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- 1. Journal of Building Engineering 67 (2023) 105970 Available online 31 January 2023 2352-7102/© 2023 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Journal of Building Engineering journal homepage: www.elsevier.com/locate/jobe Structural robustness analysis of RC frames under seismic and blast chained loads scenarios Mattia Francioli*, Francesco Petrini, Franco Bontempi Dept of Structural and Geotechnical Engineering, Sapienza University of Rome, Rome, Italy A R T I C L E I N F O Keywords: Structural robustness Multi-hazard Hazard chain Blast Earthquake A B S T R A C T In this paper the structural robustness assessment of concrete frame buildings under blast and un- der earthquake blast hazard chain scenarios is investigated. A deterministic methodology for con- necting the robustness with the blast hazard intensity and for conducting the robustness analysis under earthquake-triggered blast is presented and applied to a 3D RC frame building by imple- menting nonlinear time history analyses considering both plastic behavior and large displace- ments. A preliminary sensitivity analysis on a 2D frame is conducted to identify the critical analy- sis parameters influencing the results. The robustness curves (residual structural capacity versus the level of damage occurring in the structure), evaluated both for the blast-only and for the earthquake-blast chained cases, are compared by considering different explosion locations inside the building (location of the blast-induced structural damage). Results show that neglecting the chained load scenarios would lead to the identification of an erroneous location as critical for the structural robustness performance. 1. Introduction Structural robustness is a global attribute required to structures when subjected to a local damage which, if not confined, can spreads inside the structural system until disproportionated damage occurs with the consequent mining of the global integrity ([1]). Progressive collapse is one of the most dangerous disproportionate consequences that can occur in structures when adequate struc- tural design for robustness is not put in place ([2–4]). This topic has gained attention especially since the second half of the twentieth century, when disproportionate-progressive collapse events occurred in a number of structures. Since then, the increased interest in robust resistive mechanisms that are developed in RC and steel buildings and which allow structures to resist progressive collapse has led to the development of research ([4–7]) and standard, codes or guidelines ([8–11]), that provide indications on specific aspects and construction details which are crucial for the structural resistance to progressive collapse. The progressive collapse can be initiated by many factors such as blast loading from explosives or gas leakage, design/construc- tion errors, vehicle/debris impact, and other loadings such as fire and earthquakes ([4]). The assessment of robustness performances under specific threats, and the provision of hazard-specific robustness design solutions is crucial in modern buildings to avoid pro- gressive collapses consequences, unacceptable losses of lives or property ([7,12]). Despite to this evidence, the structural robustness is commonly assessed by a well-established procedure called “damage-presumption approach”, that is a “hazard-independent” analysis where a certain damage level is assumed for the structure (for a framed structure typically consisting in the removal of a column), and the residual capacity of the structure is then evaluated. In some guidelines, this procedure is called "alternative path analysis” (exten- sively discussed in DOD [8] and GSA [10] guidelines) and can be implemented by using both static and dynamic, linear or nonlinear * Corresponding author. E-mail addresses: mattia.francioli@uniroma1.it (M. Francioli), francesco.petrini@uniroma1.it (F. Petrini), franco.bontempi@uniroma1.it (F. Bontempi). https://doi.org/10.1016/j.jobe.2023.105970 Received 2 July 2022; Received in revised form 10 January 2023; Accepted 25 January 2023
- 2. Journal of Building Engineering 67 (2023) 105970 2 M. Francioli et al. analysis. The damage presumption approach is widely used in the literature, as the removal of a column is a typical design scenario to consider in robustness assessment [13]. Common damage-presumption approaches, indeed, do not directly and univocally link the presumed damage to a specific hazard and its intensity, as currently required by modern Performance-Based Design (PBD) approaches ([14]). Also in DOD ([8]) or GSA ([10,11]) guidelines, the reference to the explosion is made only to indicate that this hazard may be a damage-triggering cause, without specifying any direct connection between the explosion intensity and the occurred damage. This is something matching with the idea of interpreting the robustness as a structural intrinsic characteristic ([15,16]). On the other hand, some well-established guidelines for evaluating the behavior of critical facilities in case of explosions ([17–19]), are mainly referred to terrorist acts and introduce practical measures to avoid attacks on sensitive parts of the structure, and do not focus on structural ro- bustness for blast. In addition, some natural disasters occurred in recent years shown that most dangerous scenarios for structural robustness perfor- mances are those called “hazard-chains” ([20]), where a certain initial hazard strikes on the structure, which can suffer a certain dam- age from it, and then a subsequent hazard impacts as well, affecting the structure already damaged by the previous one. The initial hazard of the chain has multiple consequences which can affect the structural robustness in the hazard-chain scenario ([21]): - first, as already said, it can induce a damage who decreases (and in generally changes) the structural robustness of the structure to the subsequent hazard; - second, the initial hazard can “trigger” the subsequent one. This is the case where a strong seismic shaking can damage electrical system or gas pipes inside the building, causing earthquake-induced fires or explosions. When the behavior of structures in case of explosions is extensively discussed ([9]), the explosion is therefore not intended trig- gered by another main hazard. In this paper, the structural robustness analysis of a RC frame building under blast and under earthquake-induced blast hazard chain scenario is conducted. Although probabilistic approaches have been developed in literature for robustness analysis of structures (e.g., Ref. [22]), all the analyses of this paper are conducted in deterministic terms, in case of hazard chains it is assumed that a high intensity earthquake striking the structure causes a subsequent explosion compromising the integrity of the structure. Probabilistic as- pects of the problem are not taken into account, then any reference is made to the occurrence of the single hazards, or to the probabil- ity of a fixed intensity earthquake to trigger an explosion inside the structure, or to the probability of an earthquake or an explosion to cause a damage; all these events are deterministically assessed or assumed. The above-mentioned missing link between the blast-induced structural damage to the blast intensity in robustness assessment context, is the first literature gap covered by this paper: specific sensitivity analyses are carried out to highlight the issues arising when the robustness performances must be correctly linked to the considered hazards, by making specific reference to the blast- induced damage, and by introducing novel “blast scenario-dependent robustness curves”. The second point addressed by the paper concerns the robustness analysis under hazard-chain scenarios: the design implications related to the consideration of earthquake in- duced explosion scenarios are highlighted by carrying out robustness analyses under hazard-chain scenarios and providing new in- sights regarding the multi-hazard structural design. The purpose of this article is not to provide design indications aimed at preventing progressive collapse, for which the reader is re- ferred to the various guidelines present in the literature. The intention here is to highlight the importance of the concatenation of events for the design purposes. Thus, the goal of the paper consists in giving new insights on the robustness implications related to the possible occurrence of haz- ard-chain scenarios, and to propose a deterministic procedure to assess them. There are significant studies in literature relating the blast intensity with the damage/destruction of RC columns (e.g., Ref. [23]) or with the progressive collapse of RC frames (e.g., Ref. [24]), but they do not evaluate the robustness of the structure in terms of the so-called “robustness curves” ([25]) which, as detailed below, are a straightforward tool to extend the robustness analysis at hazard-chain scenarios. With specific regard to the progressive collapse event, as a matter of fact it is not analyzed in the present paper, but it is rather indicated to potentially occur when some thresholds are exceeded by appropriate structural response indicators (excessive values reached by vertical relative or absolute dis- placement of the nodes at the top of the blast-destroyed columns). 2. Materials and methods 2.1. Numerical procedure for deterministic structural robustness quantification of RC frames by the damage presumption approach As already said, in the damage presumption approach ([26]), the dynamic response of the frame under the sudden removal of a number “n” of columns is evaluated by a nonlinear dynamic analysis (NDA) starting from the static equilibrium configuration reached by the non-damaged structure under vertical loads (generally due to the seismic “permanent+0.3*variable” mass combination). By the sudden column removal, the NDA can lead to ([27]): i) a new static equilibrium condition characterized by some residual plastic displacements after an initial damped transitory phase or, ii) the collapse of the structure. The collapse can be defined to occur when: a) the time-response and/or the load-response diagrams are unconfined by certain boundary limits ([28]) (runaway behavior), or b) the vertical relative drift (DV) between the beam-column nodes located around the removed column reaches the value of 20%. The latter is calculated starting from the vertical displacement of the node at the top of the removed column δV and the length of the beam Lb to which the node belongs. DV = tan−1(δV / Lb) (1)
- 3. Journal of Building Engineering 67 (2023) 105970 3 M. Francioli et al. Moreover, it is possible to assume that the collapse of the structure occurs when: c) the vertical displacement ratio (largest ratio of nodal vertical displacement to the story height) at the base floor, exceeds a specific threshold of 0.5 ([28]). If the outcome of the NDA is not the collapse, an incremental static nonlinear analysis of the structure is carried out under lateral forces (pushover) in order to evaluate the residual capacity of the damaged structure ([25]). Although common approaches use the pushdown analysis to assess the residual capacity of the structure under gravity incremental loads ([12,29]), the reasons for using pushover instead of pushdown is that pushover can be more significant than pushdown in case of multi-hazard scenarios/studies, in which one of the involved hazards acts on the frame by horizontal load patterns [30]. For example, by carrying out a pushover, the damage and the capacity under earthquakes can be coherently considered in the robustness analysis. This is particularly significant in earthquake-induced explosions, where the evaluation of the residual capacity of the blast-damaged structures under earthquake after- shocks is crucial. In this way, each damage level “DL” (i.e., number “n” of simultaneously removed columns) is associated to a resid- ual lateral force capacity (λu) as evaluated by the pushover and expressed as percentage (λu/λ %) of the pushover capacity (λ) of the non-damaged structure. As a result of the aforementioned numerical outcome, the deterministic robustness of the structure can be quantified and effi- ciently represented by the so-called “robustness curves” as introduced by Olmati et al. (2013) [25]. Robustness curves are represented on a cartesian plane in which on the x-axis there is the DL suffered by the structure (“n” in the text above), while on the y-axis the cor- responding residual force capacity percentage (λu/λ % in the text above) is reported. See for example the robustness curves repre- sented in Fig. 1, where different colors and markers represent different locations along the structure for the presumed damage initia- tion. The main indications about the global robustness performances which can be obtained by the robustness curves are: i) the maxi- mum DL that the structure can suffer without collapsing, and ii) the residual capacity of the damaged structure at different DLs. Other considerations can be made by focusing on the steepness of the curve: the steepness of the robustness curve when the DL is incre- mented, is proportional to the decay of the residual capacity of the structure. In other words, starting from a certain DL (and a certain residual capacity), the greater is the steepness of the robustness curve when the damage is incremented, the larger is the decrement of residual capacity suffered by the structure due to the damage increment. By these considerations, and with reference to the robustness curves examples in Fig. 1, it can be said that for n < 2 the location 2 is more critical for robustness than the location 1, while for n = 3 the location 1 become critical, leading to the collapse of the structure (no residual capacity). The response to the initial NDA (typically represented by the time history of the vertical displacement of the node at the top of the removed column), the successive pushover analysis and, consequently, the obtained robustness curves, are strongly influenced by several parameters regarding the nature of the damaging hazard, the analysis procedure, or the structural model. One of these para- meters is the removal time interval (Δtd) for the column: the less is the Δtd, the more severe is the consequent structural response, and the less is the residual capacity obtained by the pushover [27]. In this view, the correct identification or setting of the Δtd for a certain “n” would be of value. 2.2. Numerical model and robustness sensitivity analysis To understand the importance of the Δtd parameter related to the procedure described above, the RC 2D frame test structure shown in Fig. 2a is considered. The structure is modeled using SAP2000® commercial code. The nonlinear behavior is implemented using the approximation of plastic hinges, which are obtained from the bending moment-rotation relationship (M-θ) evaluated from the equations provided by the Italian Standards NTC2018 (2018) [31] by considering the influence of the axial load as shown in Fig. 3. Moreover, geometric nonlinearity is considered by a large displacement analysis formulation. Fig. 1. Example of robustness curves.
- 4. Journal of Building Engineering 67 (2023) 105970 4 M. Francioli et al. Fig. 2. (a) 2D RC frame structure (sizes in m) and (b) Pushover curves after different locations of damage (please note that the maximum displacements reached in pushover analyses are unrealistic and are obtained by forcing the numerical convergence). Fig. 3. Example of bending moment-rotation relationship for plastic hinges. The sensitivity analysis is carried out by the damage presumption approach, then it contemplates both a global NDA and the sub- sequent pushover analysis, the last one allowing the evaluation of the residual capacity of the structure after the considered DL (where DL = number of removed columns). The NDA is conducted by the following steps (see Fig. 4): 1. First, a static analysis under vertical loads is conducted to evaluate the reactions at the top of the columns that will be removed in the subsequent step (column-reaction forces). This will allow to develop a “surrogate” FE model of the structure in which the removed columns are substituted by the corresponding column-reaction forces, which reaches the same static equilibrium of the previous model under vertical loads. 2. In the second step of the analysis the surrogate FE model is used to conduct the NDA under sudden the columns removal. Here the two subsequent sub-steps are implemented: i) the vertical loads and the columns-reaction forces are applied by a slow ramp, and; ii) the columns-reaction forces are suddenly removed by a drop-to-zero ramp of duration Δtd (columns removal time- interval); If the structure does not experiment the collapse at step 2 above, the pushover analysis is conducted by implementing triangular forces and by bi-linearizing the response curve as shown in Fig. 6a. The numerical models used in the above-described procedures are developed by implementing well-established techniques (plas- tic hinges, pushover or nonlinear dynamic analysis), and the results have been compared in qualitative and quantitative terms with al- ready published results from the literature, especially with results provided in Refs. [28,32] (shape of the pushover curves or of the time histories) and in Ref. [27] (obtained values). Depending on the location of the damage (location of the first removed column), the results obtained by the lateral pushover analysis are different (Fig. 2b): since the internal columns (n°3, n°4 and n°5) are characterized by a large tributary area for vertical loads, the damage to one of them could cause a bigger reduction of the capacity with respect to the one caused to damages at external columns. Focusing on location 3 (see Fig. 2a), some sensitivity analyses have been carried out by varying the columns removal time interval Δtd; the variation of this parameter influences the behavior of the structure in terms of residual displacement after column removal as
- 5. Journal of Building Engineering 67 (2023) 105970 5 M. Francioli et al. Fig. 4. Two step procedure for the sudden column removal analysis: static analysis under vertical loads (a); NDA under sudden column removal with columns removal time-interval Δtd. Fig. 5. Displacement of the node at the top of column 3: effect of variation of Δtd (expressed in seconds). Fig. 6. Bi-linearization of the pushover curve (a) and pushover curve of 2D RC frame structure – DL = 1: effect of variation of Δtd (expressed in seconds) (b).
- 6. Journal of Building Engineering 67 (2023) 105970 6 M. Francioli et al. obtained by the NDA, the ultimate load and deformation as obtained by the pushover analysis, and then it changes the resulting ro- bustness curve. Figs. 5 and 6 show the effect of the variation of Δtd for DL = 1 at location 3. As it appears in Fig. 5, which shows the results of the NDAs after the column removal, the effects of the dynamic amplification in terms of displacement are captured since the peak and residual displacement of the node at the top of the removed column increases as Δtd decreases. There is a small difference between the cases with Δtd = 0.5 or 0.3 s and between Δtd = 0.01 or 0.02 s. It is also noted that the value of Δtd has a certain influence on the amplitude of the oscillations around the residual displacement and on the damping time shown in the time histories. The capacity curves obtained by the pushover analysis, are bi-linearized for sake of simplicity, and the maximum capacity of the damaged structure FY is obtained by imposing the equality of the areas underpinned by the bilinear curve and by the pushover capac- ity curve for a fixed “collapse” displacement dU (Fig. 6a). When the maximum lateral displacement value dmax reached by the pushover analysis is lower than the fixed dU, then the curves are bi-linearized with reference to dmax. Fig. 6b shows the bi-linearized results of pushover analyses for location 3 with DL = 1 and how the variation of Δtd affects the capacity of the damaged structure. Al- though there are just slight differences between the curves for the considered cases, it is shown that if the Δtd value decreases, the overall capacity decreases as well. It is important to understand the effect that different values of Δtd have in terms of decreasing the capacity of the structure because this parameter can be used to simulate the damage induced by different blast scenarios. In Fig. 7, the effect of the variation of Δtd on the obtained robustness curves for damage at location 3 is shown: the decreasing of Δtd determines a decrease of the robustness. The DL = 2 (i.e., two columns simultaneously removed) always determines the progres- sive collapse of the structure due to the spontaneous collapse of the column adjacent to the initially removed one, while DL = 1 causes a drop in initial capacity of about 20%. In the figure, DL = 0.5, corresponds to the loss of the 50% of the column cross-section; this means that the explosion results in a loss of element stiffness and capacity, but not in a collapse. The decreasing of the pushover residual capacity at shorter Δtd it is due to the larger damage suffered by the elements adjacent to the removed column (e.g., beams) as a consequence of the large vertical displacement experimented by the nodes at the top of the re- moved columns (as shown in Fig. 5). 2.3. Blast-scenario dependent robustness (BSR) curves As already said, the robustness curves obtained with the damage-presumption approach: i. Are sensitive to the Δtd value; ii. Are not “hazard-specific” In order to take into account for the above-mentioned sensitivity of the robustness to the column removal time interval (Δtd) and to pertinently connect the robustness with the blast hazard, Francioli et al. (2021) [27] proposed a procedure for determining the so- called “blast-scenario dependent robustness (BSR) curves” for RC frame structures, which take into consideration the parameters of the specific blast hazard, like the blast intensity and impulsive rate. The BSR curves are obtained by a two-stage analysis where, as a first step, the vulnerability of the frame's columns is assessed by capacity (pushover) and demand (nonlinear dynamic) analyses car- ried out by a “local” model (single column under lateral load). By the local NDA of the column, each damage level DL is associated to damage time interval Δtlocal, the last one being defined by the time interval ranging from the explosion instant to the instant in which the considered damage occurs (column lateral deformation is larger than a fixed threshold value dU). The damage time interval Δtlocal is then strictly related with the intensity parameters of the blast: for detonations, pertinent intensity parameters are the equivalent Kgs of TNT joint with the stand-off distance, while for deflagrations, peak pressures acting on the blast-invested elements should be considered as intensity parameter ([33]). In both cases, lower Δtlocal values can be reasonably associated with higher blast intensities. By assuming that Δtd = Δtlocal, if the previous mentioned local analysis is not carried out, the reasonable assumption where lower Δtd values are associated to larger DLs can be made in conducting the successive global analysis step (described below). To clarify this point, reference can be made to Fig. 8, where a generic location inside a 3D frame structure a structure is depicted in which an explo- sive scenario can occur: the damage level DL = 1 is associated with one removed (i.e. destroyed by the explosion) column (i.e., C1), Fig. 7. Robustness curves for 2D RC frame structure for location 3: effect of variation of Δtd (expressed in seconds).
- 7. Journal of Building Engineering 67 (2023) 105970 7 M. Francioli et al. Fig. 8. Example of a generic location and a stand-off distance. while DL = 2 involves the simultaneous removal of C1 and C2; consequently DL = 3 corresponds to the failure of C1, C2 and C3) while DL = 4 involves the loss of all the four columns and thus of the capacity of the structure. By assuming that, when more than one column is simultaneously removed (cases with DL > 1), the stand-off distance (minimum distance of the explosion from the removed structural elements) is kept constant, in order to have an increasing damage level, it is necessary to increase the explosion intensity, which in other words involves having a shorter removal interval associated with the ele- ment C1 (nearest column to the explosion location). Then, as already said, when the Δtd associated with DL = 1 is identified, larger DLs are associated to shorter Δtds. As a second step, the BSR curve is obtained by the damage presumption approach, where each DL is analyzed by a “global” NDA (whole building under sudden column removal) and using a removal time interval (Δtd) for the columns, previously associated to each of the considered DL. The procedure for the evaluation of the BSR curve for a certain damage initiation location inside the build- ing in case of detonation explosions is shown in the flowchart reported in Fig. 9. Given a certain DL = n, the DL = n+1 is obtained by removing an additional column which is adjacent to the “n” columns removed at the previous DL. The “n” columns in a certain damage scenario are removed simultaneously by using the Δtd = Δtlocal already associated to that DL = n at the first step. The proce- dure is carried out for different locations to obtain a set of BSR curves. The BSR curves, on the contrary of those obtained by the classi- cal damage presumption approach, are hazard-dependent, in the sense that each DL is joint to the intensity of the blast hazard, which is directly associable to (and identified by) the parameter Δtd (= Δtlocal). 2.4. Robustness analysis under hazard-chain scenarios The scenarios when two chained hazards impact on the structure is one of the most demanding for robustness requirements. A typ- ical chain of hazards is the one implying the sequence of earthquake as triggering action and explosion and/or fire as chained or trig- gered one. This is what can occur when a big earthquake strikes on the structure: it may cause structural damages and, at the same time, damages to electrical/gas pipes to generate an internal fire or explosion. It would be important to understand to what extent the blast robustness of a structure is changed from the damage suffered by an earthquake, and if the considerations of such a hazard chain scenario provides new design indications. As already said, it is not the goal of this paper to investigate probabilistic aspects of the haz- ard chain scenarios (e.g., probability for an earthquake to trigger an explosion), while the goal here is to investigate the structural consequences of the earthquake-induced damages on the blast structural robustness of an RC building. For this reason, the robustness analysis under hazard chain scenarios is conducted in deterministic terms. The robustness deterministic analysis under blast after earthquake is similar to the one under the blast hazard only, but it is conducted on the structure when it is already damaged by the earthquake: thus, a nonlinear time history seismic response analysis must be carried out before applying the procedure outlined in previous sections. The robustness analysis under earthquake-triggered hazards make sense only for high-intensity earthquakes, which can induce the damage in the structure before the blast occurs. A fruitful representation of the robustness under hazard chains is on the same graph of the “blast only” robustness curve of the same structure, meaning that the residual capacity percentage λu/λ % should be factorized by the residual capacity of the undamaged structure (see Fig. 10). This is something allowing the correct quan- tification of the robustness decrement due to the earthquake-induced damage. 3. Calculation: application to a 3D RC frame The procedure for the BSR curves evaluation has been applied to an existing 3D RC structure serving as hospital building (Fig. 11). The building has a floor extension of about 42x35 m and a global height of 28 m (7 floors). It is a RC structure made of concrete C28/ 35 and steel B450C under the classification of Italian Standards ([31]), the increasing of the concrete elastic modulus (Ec) and of the
- 8. Journal of Building Engineering 67 (2023) 105970 8 M. Francioli et al. Fig. 9. Flowchart of the procedure to evaluate the BSR curves in case of detonations ([27]). concrete strength (fc) under impulsive load (strain rate effect) has been taken into account by assuming Ec = 42510 MPa and fc = 41.73 MPa for a C28/35 concrete. The steel rebars elastic modulus Es is taken equal to 210000 MPa, while the yielding stress fy (increased under impulsive load) is 510 MPa (B450C steel grade). The structure has been designed 40 years ago under the Italian Standards both for vertical and seismic loads. The seismicity of the site has been updated in Standards about 20 years after the construction, something that required a successive seismic vulnerability check in consideration of the increased seismic hazard. The rebars presents some specific details provided for shear strenght at the beam-column connections as shown in Fig. 12, something implying the changing of the longitudinal rebars in beams spanning from the nodal regions towards the midspan as shown in the figure. This has been coherently taken into account in the evaluation of the beam bending capacities in the FE model. Being the structure an Hospital, Serviceability Limit States (SLSs) requirements were very strict, both for interstorey drift ratios (limitation of damage in non-structural elements) and for floor accelerations (protection of clini- cal equipments), full details of the design structural requirements are provided in Refs. [34,35]. As for the demonstrative 2D frame structure analyzed in previous sections, the finite element model has been realized by the com- mercial structural code SAP2000® using BEAM type element for beams and columns and the material nonlinearity has been taken into account with bending plastic hinges for beams and plastic hinges sensitive to the axial load for columns (modeled coherently with the Moment-Rotation (M-θ) bond required by Italian standards, as already shown in Fig. 3). Different removal time of the col- umn (Δtd) are considered, while the value of the damping ratio is fixed to 4%.
- 9. Journal of Building Engineering 67 (2023) 105970 9 M. Francioli et al. Fig. 10. Robustness under hazard-chain. Fig. 11. Case study structure: (left) Google Earth® view; (right) FE model of the structure. On the base of architectural drawings, two different locations (i.e., the position of the element removed at DL = 1) have been con- sidered, in correspondence to technical premises as shown in Fig. 13. Key structural elements (the DL can be identified as the number of the damaged key elements) are the columns at the ground floor which, at the two selected locations have 60x60 and 65x90 (cmxcm) cross-sections. The methodology used to assess the robustness performances of the structure is the one explained in Section 2: in a certain loca- tion, starting from a certain DL the structural response is evaluated by the NDA. If failure (as defined in section 2.1) doesn't occur spontaneously, first, the residual capacity of the structure is identified by a pushover analysis, then the damage level is increased (i.e., the previously removed plus an additional column are simultaneously removed) and the structural response is re-evaluated. As already stated above, during the pushover analysis the criteria identifying the residual lateral force capacity (λu) are: i) the oc- curring of the “run-away” behaviour observed in the vertical displacement time history of the nodes around the removed column; ii) the occurrence of a vertical drift ratio (DV) bigger than 20%; iii) a vertical displacement ratio bigger than 0.5. The structure is not symmetric in plan, then the pushover analysis must be conducted along the two directions (X and Y) and, for every direction two different cases must be studied (X > 0, X < 0, Y > 0 and Y < 0) by the application of the lateral forces having a spatial distribution following the first mode shape in that direction. It is possible to notice that the structure shows different behav- iours under pushover forces along the two orthogonal directions (Fig. 14). 3.1. Set of damage presumption robustness curves As per results presented in the previous section, by carrying out damage presumption (i.e., column removal) analyses at different Δtd values it is seen that the capacity obtained by the subsequent pushover analysis changes; this is what is shown in Fig. 15, where the pushover curves of the structure at the same level of damage for different values Δtd_i of the column removal time intervals (with Δtd_1 = 0.01 s, Δtd_2 = 0.03 s, Δtd_3 = 0.3 s and Δtd_4 = 0.5 s) are reported. As expected, when Δtd decreases, the structural capacity
- 10. Journal of Building Engineering 67 (2023) 105970 10 M. Francioli et al. Fig. 12. Example of reinforcement details in beams. Fig. 13. Architectural specifications: parts of the first floor with high concentration of components potentially subjected at risk of explosion; first (red) and second (green). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) obtained by the pushover decreases as well, and the corresponding robustness curves drop as shown in Fig. 15 for the case of pushover analyses along X (Fig. 15a) and Y (Fig. 15b). From Fig. 16, it can be also seen that, as anticipated, the Y > 0 and Y < 0 provide differ- ent results for robustness. 4. Results and discussion 4.1. BSR curves As said, the explosion is assumed to occur in technical areas inside the building, then the structure is subjected to deflagrative ex- plosions (of gaseous material). The pressure wave of a deflagration is qualitatively represented in Fig. 17. The deflagration overpres- sure peak P0 can reach maximum values of 8–20 bar, while the duration of the positive phase (Δtp) may vary depending on the cir-
- 11. Journal of Building Engineering 67 (2023) 105970 11 M. Francioli et al. Fig. 14. Pushover curves for the 3D RC frame along two orthogonal directions. Fig. 15. Pushover curve of 3D RC frame structure – DL = 1: effect of variation of Δtd (expressed in seconds). cumstances, with values ranging between a few milliseconds and hundreds of milliseconds ([36,37]). The typical pressure wave time history of deflagrative explosions it is usually obtained by ad hoc Computational Fluid Dynamic (CFD) analyses ([25]), which is not developed here, to explore the local effect (pressure time history on the columns) of the explosion. As explained in section 2.3, in cases where such a local analysis is not conducted, it is a reasonable approximation to assume that larger DLs are associated to lower Δtd values. In other words, the lower considered Δtd is associated to the DL that involves the re- moval of the highest number of columns, while at the maximum Δtd is linked the one that causes the removal of the lowest number of columns. With this criterion, in order to obtain the BSR curves, pertinent points are selected from the set of damage presumption ro-
- 12. Journal of Building Engineering 67 (2023) 105970 12 M. Francioli et al. Fig. 16. Robustness curves at location 1 in Y direction: effect of variation of Δtd (expressed in seconds). Fig. 17. Typical deflagration pressure wave. bustness curves shown above (obtained for all the considered Δtd and all directions). Namely, since Δtd_1<Δtd_2<Δtd_3<Δtd_4, the Δtd_1 is associated to DL = 4, while Δtd_2, Δtd_3 and Δtd_4 are associated to DL = 3, DL = 2 and DL = 1 respectively. In the present paper, a local model (single column under lateral load due to the explosion) has been developed in SAP2000® to as- sociate different values of the Δtd to different blast intensities. The model has been validated as specified in appendix A, and has been used to evaluate the deflagration load associated to the different considered Δtd values. In addition, since the structure is characterized by different residual capacities in both X direction (λu, X > 0 ≠ λu, X < 0) and Y di- rection (λu, Y > 0 ≠ λu, Y < 0), for each DL the lower capacity is chosen between the two alternative results in each direction. The result- ing BSR curves are shown in Fig. 18 for the X direction (similar graphs, here neglected for sake of simplicity, are obtained for the Y di- rection). It is noted that, if four columns are simultaneously removed (DL = 4), at least one of the threshold values of the adopted failure criteria are exceeded independently of the pushing direction, of the blast location, and of the value of the time interval Δtd. The com- parison of the BSR curves for the two considered locations in each direction is reported (Fig. 19 (a) and (b)). It appears a similar be- haviour between the two locations, either in X or Y directions; in both cases the location 1 appears more robust than location 2 for low DLs, while for a DL equal to 3, location 1 is less performing than location 2. As said, the DL = 4, here associated to the Δtd = 0.01 s, always determines the collapse of the structure. As general conclusion, it can be said that the critical location for the blast robustness of the considered structure is the location 1, with λu/λ % = 64% for DL = 3.
- 13. Journal of Building Engineering 67 (2023) 105970 13 M. Francioli et al. Fig. 18. BSR curves associated to the X direction. 4.2. Robustness under hazard chain scenarios To highlights the consequences of an initial earthquake damage on the blast-robustness performances, a single earthquake is con- sidered here, characterized in intensity for the considered structure by the spectral acceleration at the first natural period Sa(T1) of 0.37g, and a return period of 475 years. The effect of the earthquake induced damage on the structural capacity is evident from the comparison of the bi-linear capacity curves obtained by the pushover analysis of the structure at different blast induced damages (DLs) obtained without considering the earthquake or after the earthquake-induced damage (Figs. 20 and 21 for direction X and Y in case of location 1 and location 2 respec- tively). The capacity curves shown herein refer to one of the Δtd values considered in the analyses (Δtd = 0.3s) and to directions X > 0 and Y > 0. In particular, Fig. 20 shows the results of pushover analyses when the blast-induced damage is considered to occur in location 1, while Fig. 21 shows the results when the blast-induced damage is considered to occur in location 2; to understand the correct positioning of the considered locations 1 and 2 (L1 and L2), the reader can refer to Fig. 13. The effect of the earthquake induced damage on the structural robustness is also noticeable in Fig. 22, where the BSR curves ob- tained for the directions X and Y for the structure when damaged or not by the earthquake are shown. Looking at Fig. 22, it can be highlighted that the earthquake-induced damage is quantified to be 18% and 45.5% in X direction and Y direction respectively (with residual capacities after earthquake at DL = 0 being 82% and 54.5% respectively). In addition there is an important result to highlights, which can give new insights about the robustness design of RC frames against blast: when the haz- ard chain scenario is not considered, the critical location is the one identified as location 1, suggesting that eventual provisions for en- hancing the structural robustness should focus on the blast-strength improvement at that location or, alternatively, they should focus on some management measures to decrease the blast hazard at that location, for example by keeping the technical premises away from location 1 and placing them at location 2. On the contrary, if the hazard chain scenarios are considered in the robustness analysis, they lead to the identification of the loca- tion 2 as the critical one for robustness performances, meaning that moving technical premises at location 2 (one of the above- mentioned design provisions) is not appropriate because this can increase the risk of progressive collapse. The change of critical loca- tion in terms of robustness when the hazard-chain scenarios are considered with respect to the blast-only scenario suggests that the ef-
- 14. Journal of Building Engineering 67 (2023) 105970 14 M. Francioli et al. Fig. 19. BSR curves associated to the X and Y directions. Fig. 20. Pushover response curves of the 3D frame at different DLs (Δtd = 0.3 s); location 1: (a) without considering earthquake damage, (b) with earthquake damage (DL = 3 for Y > 0 is null).
- 15. Journal of Building Engineering 67 (2023) 105970 15 M. Francioli et al. Fig. 21. Pushover response curves of the 3D frame at different DLs (Δtd = 0.3 s); location 2: (a) without considering earthquake damage, (b) with earthquake damage. Fig. 22. Effect of hazard chain on the BSR curves: (a) X direction, (b) Y direction.
- 16. Journal of Building Engineering 67 (2023) 105970 16 M. Francioli et al. fect of the initial earthquake-induced damage cannot be ignored in structural design, at least for important buildings located in seis- mic regions. 5. Conclusions In this work, an assessment procedure of the robustness of RC structures as a result of explosive phenomena was first presented; then, the effect on the structural robustness of the chained concatenation of earthquake and blast damage is investigated. It is first shown that the process for assessing structural robustness involves nonlinear dynamic analyses in order to simulate explo- sion-induced damage (by considering the sudden removal of one or more columns with a removal time Δtd), and nonlinear static analyses to assess the residual capacity of the structure (both at global and local scale). Four different Δtd values are determined on the basis of local analyses conducted on the single column under explosion, and are associated with different damage effects for the structure; the approach allows the introduction of the novel concept of Blast-Scenario dependent Robustness (BSR) curve, that is a ro- bustness curve associated with a set of specific explosion scenarios. The introduction of such a BSR curves covers the literature gap due to the lack of direct connection between the blast-induced structural damage to the blast intensity in the context of numerical simulations for robustness assessment. The second novelty of the paper regards the robustness analysis under hazard-chain scenarios: the effect of the earthquake in- duced damage on the structural robustness is investigated by applying the above-mentioned procedure to the structure after it has been damaged by an earthquake. The result of this analysis show that the blast robustness of a previously earthquake-damaged struc- ture decreases as obvious, but most of that the analyses show that the critical location inside the structure for the blast-induced dam- age in terms of robustness performances is different if the hazard-chain scenario is taken into account or not, something indicating that, when buildings are located in seismic regions, these types of hazard chain scenarios cannot be neglected in the design of RC frames for robustness. All the analyses of the paper are deterministic. No references are made to the probabilistic occurrence of the hazard chain scenarios or to the probability for a fixed intensity of the earthquake to trigger explosions, a high intensity earthquake is rather deterministically assumed to cause an explosion. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability The data that has been used is confidential. Appendix. Validation of the model for the assessment of local blast-induced damage The model used for assessing the column local damage under blast action has been developed in SAP 2000® by a beam FE pro- vided with fiber-based plastic hinges. The column in a first step it is statically subjected to vertical loads, then in a second step is sub- jected to an impulsive lateral load characterized by a blast-like load time history which leads to different responses of the column de- pending on the reached peak pressure and on the time history shape, associated to the blast intensity and to the blast typology (deto- nation or deflagration) respectively. The model can be used to associate blasts to the different Δti values considered in the successive global robustness analysis and it is called “local model” in the following text and figures. The validation of the local model has been completed by comparing the results with those obtained in SBEDS®, a well-known model for beam/column time history non-linear response analysis under detonations by an equivalent SDOF model released by the U.S. Army Corps of Engineers ([38,39]). The SBEDS®’ analysis procedure has been implemented in Matlab® and the obtained blast- response solution is called “reference solution” in the following text and figures. The reason for developing the local model instead using SBEDS® for local analysis, is that SBEDS® refers to detonations only, while the developed local model allows the analyst to consider a generic time history shape for the explosion, then allowing the con- sideration of both detonations and deflagrations (focus of the present paper). As said above, the correct time history shape for a spe- cific deflagrative explosion load should be found conducting a CFD analysis, something which has not been implemented in the pre- sent work. Here the time history shape has been assumed to be similar to one of those obtained by CFD simulations carried out by the authors in the past ([25]), and shown in Figure A1. By multiplying this shape by the value of the reached peak pressures, and by slightly stretching the shape with changes in tp, the load time history to consider in the specific case can be obtained.
- 17. Journal of Building Engineering 67 (2023) 105970 17 M. Francioli et al. Fig. A1. Parameters of the deflagration time history shape. In order to provide the reader an example of the validation of the local model, the comparison between results obtained by the lo- cal model with the reference solution is shown in Figure A2 for the 650x900 cross-section column already described in Fig. 12 above, when subject to detonation blast loads from different sides. Due to the different nature of the compared models (nonlinear SDOF in SBEDS® versus nonlinear MDOFs in SAP2000®), the equivalent total damping in non-linear behaviour regime of the local model has been assessed to be approximately 4 times larger than the one experimented by the SDOF, then the damping value for the local model has been set as lower than the one used in SDOF. The local model shown to be able in satisfactory evaluating the response of the col- umn. Fig. A2. Validation of the local model in determining the column response under detonation: strong direction (top); weak direction (bottom). Finally, in Figure A3, the response time histories obtained by the local model for the cases Δti = 0.3 s and Δti = 0.5 s are shown.
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