the results of the tests performed on the materials

(paragraph 2.1). In particular the shear stiffness of the links joining

the two concrete layers was calculated by the equation

K i ¼ GAlc

hlc

ð2Þ

where G = 3.2 N/mm2 is the mean value among those obtained with

the shear tests for material characterisation, Al is the influence area

of the links, c is the internal layer thickness and the factor hl/c takes

into account the difference between the length (hl) of the links and c

(Table 7).

In Fig. 18 the load vs lateral deflection graphs obtained from

compression tests are compared with the results obtained from

the numerical analysis. The behaviour of eccentrically loaded pan-

els is well-approximated by the numerical model while for axially

loaded panels a lower agreement between experimental and

numerical results is achieved. In fact, the behaviour and the ulti-

mate loads of real panels are largely influenced by geometrical

imperfections (not perfectly flat concrete layers, variability of thicknesses, etc.) that are difficult to evaluate and take into consid-

eration in a numerical model. However the numerical simulations

may be considered satisfactory. Furthermore, in order to evaluate

the critical load P b1 of panels, a buckling analysis was also carried

out using the same numerical model but considering a linear elas-

tic behaviour of the materials. Values of the buckling loads ob-

tained with these analyses (P b1) are reported in Table 8 and also

in Fig. 18.

It can be observed that the P b1 values seem to be approached by

the curves obtained with the non-linear models, considering

approximately axial loads. In order to highlight the semi-compos-

ite behaviour of the panels, the values of the Euler buckling load

(P b2), calculated in the hypothesis of zero shear stiffness of the

links are reported in Table 8 (values of P b2 are twice the Euler buck-ling load for a single concrete layer). Furthermore, the buckling

load P b3 are reported in the same table, where P b3 was calculated

by considering the panel to be entirely made of concrete. The coef-

ficient a = P b1/P b3 was introduced to easily estimate the buckling

load of the panels. As expected, the values of this coefficient are

less than 1 and decrease as the thickness of the internal layer

increases.

However, only in the case of undesired eccentricity, buckling

loads are close to the ultimate loads. In fact, due to the pronounced

non-linear behaviour of the materials, the ultimate load of the

eccentrically loaded panels is significantly lower than the buckling

load and can be estimated only with a non-linear analysis which

considers both geometrical and material non-linearities. Obviously

the reduction in the ultimate load will be more significant forgreater values of load eccentricity. As already mentioned, the value

200

0-4

Displacement [mm]

L o a d [ k N ]

400

2

C1

0

Svf Sof

Sob Svb 200

0-4

400

20

Svf Sof

Sob Svb

5.2

Displacement [mm]

L o a d [ k N ]

(a) (b)

Fig. 16. Diagonal compression test: vertical shortening and transversal elongation of panels with (C1) and without (5.2) transversal stiffening walls.

shear

deformable

link ( K l)

rigid

link

beam

element

3 0 0 c m

1 5

1 5

axial

load

eccentric

load

hl hl

Fig. 17. Finite element model for compression tests.


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of the load eccentricity depends on the rotational restraint be-

tween the wall and floor panels.

With regard to the diagonal compression tests, the behaviour of

the specimen with the transversal concrete wall (C1) was numeri-cally simulated with an elastic finite element model using the

structural analysis programme SAP2000 [23]. The panel was mod-

elled with shell elements with a thickness equal to the overall

thickness of the two concrete layers and with an elastic modulus

equal to that of the concrete used. The transversal concrete walls

were modelled with beam elements. Vertical static loads were ap-

plied on several nodes for a total force equal to the ultimate load

experimentally obtained (341 kN). The value of the horizontal ten-

sion in the central node of the model is 2.3 N/mm2 which is nearly

equal to the ultimate tensile strength of the concrete. In Fig. 19 the

load-shortening plot experimentally obtained is compared with

the results obtained with the numerical model, considering an

elastic modulus E c and a reduced modulus 0.4E c to take into ac-

count the cracking of the concrete. Up to a load of about 100 kNthe behaviour of the specimen is typically linear elastic and is

well-simulated by the model with the original concrete elastic

modulus E c .

At this load a first crack appeared which was associated with a

distinct horizontal segment in the load and displacement plot.

After the first crack, the experimental curve is non-linear due to

the progressive cracking of the concrete. The numerical model with

a reduced elastic modulus (0.4E c ) simulates quite well the global

behaviour of the cracked panel.

The ultimate load may be estimated by simplified formulasfound in technical scientific literature. In particular for the diagonal

compression test the following formula may be used:

P u ¼ 2Bstot f ct ¼ 2 1120 70 2:3

1000 ¼ 360 kN ð3Þ

where B is the width of the panel and stot is the overall thickness of

the concrete layers. The result is close to the ultimate load experi-

mentally evaluated for specimen C1. However, in real buildings,

the strength of panels under vertical and horizontal forces involves

not only the shear resistance but also the bending resistance,

depending on the overall dimensions of the wall. Moreover, the

influence of openings for doors and windows must be considered.

4. Conclusions

The results of an experimental campaign on completed in situ

sandwich panels with no-shear connectors, used as wall panels,

have been presented. In particular, compression tests with axial

and eccentric load and diagonal compression tests were per-

formed. Some numerical simulations with linear and non-linear fi-

nite element models were also carried out.

As regards compression tests, wall panels with different inter-

nal layer thickness (WP08, WP12, WP16) and with two different

configurations (WPN and WPH) were tested. High ultimate loads,

decreasing for increasing values of the slenderness ratios, were ob-

tained. The numerical simulations indicated that the ultimate

loads of axially loaded panels are close to the buckling loads which

can be determined by performing a linear buckling analysis or byusing the coefficient a. Differently, the ultimate loads of eccentri-

cally loaded panels, which are significantly lower than the buckling

loads, can be simulated only by performing a non-linear analysis.

Additional research is needed to develop simple, effective and ra-

tional methods for predicting the ultimate load of wall panels for

different values of load eccentricity. The results of the experimen-

tal tests and numerical simulations indicated that a partial degree

of composite behaviour was attained by the tested panels even if

non-shear connectors are used in the interior layer. However, this

semi-composite behaviour is due not only to the internal layer, but

also to the reinforced concrete beams at the ends of the panels.

Additional investigations are needed to develop simple, effective

and rational methods for predicting the ultimate load of wall pan-

els for different values of load eccentricity and to study the behav-iour of panels without reinforced concrete beams.

Table 7

Characteristics of the numerical model.

WP08 WP12 WP16

hl (mm) 115 155 195

K l (N/mm) 9660 5786 4095

650

0

L o a d [ k N ]

1300

2a.1

f.e.m. 2a.1

Pb1 = αPb3

2b.2

f.e.m. 2b.2

3a.2 f.e.m. 3a.2

3b.2

f.e.m. 3b.2

650

0

L o a d [ k N ]

1300

Pb1 = αPb3

4a.1

f.e.m. 4a.1

4b.2

f.e.m. 4b.2

650

05 10 15 20

Displacement [mm]

L o a d [ k N ]

1300

25

Pb1 = αPb3

Fig. 18. Axial and eccentric load tests: comparison between experimental and

numerical results.

Table 8

Critical loads and reduction coefficient.

Specimen Mean U.L. (kN) P b1 (kN) P b2 (kN) P b3 (kN) a

WP08 742 931 92.8 3653 0.25

WP12 825 1082 92.8 7424 0.15

WP16 881 1221 92.8 13,169 0.09

2

experimental

f.e.m.

( E c)

f.e.m.

(0.4 E c) C10

Displacement [mm]

L o a d [ k N ]

400

200

Fig. 19. Diagonal compression tests: comparison between experimental and

numerical results.


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As regards diagonal compression tests, simple wall panels, pre-

stressed wall panels and panels with transversal stiffening walls

were tested. In all these cases high cracking loads were observed.

The panels also showed a high capacity for stress redistribution

thanks to the metallic mesh inside the concrete layers. However,

only one specimen with transversal stiffening walls showed a ten-

sile diagonal rupture while the other specimens showed a com-

pression failure at the load application region. The numericalsimulation of the test reaching the tensile diagonal rupture showed

that an effective concrete modulus of elasticity may be considered

to simulate the global behaviour of the cracked panel and that the

ultimate load may be estimated on the basis of the tensile strength

of the concrete. However, since in real buildings the behaviour of

the panels under vertical and horizontal in-plane forces is strongly

influenced by the overall dimensions of the wall and by openings

for doors and windows, further investigations on panels with dif-

ferent configurations are recommended.

Acknowledgments

The financial support provided by ‘‘Schnell House’’ S.p.A., based

in San Marino, is gratefully acknowledged. The technical support of

the laboratory staff at the Dept. of Architecture, Construction and

Structures, Università Politecnica delle Marche, is greatly appreci-

ated. The opinions, findings and conclusions contained in this pa-

per are those of the authors, and do not necessarily reflect the

views of the sponsors.

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