This paper is meant to deliver an overview on the damages and the following repairs of the

structures of the 26th and 27th floor due to the airplane accident occurred on April, the 18th 2002. An aircraft

112 TC Commander impacted the facade of the Pirelli skyscraper getting into the building, where its gasoline

tanks exploded. The first part of the paper focuses on a simplified analysis with the aims of evaluating the

blast impulse and the peak pressure caused by the explosion on the floors structures. Then, a description of

the realignment of the deck and the repairing operations are briefly summarized

Acito, Maurizio

Crespi, PIETRO GIUSEPPE

Franchi, Alberto

Migliacci, Antonio

This paper is meant to deliver an overview on the damages and the following repairs of the
structures of the 26th and 27th floor due to the airplane accident occurred on April, the 18th 2002. An aircraft
112 TC Commander impacted the facade of the Pirelli skyscraper getting into the building, where its gasoline
tanks exploded. The first part of the paper focuses on a simplified analysis with the aims of evaluating the
blast impulse and the peak pressure caused by the explosion on the floors structures. Then, a description of
the realignment of the deck and the repairing operations are briefly summarized

ACITO, MAURIZIO

FRANCHI, ALBERTO

CRESPI, PIETRO GIUSEPPE

MIGLIACCI, ANTONIO

Archivio istituzionale della ricerca - Politecnico di Milano

Protection of Historical Buildings, PROHITECH 09 – Mazzolani (ed)© 2009 Taylor & Francis Group, London, ISBN 978-0-415-55803-7The airplane crash at the “Pirelli” tall buildingA. Franchi, A. Migliacci, M. Acito & P. CrespiPolitecnico di Milano, Milan, ItalyABSTRACT: This paper is meant to deliver an overview on the damages and the following repairs of thestructures of the 26th and 27th floor due to the airplane accident occurred on April, the 18th 2002. An aircraft112 TC Commander impacted the facade of the Pirelli skyscraper getting into the building, where its gasolinetanks exploded. The first part of the paper focuses on a simplified analysis with the aims of evaluating theblast impulse and the peak pressure caused by the explosion on the floors structures. Then, a description of therealignment of the deck and the repairing operations are briefly summarized.1 PREMISESOn 18 April 2002, at 17.47 hours, a 112 TC Com-mander single-engine plane struck the facade of thePirelli skyscraper overlooking Piazza Duca d’Aosta(Fig. 1) between the deck beams of the 26th floor andthose of the 27th floor, near the central cross-section,penetrating inside the building (Figs 2-4).As the result of the collision and the explosion ofthe two fuel-tanks, located near the wings, the air-plane engine was severed from the fuselage, endingup outside the opposite facade that is facingVia FabioFilzi.The damage caused to the structures (Migliacci &Acito 2003, Migliacci et al. 2004b, Acito & Migliacci2004) on the 26th floor indicated cracks at mid spanof the longitudinal beams as well as at the fixed endsand along the border of the thin r.c. slab above thebeams (Fig. 5). That damages result in conspicuousdisplacements at mid span of the beams, ranging froma maximum of 25 cm of beam No. 5 to 14 cm of beamNo. 1 (Fig. 6).As regard to the deck of the 27th floor, the defor-mation evidenced an upward residual displacementFigure 1. Pirelli building before and after the crash.of 5–6 cm at mid span of beam No. 5, the mostseverely damaged one, confirming the hypothesis thatthe impulse due to the explosion of the gasoline tankswas the major cause of the permanent displacements.The problem of the realignment of the beams of the26th deck appeared to be of fundamental importanceFigure 2. Longitudinal section of the Pirelli building.537Figure 3. Floor plan of the Pirelli building.Figure 4. Floor section of the Pirelli building.Figure 5. Yield lines after concrete demolition.Figure 6. Residual displacements of the 26th floor beams.inasmuch as the mandate from the owner (PublicAdministration of Lombardia Region) was to attemptto preserve the existing structures. With this in mind,it was essential to force the beams upwards by usinghydraulic jacks on a pack prop.As the sequel of this paperwill show, achievement ofthis objective was made possible by relying on plasticdisplacement set up in the opposite direction in relationto the configuration after the explosion. Moreover, thereasoning was that it should be possible to arrive toa simple analytical model for estimating the forcesFigure 7. Plastic hinges model of a generic beam of thedeck.required for such a forcing process; such an estimationwould be necessary in order to make a suitable choiceof the jack types to be utilized on the field.2 ANALYTICAL EVALUATION OF THEIMPULSIVE LOADING DISTRIBUTIONThe analytical evaluation of the impulsive loading dis-tributionwhich has caused the deformed configurationof the 26th and 27th floors has been determined with aprocedure similar to that of a back analysis: knowingthe permanent displacements distribution of the floors,the load are evaluated back.2.1 Rigid plastic dynamic modelA simple rigid plastic dynamic model (Hodge 1959,Franchi et al. 1998) has been implemented as a possibletool for evaluating the response of reinforced concretebeams under blast loading. This model of a genericbeam is based on a three plastic hinges scheme con-necting two rigid elements (Fig. 7). The description ofthe kinematics of the system is expressed in terms ofrotation, rotation velocity and rotation acceleration ofthe plastic hinge placed at the end of the beam.By assuming that the initial conditions of motion(t = 0 s) are characterised by zero rotation and rotationvelocity of the plastic hinges and by calling with Athe cross section area of the beam, with ρ its densityand with m its mass for unit length, the definition ofimpulse loading I reads:where l is the length of the rigid link between twoplastic hinges (half of the total length of the beam).By assuming that the motion after the blast is a freedumped motion in which the rotational accelerationof the plastic hinge can be considered constant, itsvalue can be determined by imposing that, during themotion, the sum of kinetic energy K , load potential Uand energy dissipatedD is constant, or, in terms of ratequantities:538Table 1. Beams data.2l b m M+S M−S M+E M−EBeam m m kg/m kNm kNm kNm kNm1 24.69 1.31 982 945 226 785 12892 24.54 1.61 963 948 226 787 12803 24.43 1.63 963 948 226 787 12804 24.39 1.63 963 948 226 787 12805 24.09 2.46 1543 1175 269 923 1819whereW =−U is the load work. By expressing K ,Wand D as:where M+S and M−E are the absolute values of themid span positive (bottom fibres in tension) and fixedend negative ultimate moments, determined at steelyielding in tension, and g is the gravity acceleration.The time tf required to stop the dynamic process canbe determined on the basis of the value of the rotationalaccelerationobtained from the solutionof equation (2).The mid span final displacement u+f measured at timetf (Franchi et al. 2005), positive if downward, is:In the sameway themid span final displacement u−fof a beamwhich has an impulse force from the bottomto the top may be expressed as:whereM−S andM+E are the absolute values of the midspan negative and fixed end positive ultimate momentrespectively.2.2 Blast loading distributionThe essential data required for the determination of theblast impulse are resumed in Table 1 for the variousbeams (Nervi 1960, Ponti 1960). They are: span 2l,influence width b, mass per unit lengthm (comprisingself weight and dead loads) and ultimate moments.The blast impulse caused by the explosion on thebeams of the 26th and 27th floor of the building canbe determined by means of the equations (4) and (5)starting from the measured permanent deflections ofthe beams. The computed impulses range from 28 to60 kNs in downward direction, on the 26th deck, andfrom 6 to 9 kNs in upward direction on the 27th floor(Tables 2 and 3).The blast pressure can be assumed as a triangularpulse characterised by a peak pressure pm and a dura-tion td (see Fig. 8). The consequent blast impulse I ismeasured by the area under the time-force curve:Figure 8. Blast impulse pressure during time.Table 2. Blast impulses and peak pressures on 26th floor.u+f I A pmBeam mm kNs m2 MPa1 90 28.20 10.7 4.042 130 33.76 13.1 3.963 165 38.11 13.3 4.424 185 40.39 13.3 4.695 207 60.29 19.8 4.70Table 3. Blast impulses and peak pressures on 27th floor.u−f I A pmBeam mm kNs m2 MPa1 −22 6.00 10.7 0.862 −24 6.47 13.1 0.763 −35 7.90 13.3 0.924 −45 8.99 13.3 1.045 −63 7.42 19.8 0.58where A is the influence area of the floor on whichthe pressure is considered constant, assumed equalto the influence width b times one third of the spanof the beam.Assuming that the blast duration td is 1.3ms (Ngoet al. 2007) and knowing the blast impulse I , the peakpressure can be calculated from equation (6). Thedistribution of blast impulses and peak pressures onthe various beams is presented in Tables 2 and 3. Itis important to remark that only the plastic part ofthe displacements u+f and u−f has been used in thesecalculations, according to the rigid-plastic hypothesisassumed in the described analytical model.The obtained peak pressures are about 4MPa indownward direction on the 26th floor with a distribu-tion showing higher values in correspondence of theinner beams (No. 4 and 5). Lower values (about 1MPa)are applied in the upward direction on the 27th deck.3 REALIGNMENT OF THE 26TH FLOORIt has been recognized that the first operation to becarried out, before any type of consolidation of the539Figure 9. Hydraulic jacks distribution.structures, had to be the realignment of the princi-pal beams of the 26th floor. Only after the realign-ment, in fact, it is conceivable to remove the dam-aged concrete (class C20/25), substitute those partsof the bars which did suffer inelastic strains espe-cially in compression (Park et al. 1982), reconstructthe concrete sections with special self-compactingmortar (fck ,cube = 49MPa), and finally to apply a post-tensioning steel system which will increase the safetylevel of the repaired structure.3.1 Operative procedureThe operative procedure had to employN. 15 hydraulicjacks (see Fig. 9) positioned at the intersection of the5 longitudinal beams with the 3 transversal beams;other 4 jacks where added in between the previouspoints, only for beam No. 3 and 5, in order to bet-ter distribute the uplift loads, especially for the mostdamaged beams.The jacks were placed in contact to the bottom faceof the beams bymeans of provisional steel beams fixedto the floor. Close to the jack points some steel tubularprops follow the realigning process, by adapting theirheight, step by step, following the movements of thebeams.The idea was that to force the beams up to collapsein the upward direction and then to move the mid spansection of each beam over the horizontal line of a dis-placement equal to the elastic displacement caused bythe same 3 point loading forces.The evaluation of the upward loads to be appliedin order to obtain the realignment can be made byanother simple model made of three plastic hinge con-nected by rigid elements. The required displacement,upward the horizontal line, in order to obtain a flatfloor after removal of the applied uplift forces can bedetermined too.3.2 Analytical model for the evaluation of the totalrealignment forceThe simple model adopted for the realignment processis shown in Figure 10. The applied forces are threepoint loads equal to F /3, representing the action of thehydraulic jacks, and the self weightmg per unit length,the other quantities follows the same notation of theprevious paragraphs.Figure 10. Model for the realigning process.The limit load for the forcing up of the beams of the26th floor Fu may be expressed as:By applying equation (7) to the five beams of 26thfloor and summing the obtained forces, a total force ofabout 2600 kN is obtained. The experimental resultsproved the goodness of the assumptions.3.3 Evaluation of the displacementThe model for the evaluation of the beams deflectionof the 26th floor in service conditions was that of afixed end beam subjected to uniform distributed loadof 10 kN/m. The mid span displacement measured bymeans of experimental tests is around 2 cm, for a corre-sponding average elastic stiffnessEJ = 432000 kNm2,comprised between the un-cracked and fully crackedstiffness.The actual elastic stiffness, reduced by the degra-dation due to the explosion and the realigning pro-cess, should be assumed about half the original, i.e.200000 kNm2, due to the fact that only the criticalsections suffered limited damages.By assuming a fixed end beam subjected to threepoint loads equal to F /3, the elastic displacement f atmid span up to the horizontal line will be:Therefore the target displacement during the upliftprocess for beam No. 5 was fixed into 12 cm.3.4 Realignment processForcing operations have been obtained by applying theuplift loadsF evaluated in section 3.2.The load controlsystem consisted of an oil distributor which controlled,through suitable valves, the oil flow in each jack. Thesequence of the loading history has been organized sothat the final load was subdivided into steps: insideeach loading step, single beams have been loaded oneafter the other starting fromNo. 5 to finish with No. 1.The loading of each single beam has been obtained, ateach loading step, by increasing the lateral loads andkeeping the central load constant, and then vice versa,540Figure 11. Prop and hydraulic jacks at beam No. 3.Figure 12. Displacement monitoring during forcing pro-cess.i.e. increasing the central load by keeping the lateralloads constant.The 25th, 24th, 23rd and 22nd floors have been stiff-ened provisionally by pack props in order to distributeon four floors the total uplift load applied to the 26thdeck, such that each floor would take no more than theallowable service load.The displacement monitoring system was made ofNo. 15 displacement electrical transducers connectedto a digital data acquisition system. The displacementtransducers were located between the 26th and 27thfloor, by considering the 27th floor as a fixed refer-ence system. The total time required for the realigningprocess was about 30 hours. Figure 11 shows a pic-ture of the hydraulic jacks and props system, whileFigure 12 shows the real time displacement diagramduring forcing phase.A picture of the mid span plastic hinge of beamNo. 5 during the final stages of the process is shown inFigure 13. It is remarkable the buckled configurationof the compressed bars.4 STRENGTHENING OF THE BEAMS BYMEANS OF POST-TENSIONIG TENDONS4.1 Post-tensioning designA post-tensioning system, acting on the 5 damagedbeams of the 26th floor, has been conceived in orderto improve the flexural stiffness of the entire floor(Migliacci et al. 2004a).Figure 13. Plastic hinge at mid span of beam No. 5.Figure 14. Post-tensioning geometry, equivalent loads andbending moments.The system is made, for each beam, of two post-tensioned cables (each one made of 5 strands of 0.6′′,placed on one side of the beam) acting as a reversearch, with the anchorages located near the transversebeams and the crown in the middle of the longitudinalbeams where the cable deviation device is positioned.The post tensioning cable inclined geometry is pre-sented in Figure 14 together with the system of staticloads equivalent to the post-tensioning action. AxialN , shear V and bending M actions are applied incorrespondence of the anchorage, while, due to theparticular selected geometry of the cable, an upwardload 2V is applied in the middle of the beam. Thebending moment diagram induced by post-tensioningon the statically indeterminate scheme of the longitu-dinal beam, with built-in ends in the core walls of thetower, is also shown. The diagram shows the goodnessof the designed post-tensioning in order to carrying onthe applied load.The architectonic implications have been also con-sidered in the selection of the tendon geometry.4.2 Anchorage devices: “noses”The tension load present in the cable is transferred tothe longitudinal beam by means of a couple of anchor-age devices, placed at both sides of the beam web.The anchoring devices, named “noses”, are made of aparticular reinforced concrete characterised by an highworkability, cast in place in a stainless steel (AISI 304)formwork. The idea is that to fix the noses to the sidesof the beam web, as is usually done in the externalcables post-tensioning system, by means of anotherpost-tensioned joint.541Figure 15. Tensioning of the threaded bars fixing the noseto the side of the longitudinal beams.Figure 16. Cables deviation device.Themonolithic connection betweennoses andbeamis realized by the tensioning of 8 threaded bars whichcross the beam web, in which has been previouslydrilled 8 holes, and the two noses placed on its sides,connecting each other (Fig. 15). The threaded bars aremade of duplex stainless steel with an yield strengthof fyk ,0.2 ≥ 700MPa and a 30mm diameter.4.3 Deviation deviceThe device placed in the middle of the longitudinalbeamwhich allows the deviation of the post-tensioningcables, developing the vertical sustaining load 2V , ismadeof stainless steel (AISI 304)weldedplates havingthe particular geometry shown in Figure 16.5 CONCLUSIONSThe realigning process has requested, or stimulated,to solve several problems and, among the others, thefollowing are the most significant:– the evaluation of the impulse and peak pressuresdistribution during the explosion of the aircrafttanks which has caused the severe damages of thestructures of the 26th and 27th reinforced concretefloors;– the evaluation of the total force to be applied byhydraulic jacks in order to obtain the realignmentof the 26th floor;– the evaluation of the maximum displacement overthe horizontal line necessary in order to obtain a flatplane after removal of the forcing jacks action, witha maximum error of 2–3 cm;– the operative technique able to implement and tocontrol the forces and displacements during therealigning process;– the design of a post-tensioning system of exter-nal cables capable to increase the strength of thedamaged beams of the 26th deck, comprising theanchorage and deviation devices;– the restoration of the plastic hinges sections in themiddle and at the end of the beams with concreteand reinforcing steel similar to the original one.ACKNOWLEDGEMENTThe authors acknowledge the high professional skillsof P&P Consulting Engineers, ISMES-LMC andGrassi & Crespi technical staff which has greatlycontributed to the success of the whole operation.REFERENCESAcito, M. & Migliacci, A. 2004. Il progetto e la tesatura deicavi nel risanamento strutturale dell’impalcato del 26◦piano del grattacielo Pirelli. Congresso AICAP,Verona.Franchi, A., Acito, M., Crespi, P. & Migliacci, A. 2005. Ana-lytical and experimental procedures for the realigning ofthe 26th floor of the Pirelli tall Building after the air-plane crash on April 2002. Proc. IABSE InternationalSymposium, Lisbon, Portugal, 14–16 September 2005.Franchi,A., Genna, F. & Corradi, L. 1998. On the determina-tion of bifurcation and limit points. Journal ofEngineeringMechanics, 124 (8): 866–874.Hodge, P.C. 1959. Plastic Analysis of Structures. Mc GrawHill.Migliacci, A. & Acito, M. 2003. La Vicenda del grattacieloPirelli – il restauro. Roma: Mancosu Editore.Migliacci, A. et al. 2004a. Modelli numerici perl’interpretazione del comportamento del c.a. in regimenon lineare: la vicenda del grattacielo Pirelli. CongressoCTE, Bari, Italy.Migliacci, A., Acito, M. & Franchi, A. 2004b. Il progetto dirisanamento delle strutture del grattacielo Pirelli. Studiesand ResearchesV. 24. Italcementi S.p.A., Bergamo.Nervi, P. 1960. L’ossatura. Edilizia Moderna, n. 71.Ngo,T.,Mendis, P., Gupta,A.&Ramsay, J. 2007. Blast Load-ing and Blast Effects on Structures – An Overview. EJSESpecial Issue: Loading on Structures: 76–91.Park, R., Priesley, M.J.N. & Gill, W. D. 1982. Ductilityof Square Confined Concrete Columns. Journal of theStructural Division, ASCE Vol.108 n◦ST4: 929–950.Ponti, G. et al. 1960. Il Progetto. Edilizia Moderna, n. 71.542

The airplane crash at the "Pirelli" tall building

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The airplane crash at the "Pirelli" tall building

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