Steel Storage Pallet Racking Systems in Seismic Areas: Full Scale Push-over Tests and Numerical Simulations

Politecnico di Milano, ABC department Piazza Leonardo da Vinci, 32

20133 Milan, Italy Tel: +39 335 6166318

E-mail: carlo.castiglioni@polimi.it

Abstract This is a presentation of the results of full scale push over tests on steel storage pallet racking systems which have recently been managed and carried out by Politecnico di Milano. Some of these tests have been performed within the EU research project SEISRACKS2: “Seismic Behaviour of Steel Storage Pallet Racking Systems”, RFSR-CT-2011-00031 (Castiglioni et al., 2014), while the other ones for consulting of industrial producers. The execution of these last tests is a key indicator of the relevant impact of the “SEISRACKS2” project on the industrial sector of storage racking system. The results of full-scale push-over tests allow to validate experimentally the design choices and calibrate the numerical models. As a consequence, numerical models able to simulate the real behaviour of whole steel storage racking system can be developed. Keywords: steel storage rack, push-over test, full-scale specimen

Introduction Experimental studies, which have been carried out on steel storage pallet racking system so far, are mainly limited to experimental characterization at a component level (e.g beam-to-column and base plate connections or compression and tension tests on structural elements) (Blume, 1973; Krawinkler et al., 1979; Chen et al. 1980; Bernuzzi and Castiglioni 2001; Castiglioni, 2003; Castiglioni et al., 2007; Castiglioni, 2008). Full scale experiments performed previously gave only limited indications, for a few types of rack configurations. Many questions remain open regarding ductility and energy dissipation capacity (and consequent q-factors) of several rack configurations adopted widely in the industry. With the aim to meet these needs, the research project SEISRACKS2 ("Seismic Behavior of Steel Storage Pallet Racking Systems") (Castiglioni et al., 2014) was initiated in 2011 thanks to a financial support of EU Research Fund for Cool and Steel (RFCS). Within this project, for the first time in Europe, push over tests have been performed, considering real-scale geometry and loading on different pallet racking systems, provided by four world-wide renowned industrial producers, which were the industrial partners in the research project. Thanks to these tests, ductility performances of several types of racks (braced and unbraced) have been investigated and some indications have already been introduced in the normative document EN 16681, the first European Standard for the design of the racking systems in seismic zones.

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(*Bildirinin Türkçe tercümesi 481 nolu sayfadadır.)

Carlo Andrea CASTIGLIONI, Alper KANYILMAZ, Giovanni BRAMBILLA, Gian Paolo CHIARELLI

In particular, the results of this project showed a significant discrepancy between the expected ductility performances of racks in the design phase and their experimental behaviour as verified during the testing program. One of the main outcomes of “SEISRACKS2” project (Castiglioni et al., 2014) is that the execution of full-scale push-over tests is strictly required when a ductile design is adopted, because it is the most effective way to verify the real structural behavior of racks in terms of global resistance, stiffness and ductility. According to these outcomes of “SESISEACKS2” project (Castiglioni et al., 2014), full-scale experimental tests have recently been performed by Politecnico di Milano for consulting of two industrial producers, which are specialized in analysis, design, production and distribution of steel storage pallet racking system. The aim of industrial producers was to validate their design choices and calibrate the numerical models. This article presents the results of all push-over tests on full-scale unbraced and braced rack systems, performed by Politecnico di Milano so far.

Experimental setup and test procedure

All tests were performed in a compound owned by Marcegaglia Buildtech, and managed by Politecnico di Milano. In total, 14 full scale specimens have been tested, as summarized in Table 1.

Table 1 Summary of test specimens.

Industrial Producers Specimen Configuration

of Rack

"SE

ISR

AC

KS2

" PR

OJE

CT

IP A IPA1 unbraced IPA2 braced

IP B IPB1 unbraced IPB2 braced

IP C IPC1 unbraced IPC2 braced

IP D IPD1 unbraced IPD2 braced

CO

NSU

LT

ING

S A SA1 unbraced SA2 braced SA3 braced

S B SB1 unbraced SB2 unbraced SB3 braced

All specimens are composed of four levels and two bays. Each level height is 2 m, and there are four loaded levels. The total height of the specimens is approximately 8 meters, and the upright width varies between 1.2 and 1.1 meters, which is representative of most common configurations on the market. These dimensions are not constrictive, and modifications of test set-up can be easily implemented in case of different specimen sizes and/or loading direction (down or cross aisle). Specimens have been designed

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using the design parameters of FEM 10.02.08. Two typical configurations of specimens are shown in Figure 1.

a) Unbraced system (IPA1). b) Braced system (IPD2). Figure 1 Test frame and assembly.

As shown in Figure 1, the specimens are made of: • Upright frames: Composed of two uprights in perforated thin gauge cold formed

profiles linked together by a system of bracing elements. • Pallet Beams: Members linking adjacent upright frames, lying in horizontal

direction parallel to the operating aisle (down aisle direction). They are closed built up sections made of two cold formed “C” elements, welded to end plate connectors hooking in the holes of the uprights.

• Beam to upright connections: Beam to upright joint is a proprietary system consisting in a hook-in connector welded to the beam ends, fitted into dedicated regularly spaced slotted holes in the uprights.

• Base plate connections: Base plates are L-shaped components that connect the upright bases to the foundation. The uprights are connected to the base plates, through slotted holes in the vertical part of the base plate, using bolts. The horizontal part of the base plate is connected to the foundation, using bolts. Number and configuration of bolts depend on designer's choice.

In braced configuration there are also: • Vertical bracings: Bracings in the vertical plane in down aisle direction, linking

adjacent upright frames. Spine bracings are located on the rear side of the rack, for single entry racks, and they are made of vertical uprights, transverse members (mainly compressed) and diagonal members working mainly in tension.

• Horizontal bracings: Diagonal bracing elements are placed between beams, which transfer the horizontal forces from the unbraced vertical plane of upright to the braced vertical plane.

Table 2 and 3 show the main geometric properties of element sections and the stiffness of joint connections for the specimens of “SEISRACKS2” project, defining: kb*= E*Ixu/Lu kj*= E*Ixb/Lb

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where: − E is the Young’s modulus of steel elements − Ixu is the x-axis moment of inertia of upright section − Ixb is the x-axis moment of inertia of beam section − Lu is the one inter-storey upright length − Lb is the length of pallet beam

Table 2 Geometry properties of upright section and stiffness of base plate connections

for specimens of “SEISRACKS2” project.

Table 3 Geometry properties of beam section and stiffness of beam-to-upright joint for specimens of “SEISRACKS2” project.

Vertical loading of the specimens are provided by concrete blocks of 8 KN, which is a medium-high value for the merchandise used commonly in practice. The blocks were loaded on the wooden pallets on the ground, and 3 loaded pallets are placed on each bay of the rack specimens (see Figure 1). This vertical loading configuration reproduces one of the most critical warehouse operating conditions with high axial stress in the uprights and bending stresses in the beams. Seismic loading is applied as a static horizontal load, applied with a triangular pattern (Figure 2), proportional to the dynamic inertia of each level, which is the typical force-controlled loading condition.

Upright section and base plate connection Specimen α β

t [mm]

A [mm2]

Ix [mm4]

Iy [mm4]

kb/kb*

IPA1 30,0 60,0 2 674 22430 7160 219,5IPB1 41,0 50,0 2 686 20100 11900 233,4IPC1 37,5 45,0 2 460 13438 7024 142,5IPD1 32,5 42,5 2 424,7 12426 6530 137,9IPA2 30,0 60,0 2 674 22430 7160 219,5IPB2 33,8 50,0 2 536 14300 5590 276,3IPC2 40,0 40,0 2,5 784 24308 14844 130,8IPD2 37,0 48,0 2,5 979,6 31714 19036 78,4

Beam section and beam-to-upright joint Specimen γ δ

t [mm]

A [mm2]

Ix [mm4]

Iy [mm4]

kj/kj*

IPA1 100,0 30,0 1,5 762 32670 12125 53,9IPB1 86,7 33,3 1,5 659 24200 4390 79,6IPC1 67,0 26,7 1,5 588,7 17122 7556 87,6IPD1 83,3 33,3 1,5 516 16415 9862 85,8IPA2 80,0 30,0 1,5 641 22500 9664 82,6IPB2 73,3 26,7 1,5 600 19100 4290 89,4IPC2 73,7 33,3 1,5 645 21775 10300 69,6IPD2 74,3 28,6 1,75 816 31270 13120 66,1

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Figure 2 Triangular load pattern.

The tests consists in “pulling” the structure until it collapses, or a controlled deformation parameter reaches a predefined limit value. The pulling effect is obtained by applying a progressively increasing load pattern and plotting the total applied load versus the monitored point’s displacement to obtain a characteristic force-displacement graph named “capacity curve”. To obtain accurate three-dimensional deformed shapes of the specimens, the displacements are monitored at each loading level and at both upright columns, by means of twelve potentiometric displacement transducers. 8 transducers were placed to measure down-aisle displacements and 4 transducers at the second and fourth floor on the left and right side of the specimen, to measure the displacements in cross aisle direction. This configuration was adopted to register the out-of-plane displacements due to torsional deformations. All this assembly and testing procedure is described in detail in a specific protocol developed within “SEISRACKS2” project (Castiglioni et al., 2014).

Data processing method Due to a nondisclosure agreement, the graphs, that will be presented, have been normalized following the procedure reported below :

• Base shear force (F) has been divided by the yield shear force (Fy): F / Fy = F*

• Displacements (d) have been divided by the yield displacement (dy): d / dy = d*

After a careful analysis of the different approaches, proposed by ECCS45 and other international standards, to evaluate the conventional limit of elastic range (Fy,dy), the following definition is assumed as the most suitable one (Brambilla and Chiarelli, 2014). From the recorded base shear – top displacement curve (F-d curve), the yield shear force Fy and the corresponding displacement dy may be deduced as follow (Figure 3):

− evaluate the tangent at the origin of the F-d curve; it gives a tangent modulus E = tgαy

− locate the tangent that has a slope of E/10; − the intersection of the two tangents defines the level of Fy; − dy is the displacement corresponding to that intersection.

In order to calculate the q-factor, the pushover curves have to be linearized. The q-factor is defined as the product of the overstrength Ω, and the ductility ratio μ = q0.

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𝛀 =𝑭𝒖𝑭𝒚

; 𝒒𝟎 =𝒅𝒖𝒅′

; 𝒒 = 𝛀 ∙ 𝒒𝟎

where: − Fy is obtained from the above mentioned definition − Fu is the maximum reached load − du is the displacement associated to the maximum reached load − d’ is the displacement associated to the intersection between the tangent at the

origin and the Fu limit (see Figure 3b)

(a) (b) Figure 3 Definition of yield force by ECCS45 (a) and its application on test results of

IPA1 (b).

Test results General remarks From the global displacement-force response of the specimens, two major types of global behavior have been observed: ductile or brittle. In first case the capacity curve is characterized by non-linear shape with gradual increase of load and displacement until the plateau (indicating maximum capacity load) is reached. After this, the structure is unloaded; the relative branch of capacity curve is parallel to the initial loading one, showing recovery of elastic deformations and residual permanent ones (Figure 4). It can be deduced that the racks still have some plastic resources at least in terms of displacement.

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Figure 4 Schematic ductile capacity curve

Instead, in case of brittle behaviour, the capacity curve is characterized by a linear branch until the maximum capacity load is reached. A short plateau can be observed before collapse occurs. The structural failure is highlighted by the absence of the elastic unloading branch that is replaced with an accumulation of lateral displacement during the unloading phase (Figure 5).

Figure 5 Schematic brittle capacity curve

Unbraced racks Specimens IPA1, IPB1, SA1 (Figure 6a), SB1 and SB2 showed a moderately ductile behavior thanks to the diffuse exploitation of plastic resources in their connections, and robust base plate connections. On the other side, global behavior of the specimens IPC1 (Figure 6b) and IPD1 were brittle.

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(a) SA1 (b) IPC1 Figure 6 Unbraced racks, typical global ductile behaviour (a) and brittle one (b)

There were two major reasons of this poor global behavior:

• First one is the formation of plastic hinges at the first loading level, which led to a soft storey mechanism (Figure 7a). In fact, inter-storey drifts of the upper levels of IPC1 and IPD1 were smaller than those of the first level, which is a characteristic of the soft storey collapse mechanism that may also lead to global instability, due to second order effects. In the case of specimens IPC1 and IPD1, poor base plate details were major causes of such behaviour. This insufficient connection detail caused a “hinge” behaviour at the bases, and the rotation demands were concentrated at the “more rigid” first floor beam-upright connections.

• Second reason is the distortional buckling phenomena, spread on the first two inter-stories of the structures. Strong localized pressures led to a distortion of upright sections in correspondence with the beam-upright joints of the first two levels, and this also caused a soft story mechanism (Figure 7b). The distortional buckling of the uprights occurring in correspondence with the beam levels, could have been avoided by connecting the upright bracing members to the joint locations corresponding to the pallet beam connectors.

(a) IPC1 (b) IPD1 Figure 7 Examples of brittle behaviour causing global collapse.

As confirmation of these observations, in the Table 4 the q-factor values of the unbraced structures and the corresponding values defined by producers are reported.

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Moderately high values of q indicate reasonable ductility of these structures, while low values of q indicate lack of global ductility as verified in case of IPC1 and IPD1. It must be noted that for these two case q-factor value assumed in design phase is lower than the effective ones.

Table 4 q-factor values of unbraced racks.

"SEISRACKS2" Project CONSULTING

IPA1 IPB1 IPC1 IPD1 SA1 SB1 SB2

Ω 1,27 1,12 1,08 1,02 1,14 1,20 1,14

q0 3,03 1,90 1,66 1,22 2,29 2,56 2,03

q 3,85 2,13 1,79 1,24 2,62 3,08 2,30

q DESIGN 2,00 2,00 2,00 2,00 2,00 2,00 2,00

Braced Racks Braced specimens IPA2, SA2 (Figure 8a), SA3 and SB3 showed a global ductile performance, thanks to the exploitation of plastic resources of its vertical bracings that are efficiently connected to the rack joints. Figure 8b displays that global stiffness of the specimen IPB2 had a sudden decrease, when the compression diagonals were buckled. After this point, no plastic behavior occurred until the shear failure of the bolt, which did not permit the rack to take any advantage of plastic resources of the diagonals. The shear failure took place at the connection of the ground level spine bracing to its base plate (Figure 9a).

(a) SA2 (b) IPB2 Figure 8 Braced racks, typical global ductile behaviour (a) and brittle one (b).

Instead IPD2 has shown very low ductility because of the connection failure of the vertical bracing at the first level (Figure 9b). This failure was caused by the bending of the bolt that was connecting the first level bracing to the bracing joint at the first floor. For specimen IPC2, test was stopped due to the capacity of the hydraulic jack, although design elastic limit of the structure was reached at the end of the test. This specimen could only slightly exploit the plastic resources of its spine bracings. Based on these results, it has been seen that structural response of braced specimens is highly influenced by the connection details of their spine bracing elements. Plastic

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resources of spine bracings could only be exploited if their connections were resistant enough to not to cause an early brittle local failure.

a) IPB2 – bracing connection failure. b) IPD2 – bolt bending failure. Figure 9 Deformation of braced specimens.

Table 5 summarizes the q-facors obtained by the various tests and shows that in case of brittle behaviour the q-factor is equal to 1,0. As for unbraced racks not all the structures ensured experimental q-factor values equal or larger than the design ones.

Table 5 q-factor values of braced racks.

"SEISRACKS2" Project CONSULTING

IPA2 IPB2 IPC2 IPD2 SA2 SA3 SB3

Ω 1,13 1,00 1,00 1,00 1,18 1,15 1,11

q0 2,19 1,00 1,00 1,00 2,22 2,17 1,98

q 2,48 1,00 1,00 1,00 2,61 2,50 2,20

q DESIGN 1,50 1,50 1,50 1,50 1,50 1,50 1,50

Numerical modelling

Simulation approach The behavior of the different racking systems has been simulated through finite element software, and a comparison of the response of these models under static non linear pushover analysis with the results of full scale pushover tests performed on each racking system has been carried out. A commercial software package (SAP2000), commonly adopted by professional engineers in their daily practice applications, was used, avoiding special sophisticated software, specifically developed for cold formed thin walled profiles. The general approach followed for numerical modelling consists in simulating of the different components of the racks (uprights, beams and diagonals) with frame elements. Once defined the structural geometry and the member properties, attention is focused on the material and geometric non linearity. By a state-of-the-art review (Castiglioni, 2008) it can be noticed that almost all plastic deformations in the rack systems are concentrated in four types of structural elements: beam-end connectors, column base connection, diagonals (for the braced racks) and on the top of the first inter-storey.

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Beam-end connectors, column base connection and diagonals are modelled by means of non-linear link elements. Their behaviour is defined according to experimental results of component tests. At the top of the lower level of the uprights, plastic hinges are introduced, in order to take into account the interaction between axial force and moment resistance of the uprights. The constitutive laws for these hinges were derived based on the results obtained from Abaqus FEM analysis. Simulation results Figure 10 shows two example of comparison between numerical push-over curve and corresponding experimental one. A very good experimental-numerical agreement can be noted, without requiring any calibration. It’s important to highlight that in both cases all the experimental data of the component tests were available. Therefore, a correct numerical estimation of real behaviour of the rack can be obtained if all results of component tests, required to characterize the non-linear behaviour of racks, are provided and integrated into numerical modelling.

a) Unbraced frame, IPA1. b) Braced frame, IPA2.

Figure 10 Numerical to experimental comparison.

The development of numerical models, able to simulate the real behaviour of racks, enables producers to perform safe design of whole steel storage racking systems with the same types of tested structural elements and connections, or racks with elements whose geometric properties are comparable to those tested ones. Moreover, after validating or eventually calibrating the numerical model, it can be used for further advanced analysis, such as the Incremental Dynamic Analysis (IDA), with the aim of investigating the dynamic behavior of the rack under different levels of seismic intensity. These dynamic non-linear analyses allows a more realistic assessment of the structural capacity under seismic actions, but they can be reliable only if the numerical model was validated.

Conclusions Performance of fully loaded steel storage pallet racks has been investigated under severe horizontal loading. Rack specimens produced by international rack producers have been tested in their down-aisle (longitudinal) direction. From the global displacement-force response of the specimens, two major types of global behavior have been observed: ductile or brittle. In case of unbraced racks, brittle behavior is due to onset of soft storey mechanism, while, in case of braced racks,

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sudden failure of bracing connections did not allow the development of a diffused plasticity mechanism. The brittle behaviour is not in accordance with the design assumptions. With this regard, it is enough note the discrepancy in some cases between design q-factors and experimental ones. Therefore, if ductile design is adopted, a validation of the real structural behavior of racks in terms of global resistance, stiffness and ductility becomes a urgent need for industrial producers. This article shows how the execution of full-scale push-over tests on rack specimens is an effective and easy-feasible practice to respond to this demand. Moreover, test results can be used to validate and calibrate the numerical models of racks. Starting from numerical models able to correctly evaluate the real behaviour of racks, industrial producers can perform safe design of whole steel storage racking systems with the same types of tested structural elements and connections, or racks with elements whose geometric properties are comparable to those tested ones. The testing methodology developed within the “SEISRACKS2” project has been organized into a test protocol. Following such test protocol, recently, tests have been carried out in the laboratory facilities managed by Politecnico di Milano for two industrial producers, demonstrating the immediate impact of “SEISRACKS2” project into industrial sector of steel storage racking system.

References John A. Blume & Associates (1973) Seismic Design Examples of Industrial Storage Racks, Report prepared for the Rack Manufacturer’s Institute, San Francisco CA. Krawinkler H., Cofie N.G., Astiz M.A. and Kircher C.A. (1979) Experimental Study on the Seismic Behaviour of Industrial Storage Racks, Report No. 41, The John A. Blume Earthquake Engineering Center, Department of Civil Engineering, Stanford University, Stanford CA. Chen C.K., R.E. Scholl and J.A. Blume (1980) Seismic Study of Industrial Storage Racks, Report prepared for the National Science Foundation and for the Rack Manufacturers Institute and Automated Storage and Retrieval Systems (sections of the Material Handling Institute), San Francisco CA. ECCS N° 45 (1986) Recommended Testing Procedure For Assessing the Behaviour of Structural Steel Elements under Cyclic Loads, Technical Committee 1 – Structural Safety and Loadings Technical Working Group 1.3 – Seismic Design. Bernuzzi C. and Castiglioni C.A. (2001) Experimental Analysis on the Cyclic Behaviour of Beam-to-Column Joints in Steel Storage Pallet Racks, Thin-Walled Structures, (39), 841-859. Castiglioni C.A. (2003) Dynamic Tests on Steel Pallet Racks, Costruzioni Metalliche, 55 (3), 35-44. FEM 10.2.08 (2005) Recommendations for the Design of Static Steel Pallet Racking in Seismic Condition, Federation Europeen de la Manutention.

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Castiglioni et al. (2007) SEISRACKS RFSR-CT-2004-00045, Final Report, Research Program of the Research Fund for Coal and Steel RTD. Castiglioni C.A. (2008) Seismic behaviour of steel storage pallet racking systems, Structural Engineering Departement of Politecnico di Milano, Milano. Computers and Structures Inc., CSI Analysis Reference Manual for SAP2000®, ETABS®, and SAFE®, Berkeley CA. (2010) Brambilla G., Chiarelli G.P. (2014) “Steel storage pallet racking systems: full scale pushover testing and numerical analyses”, MSc Thesis, Politecnico di Milano. Castiglioni et al. (2014) SEISRACKS2 RFSR-CT-2011-00031, Mid-term Report, Research Program of the Research Fund for Coal and Steel RTD. EN16681 (2015) Steel static storage systems — Adjustable pallet racking systems — Principles for seismic design, CEN.

Annex

A- Experimental curves for “SEISRACKS2” project: Unbraced racks:

a. IPA1 b. IPB1

c. IPC1 d. IPD1

Figure 11 Experimental capacity curve of unbraced racks for “SEISRACKS2” Project.

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Braced racks:

e. IPA2 f. IPB2

g. IPC2 h. IPD2

Figure 12 Experimental capacity curve of braced racks for “SEISRACKS2” Project.

B- Experimental curves for consulting to SA :

a) SA1 b) SA2

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b) SA3

Figure 13 Experimental capacity curve of braced racks for consulting to SA

C- Experimental curves for consulting to SB :

a) SB1 b) SB2

b) SB3

Figure 14 Experimental capacity curve of braced racks for consulting to SB.

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