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PERFORMANCE OF BASE-PLATE CONNECTIONS

OF STEEL STORAGE PALLET RACKS

N. Baldassino1

and R. Zandonini2

ABSTRACT

Pallet racks from the structural point of view, can be considered typical steel framed

structures. Their design is usually performed adopting 2-D simplified models related to the

main frame directions: i.e. down-aisle and cross-aisle direction. The frame stability in down-

aisle direction, where usually bracing systems are missing, is ensured by the degree of

continuity provided by joints (i.e. beam-to column and base-plate joints). The rack joints

behaviour appears quite complex and the main Standards for pallet racks (i.e., AS, FEM,

RAL, RMI) suggest specific tests to suitably characterise their performances.

At the University of Trento, in the framework of a research work of pallet racks, a

research on the response of base-plate joints under axial eccentric load (base-plate

connection test) is currently in progress. In this paper the on-going activity is presented and

the preliminary results are briefly discussed.Keywords: steel storage pallet racks, experimental and numerical analysis, Codes, joint

response, joint model.

1. INTRODUCTION

Pallet rack structures are one of the more common industrial storage systems for

palletised goods. They are generally made up using cold formed steel profiles which allow a

considerable weight reduction and a high design flexibility due to the great variety of section

profiles. From the structural point of view steel storage racks can be considered framed

structures, whose behaviour is further complicated by the stability problems associated to thin

walled profiles, by the presence of perforations in the upright and by the type of mechanical

connections adopted [1, 2]. As in traditional framed structures, the design of pallet racks is

1

Assistant Professor, University of Trento, Trento, Italy2Professor, University of Trento, Trento, Italy

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performed by adopting 2-D sub-models related to the main directions of the rack system, i.e.

down-aisle and cross-aisle direction. In cross-aisle direction the model of braced frame can be

adopted. In down-aisle direction bracing systems cannot be used and the model of semi-

continuous sway frame seems the most adequate. The lateral frame stability is hence ensured

by the degree of continuity provided by beam-to-column joints and base-plate connections.

The rack joints behaviour appears quite complex. Beam-to-column and base-platejoints (Fig. 1), which are made by adopting mechanical devices, are characterised by a non-

linear response and by a semi-rigid nature [3]. In addition, beam-to-column joints show in

many cases non negligible initial lack of fits and base-plate connection performances depend

on the level of axial load applied to the column.

Fig. 1 - Typical beam-to-column and base-plate joints.

The design philosophy adopted by the main Codes for pallet racks [4], [5], [6], [7],

makes reference to a design approach which combines the results of tests on components (i.e.

columns, beams, joints, sub-assemblies) with the theoretical criteria developed and codifiedfor traditional cold-formed members. Among tests, a great attention is devoted to the

mechanical characterisation of joints. All Codes for pallet racks, in fact, prescribe specific

tests to analyse the behaviour of beam-to-column joints mainly in bending condition, while

only the FEM Recommendations suggest, non mandatory tests on base-plate connections [8].

The lack of data on base-plate joint responses is generally reflected in the adoption of

simplified models of analysis (i.e. frame model with hinged base).

Numerical studies focused on the influence of joint modelling on the rack performance

have pointed out the non-negligible influence of the base-plate joints on the overall rack

response and, as a consequence, the need for test data to suitably assess the key parameters

which characterise the behaviour of the column base restraint [9].Since 1992, at the University of Trento studies on the behaviour of pallet racks have

been carried out. In the framework of these activities, a research on base-plate rack joints under

axial eccentric load (base-plate connection test) is currently in progress to extend suitably the

state of knowledge on racks. Tests on base-plate joints have been carried out with the purpose to

measure the moment-rotation characteristic (Mb-b) of the connection between the upright andthe floor in the range of axial loads of practical interest. Further issues have been considered

such as the influence of the axial load on the joint response, the interaction between base-plate

and upright and the influence of the base fixings on the joint behaviour. Moreover, test results

are currently under study, with the aim to define a suitable design model of the moment-

rotation joint response. In the following, the testing rig and some results of this study are briefly

presented and discussed, also with reference to their evaluation in view of the development of areliable design model.

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2. THE EXPERIMENTAL STUDY ON BASE-PLATE CONNECTION

Despite base-plate joints response plays a key role in rack performance, the knowledge

about their structural behaviour appears quite limited. The on-going study on base-plate

connections recently started at the University of Trento consists of an experimental and a

numerical phase. The experimental phase comprises of 163 tests on 23 different types ofcommercial connections carried out following the general scheme and the procedure

suggested by the FEM Recommendations.

2.1 The tests set-up

A test set-up was specifically designed and it is schematically presented in Figure 2.

Fig. 2 - Test set-up for base plate joint test.

The specimen is composed by two stub columns symmetrically connected to a

concrete cube by using the same fixings as in the structure they are supposed to represent. The

friction between the cube and the testing apparatus is minimised by interposing two layers of

teflon to make possible its movement in a horizontal plane. Two hydraulic jacks apply the

loads to the specimen: jack 1 simulates a concentric axial load F1 on the upright, while jack 2

applies a transverse load F2 to the centre of the cube. After a preliminary alignment phase, the

force F1 is increased to the chosen value and held constant during the test. The force F2 is then

increased up to the collapse of the specimen. The rotation of the column bases with respect to

the concrete cube, the horizontal cube displacement and its possible rotation are measured

during the test (Fig. 3). Figure 4 shows the testing-rig during a test.

The applied moment at the base joint (Mb) is evaluated as indicated in Equation (1)

while the base joint rotation (b) is computed as the mean value of the base rotations of thetwo uprights (Eq. 2).

1

F2

2F

Mb +=l

(1)

Concrete cube

Upright Upright

Base plate

Hydraulic jack2 Hydraulic jack 1

Base plate

F2

F1 F1

Hinge

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+

=

34d

43

12d

21

2

1

b

(2)

where = mean displacement of the concrete cube;i = displacement on the i transducer;l = (l1+l2)/2 (Fig. 3);d12, d34 = distances between transducers (Fig. 3).

Fig. 3 - Measured displacements in base-plate joint test.

Fig. 4 - Test on base-plate joint at the University of Trento.

In order to verify the influence of the axial load on the joint response, tests have been carried

out by considering at least two representative values of force F1. Furthermore, in

correspondence of each values of F1 three tests have been executed, i.e. for each base-plate

joint at least six tests have been performed.

F1 F1

F2

J1

J2

1

42

3

5 6

d34d12

l1 l2

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2.2 The test results

The test data have been analysed in accordance to the criteria previously indicated.

Figure 5 shows a typical outcome in terms of moment-rotation relationships (Mb-b) for twodifferent levels of axial load. These curves confirm the non negligible influence of the axial

load on the connection response in terms of both ultimate moment and stiffness. There ishence a clear need to perform tests at values of the axial load which are significant and

consistent with the purpose to characterise completely the base joint response.

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

0 10 20 30 40

b [mrad]

Mb[kNm]

Fig. 5 - Moment-rotation relationship of base-plate joints.

Collapse was never achieved at the base fixings but it was associated with an interaction of

plasticity and instability of the upright near to the base-plate. Furthermore, collapse modes

showed that the interaction between the base-plate and the upright could affect remarkably the

whole joint response.

The tested specimens were characterised by a large variability of the type of base-plate

element and of its fixing to both the concrete block and the upright. Therefore, it is difficult to

make a direct comparison between test results. The meaning of such a comparison would also

be generally questionable. Some examples of connections are presented in Figures 6(a)-6(f).

The restraint offered by the base-plate element to the upright appears quite different: in some

cases the base-plate does not restrain effectively the column deformability (Figs 6(a)-6(c)), in

other cases (Figs 6(d)-6(f)) the base-plate element stiffens the upright at the end near theconnection. This is reflected in the collapse mode. In the former case noticeable deformations

occurred at collapse: distortional deformation of the upright section or localised deformations

on the upright (i.e., hole ovalisation, local buckling of the upright) were observed. On the

other hand, when the base-plate element restrains the upright, collapse was generally localised

in correspondence of the connection between base-plate and upright.

Attention was hence focused on the influence on the moment-rotation relationship of the

interaction between the connection element and the upright. Preliminary tests were hence

performed on two sets of specimens characterised by the same upright, the same fixing system

to the concrete cube but different restraint provided to the column. In the following the two

typologies of specimens are identified as A and B respectively. Specimens type A had a base-

plate element as indicated in Figure 6(e), while for specimens type B base-plate as in Figure

6(b) was adopted. The former base-plate differs from the latter only for the restraint imposed

N = 40% Nu

N = 25% Nu

Nu = upright squash

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to the free deformation of the upright. Tests were performed by considering only one level of

axial load F1 approximately equal to 45% of the ultimate resistance of the upright. For each

type of specimen three test were carried out. Test results are presented in Figure 7, where it

can be noted a different behaviour of specimens A and B mainly with reference to the

stiffness, while a limited increase on the collapse moment is showed.

(a) (b) (c)

(d) (e) (f )

Fig. 6 - Typical base-plate connections.

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

0 10 20 30

b [mrad]

Mb[kNm]

Fig. 7 - Influence of the interaction between the base-plate element and the upright.

The effect of the restraint to the upright deformation in the zone close to the base

connection, is reflected in a higher stiffness of the connection. On the basis of these

preliminary results it appears that the performance of the base-plate joint depend in a non

negligible way by the interaction between the upright and the base-plate element and in a

limited mode on the fixing systems to the concrete cube.

As a preliminary conclusion it can be observed that the moment-rotation experimentalrelationship takes into account both the behaviour of the base-plate connection and the

Specimens type A

Specimens type B

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interaction between the upright and base-plate element. As a consequence, tests enable to

determine the overall response of the final part of the uprights near to the base-plate.

3. BASE-PLATE JOINT MODELLING

The on-going phase of the research is dedicated to the modelling of the base-plate jointresponse. The main aim of the study is to define a simplified design model to represent in a

sufficiently accurate way the experimental response of the base joint.

To this aim the FEM Recommendations alternatively to a multi-linear relationship, adopt the

simplified model showed in Figure 8 which consists of a bi-linear moment-rotation

relationship. Conditions are imposed to the secant stiffness with the purpose to minimize the

difference between the model and the experimental curve and to ensure consistency in terms

of elastic energy.

Fig. 8 - Base-plate connection model as in FEM Recommendations.

With reference to a joint and to the tests related to the same level of axial load, the

FEM criterion can be summarised as follows:

1- calculation of the design moment of the joints as Mk/m, where Mk is the characteristic

value of the moment and m is a partial safety factor (m = 1,1). The Mkvalue is evaluatedas:

====

n

1imti

n

11timk )MM(

)1n(

1kM

n

1ksMM (3)

where Mm is the mean value of the joint collapse moments, s is the standard deviation, n is

the number of specimens, k is a parameter function of n (i.e, for n=3 k=3,15);

2- approximation of each experimental curve by means of a polynomial relationship (i.e. by

adopting the least squares fit polynomial);

3- for each polynomial curve definition of the secant stiffness kti by imposing theequivalence of the areas A1 and A2 (see Fig. 8), as:

Rotation

Moment

Slope

A 1=A 2A 1

k ti=

A 2

0 ki

M km

M ti

1.15

ki

MRd=

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( )=

ki0 tikiRd

2

Rdti

d)(MM2

Mk

(4)

4- check on the value of the secant stiffness:

mki

kti

M15,1k

(5)

5- if Equation (5) is not verified, redefinition of the stiffness (Final Stiffness in Fig. 9) sothat the condition (5) is met.

Fig. 9 Redefinition of the base-plate connection stiffness as in FEM Recommendations.

The design value of the stiffness (kd) associated to the base-plate connection and to the

considered level of axial load, shall be taken as the average value of the kti values:

== =

n

1itimd kn

1

kk (6)

The FEM criterion results in a simplified model which can be easily implemented in

numerical analysis.

The first attempt to evaluate the test data has been carried out following the simplified

criterion proposed by the FEM Recommendations (Figs. 8 and 9). It has been noted that:

- the dispersion of test results within the same set of data appears generally quite limited: the

average coefficient of variation results approximately of 8%. A high value of the

coefficient of variation leads to a great reduction of the design moment with respect to the

mean value of the collapse moment.

- both the FEM requirements (Eq. 4 and Eq. 5) are fulfilled at the first step only in a verylimited number of cases, in fact:

Rotation

Moment A1

A2

0 ki

Mkm

Mti

MRd =

---- Initial Stiffness(A1 = A2 but kti 1,15 MRd/ki)

Final Stiffness(A1 !A2 but kti = 1,15 MRd/ki)

kti

kti

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only 4% of the considered curves meet at the same time the requirements of Equations

(4) and (5). This happens only in cases where it has been observed a greater dispersion

of test data (high value of the coefficient of variation). This means that the MRd value

appears remarkably reduced with respect to the experimental moment of collapse.

the remaining cases do not satisfy simultaneously the requirements in Equation (4) and

Equation (5). The related experimental curves appear characterised by a remarkably nonlinear behaviour and by a rapid reduction in stiffness which does not allow to meet the

FEM requirements. As a consequence a redefinition of the stiffness has to be considered

(Fig. 9). Test results have showed that assuming as reference the stiffness values which

fulfil the equivalence of the areas (Initial Stiffness in Fig. 9), the stiffness reduction

range between 0,3 % and 69% with a mean value of 19,6%. The relevant reduction of

elastic energy with respect to experimental response appears quite remarkable. The

mean value of the reduction is of 12,5% with a maximum of 34,7%.

These preliminary results point out that in base joints characterised by a notable non linear

behaviour the FEM procedure appears quite penalising in defining the design model of

analysis if compared to the experimental joint response.

A parametric study on the influence of the base-plate response on the overall frame

behaviour [9] showed that both the design moment of resistance and the joint stiffness

associated to a bi-linear joint model influence remarkably the rack performance in terms of

ultimate load carrying capacity and service load. As an example, a reduction of 50% of the

joint stiffness gives a 6% reduction of the ultimate load carrying capacity and of the service

load. Furthermore a reduction of 80% in joint stiffness produces 15% reduction of the frame

performance with respect to both ultimate and service limit states conditions. It is necessary to

point out that the aforementioned results have been carried out by considering a particular

rack frame and hence they can not be extended to other rack configurations.

As a consequence of these preliminary results, further studies have been planned with

the purpose to deeply investigate the influence of the base-plate joints modelling on the frameresponse and to identify alternative simplified criteria for the definition of a reliable design

model for base-plate joints. In the framework of these activities test data analyses are currently

in progress following the criteria suggested by the prEN 1993-1-8 document [10] for the

traditional steel column base joints. Classification of base-plate joints and definition of a

simplified bi-linear model are the main feature up to-now investigated. However, it is

apparent that it would be more efficient to approach the problem by means of the

methodology of the component method. The potentially unlimited variety of connection

types makes this method more difficult to be implemented in a standard form than for

traditional frameworks. The concept is in any case the way to be followed also in view of the

definition of overall Mb-b curves.

4. CONCLUSIONS

At the University of Trento a research on the behaviour of base-plate connection of

steel storage pallet racks under eccentric load is currently in progress. The study which

comprises of both an experimental and a numerical phase has been briefly summarised in this

paper. The testing rig and the test procedure have been presented. The attention has been

focused on test results which have highlighted the main factors affecting the base-plate joint

response. The level of the axial load applied to the upright, the behaviour of base-plate

connection and the interaction between upright and base-plate element play a key role in joint

response. Their influence on the joint performance is captured globally by tests, which appearable to represent the overall response of the end part of the uprights near to the floor. With

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reference to the base-plate joint model to be adopted in numerical analyses, it has been applied

the simplified bi-linear relationship suggested by FEM Recommendations. The preliminary

results point out that in base joints characterised by a notable non linear behaviour the FEM

procedure appears quite penalising in defining the design model of analysis if compared to the

experimental joint response. On the basis of these results future activities have been planned

with the purpose to define alternative criteria to select a model for the joint response whichcombine simplicity and accuracy.

ACKNOWLEDGMENTS

The data related to the pallets rack frames have been kindly supplied by some Italian

Companies involved in the activities of ACAI-CISI (Italian Association of Steel Constructors

Rack Manufacturing Companies Group).

The authors greatly appreciate the skilful work of the technical staff of the Laboratory of

the Department of Mechanical and Structural Engineering of the University of Trento for

assistance during the tests.

The authors wish to tank eng. Davide Vulcan for his help in test data analyses.

REFERENCES

[1] Godley, M.H.R., Storage Racking, chapter 11 of Design of Cold Formed Steel

Members, Rhodes ed., , 1-399, 991.

[2] Hancock, G.J., Distorsional Buckling of Steel Storage Rack Columns, Journal of

Structural Engineering, ASCE, 111(12), 1985, pp. 2770-2783.

[3] Davies, J.M. and Jiang, C., Design for Distortional Buckling, Journal of

Constructional Steel Research, 46, (1-3), 1998.

[4] AS, Steel Storage Racking AS4084, Australian Standards,1993.[5] FEM, Reccomandation for the Design of Steel Pallet Racking and Shelving, Section X of

the Federation Europeenne de la Manutention, 2001.

[6] RAL, Storage and Associated Equipment, Deutsches Institut fur Gutersicherung und

Kennzeichnung German Institute for Quality Assurance and Marketing, 2002.

[7] RMI, Specification for the Design, Testing and Utilization of Industrial Steel Storage

Racks, Rack Manufactures Institute,1997.

[8] Baldassino, N. and Zandonini, R., Industrial steel racks: tests, design and Codes,

Proceedings of the Conference on Advances In Structures: Steel, Concrete,

Composite and Aluminium,ASSCCA '03, Sydney, Australia, June 2003.

[9] Baldassino, N. and Bernuzzi, C., Analysis and Behaviour of Steel Storage PalletRacks, Thin Walled Structures, vol. 37, n. 4, 277-304,2000.

[10] prEN 1993-1-8, Eurocode 3: Design of steel structures Part 1-8: Design of joints,

December 2003.