Optimization of Intervention Strategies for Masonry Buildings Based on CLT Components

Abstract

Unreinforced masonry has been for centuries one of the most widespread constructive techniques for both massive structures and civil buildings (e.g., palaces, hospitals, houses), for the most still standing nowadays. Their future conservation relies on (i) their protection from main natural threats (e.g., earthquakes) and (ii) updating to current functionality and hygrothermal standards. In the former framework, existing masonry buildings proved to have some intrinsic vulnerabilities, depending on composition (units and binder) and structural typologies. Based on experience gathered from seismic events, various retrofitting techniques have been proposed. In such a context, the use of cross-laminated timber (CLT) components is a very promising solution, in terms of compatibility with built heritage and integration of seismic and hygrothermal performances. This paper aims at improving the knowledge of the structural performances of compound timber–masonry interventions by numerical simulations carried out at (i) pier scale and (ii) building full scale via finite element modeling and nonlinear static analyses (pushover). First, a coupled timber–masonry wall was simulated and underwent sensitivity analyses with the properties of both components varying; then, the optimized solution was applied to a case study to assess the intervention benefits, and the results were also cross-checked with those of more traditional interventions (e.g., grout injections).

1. Introduction

The built environment has grown over time, along with the development of technology and people’s wealth. Nowadays, multi-story reinforced concrete and steel structures arise next to historical buildings made of unreinforced masonry (URM) and timber components. Masonry, made of clay brick or stone units, has been for centuries one of the most widespread constructive techniques for vertical structures, due to the easiness of material supply and assembling. Instead, horizontal structures have been mainly made of timber components such as beams, truss beams, and planks. Moreover, the use of timber frames coupled with masonry or earthen infills has been adopted, especially within low-income areas [1,2].

After the introduction of steel carpentry and reinforced concrete (RC) in the 19th and 20th centuries, masonry and timber components were gradually put beside and substituted. The use of RC components has been integrated within new-built masonry structures and for the retrofitting of the existing ones, especially for substituting traditional timber floors. However, such prescriptions were proven to cause a worsening effect on the seismic behavior of existing unreinforced masonry (URM), due to the high density of RC components and related increase in seismic forces, especially in the case of poor masonry quality. Consequences were dramatically shown by the earthquakes that occurred in Italy in the last decades, e.g., L’Aquila-Abruzzo 2009 [3,4,5,6,7,8], Emilia 2012 [9,10], and Central Italy 2016 [11,12,13,14,15].

The observation of damage suffered by URM buildings that underwent incompatible interventions and the need for sustainable construction materials with URM-compatible mechanical and physical properties led to a rediscovery of traditional timber construction techniques.

Research aimed at improving the stiffness of traditional timber floors (monodirectional joists with overlaying plank) by the use of dry timber techniques was extensively carried out; examples include the studies on retrofitting by overlaying of plywood panels [16,17,18], oriented strand boards (OSBs) [19], CLT panels [20], or planks oriented at 90° [21] or 45° [22,23,24,25].

The beneficial effect of those timber-based strengthening techniques, compared to RC ones, was proved by numerical analyses, in the finite element (FE) [26,27,28,29] and discrete element (DE) environments [30].

The use of timber for retrofitting was also extended to URM walls. The coupling of the latter ones with strong-backs [31,32,33,34] or massive panels (CLT) [35,36,37,38,39] proved to be capable of improving both the in-plane (IP) and out-of-plane (OOP) performances of piers.

The success proved by timber for the strengthening of URM buildings leads to the possibility of combining its structural performances with the well-known good hygrothermal properties of wood. An innovative integrated strategy based on the use of CLT panels and floors for the structural, hygrothermal, and functional refurbishment of existing URM buildings, named Nested Building (NB), was presented in [40]. It consists in the demolition of obsolete inner structural components (i.e., walls and floors) and the insertion of a CLT endoskeleton, to which the bearing of loads and the improvement of hygrothermal performances are addressed. This results in the addition of new CLT floors and the coupling of panels to the existing walls.

In such a context, the design of the coupled URM–CLT walls becomes a key factor. In this paper, a pier-scale characterization of IP lateral performance of such components with the variation of the mechanical and geometrical parameters, of both URM and retrofit components, was carried out via FE modeling in DIANA FEA v. 10.5 [41]. Based on the outcomes, the study was extended to the full building scale by simulating the NB intervention on a case study (i.e., a mixed-masonry building called Cattedra, located in the Italian Alpine Region). The results were compared with the performances obtained using more traditional intervention approaches for walls, i.e., the grout injection technique.

1.1. Techniques for the Retrofitting of URM Walls

The retrofitting of masonry walls aims at increasing both OOP and IP mechanical behaviors, especially when the structural material has poor mechanical characteristics and cannot satisfy the current requirements. According to the Italian seismic code [42], the intervention should be designed with respect to the masonry type and conservation status. The main aspects of common improvement techniques are briefly summarized in the following.

Grout injections aim at making masonry behavior monolithic by filling the possible voids between units or leaves [43,44,45,46,47]. They can be very effective in masonry with a connected net of inner voids. Particular attention should be paid to the injection pressure, especially in poor masonry quality, to avoid local damage or gatherings. The success of the intervention should be checked by comparison with pre-intervention status, for instance by sonic pulse velocity tests [48,49].

The structural repointing of masonry consists of the embedment of steel or composite bars within binder horizontal joints. It usually aims at increasing the confinement of units and at contributing to the tensile strength of masonry. However, it should be noted that its effectiveness is concentrated at the outer surfaces of the walls, and hence it is particularly addressed to double-wythe panels [43,50,51,52]. A further upgrading of the repointing technique to a steel wire net, named Reticulatus, was developed and discussed in [53].

In the case of sound masonry (monolithic behavior assumed), composite strips (i.e., polymer matrix with fibers) can be applied to wall surfaces to increase their tensile strength [54]. Composites have gained increasing success in the retrofitting and refurbishing of architectural heritage, due to their high strength/weight ratio and the easiness of application. However, they should be preferably installed with compatible mortars instead of resins, to provide more reversible and durable interventions [55,56,57]. Further details and test results can be found in [58,59,60].

Post-tensioning consists of the insertion of post-tensioned vertical tie-rods inside a URM panel to furtherly increase its tensile and flexural performances. However, this intervention needs a relevant URM strength when bearing the compression stress increment.

Jacketing is usually implemented in case of incoherent or seriously damaged walls, whose bearing capacity is extremely compromised. Jacketing layers, made of cementitious or hydraulic lime grouts and steel or composite fiber reinforcements, should be placed at both outer surfaces, anchored to each other by transverse connectors (e.g., steel or composite bars) [61,62]. However, particularly with traditional concrete layers, this intervention results in mass and stiffness increase, which can alter the original structural behavior and eccentricities of the building, if not properly designed.

Therefore, some of the above-mentioned URM wall retrofitting techniques can have some disadvantages and drawbacks, especially related to structural and/or aesthetic aspects. Recently, various techniques based on timber components have been studied, designed, and tested. They mainly provide the coupling of URM walls with wooden elements, such as strong-backs or panels. Based on the positioning of such elements [36], interventions can be subdivided into two main categories, above the (i) outer or (ii) inner façades.

The optimal solution must be chosen according to (i) hygrothermal properties, (ii) structural properties, (iii) mechanical and chemical compatibility, (iv) reversibility, (v) recognizability, (vi) assembling easiness, (vii) durability, and (viii) sustainability (also intended as low obtrusiveness). Based on [40],

Table 1. Properties of strengthening materials for URM retrofitting.

Material	Structural Properties	Hygrothermal Properties	Compatibility	Construction Site	Reversibility	Recognizability	Durability	Sustainability
RC frame	High stiffness Good connection to existing walls	High thermal conductivity Risk of thermal bridges	High weight	Difficult casting stage due to presence of existing structure	Irreversible	Frame is usually hidden (e.g., covered by drywall)	Good	Poor
Steel frame	High strength and stiffness Prestress required to ensure structural collaboration with existing portion	High thermal conductivity Risk of thermal bridges	High weight	Not always simple handling of prefabricated elements Fast installation	Reversible due to dry connections	Frame is usually hidden (e.g., covered by drywall)	Excellent	Poor
Composite materials	High tensile strength	-	Overabundant tensile strength could alter URM behavior	Easy handling	Limited	Hidden within grout and plaster	Good	Poor
Wooden frame	Great lightweight characteristic Low stiffness and great deformability Suitable only for thin masonry panels	Good thermal properties but not covering building envelope	Optimal	Easy handling due to light weight Fast installation	Reversible and recyclable (sustainable)	Frame is usually hidden (e.g., covered by drywall)	Reduced when exposed to prolonged moisture	Good
CLT panel	Significant lightweight characteristic High stiffness to density ratio	Good thermal properties	Optimal	Facilitates handling due to light weight, possible difficult in-site assembly movements due to presence of existing structure Fast installation Prefabrication	Reversible and recyclable (sustainable)	CLT can be fair-faced	Reduced when exposed to prolonged moisture	Good

reports and compares the main characteristics of traditional and timber-based materials for retrofitting. Among the various solutions, the use of CLT panels emerges as the most balanced solution. They can improve stiffness and strength with reduced mass addition or even decrease it (if demolished elements are made of heavier RC or URM). However, mass variations should be carefully controlled, since the URM shear behavior could benefit from vertical compression stresses. Compared to timber frames, CLT panels have higher stiffness, easier assembling thanks to prefabrication, and better hygrothermal performances (i.e., 2-D development permits to cover existing wall surfaces).

The effectiveness of coupling URM walls with CLT panels by fastening at floor levels, i.e., bottom and top wall edges, was assessed for clay bricks [35] and hollow block masonry [36]. It was found that such configurations can significantly improve the load capacity of URM piers to 34% and 129% and the ultimate displacement to 100% and 57%, respectively.

The spread of URM–timber dry connectors for the coupling of walls was also experimentally assessed. Riccadonna et al. (2019) [63] performed shear tests on dry steel bars (five types) connecting a timber panel to a 19th-century masonry wall. Stone elements could bear higher loads than clay brick, although with high CoV, due to stone geometry and consistency irregularity. Failure of stone elements occurred for tensile cracks or crushing of mortar joints, due to the poor resistance of the historical lime mortar. Clay units showed crushing or splitting failures. Although fastener choice should have low impact, mild steel ones should be used in the case of bricks due to the earlier rise of the plastic hinge and better dissipative capacity; hardened carbon steel fasteners could limit the risk of brittle failure in stone units instead. If stone masonry has thick and irregular binder joints, grouted connectors should be considered. In such a context, CLT panels are a viable option due to their good mechanical performances and insensitivity to load angles.

Giongo et al. (2021) [38] performed destructive tests on URM specimens cut and isolated within load-bearing walls in a decommissioned URM building (i.e., 19th century Comano Terme bath area in northern Italy). Masonry was made of clay bricks (about 200 × 100 × 50 mm) and lime mortar. Three wall specimens of 1.8 × 1.8 m were isolated. In the case of timber strengthening, fasteners were inserted within pre-drilled holes, with attention being paid to targeting clay bricks instead of mortar joints. Two types of fasteners were used, i.e., A (Ø 12 × 180 mm partially threaded screws) and B (Ø 10 × 230 mm full-threaded screws). The experimental campaign was not limited to characterizing the timber reinforcement behavior of strengthening masonry, but also evaluated its effectiveness as a repair intervention (post-damage implementation). Three types of configurations were analyzed: (i) as-built (bare specimens AsB-1 and -2), (ii) repaired (addition of timber panel after testing, RP-1 and -2), and (iii) retrofitted (addition of timber panel to bare specimen, before test, R3). Number and type of fasteners varied as follows: RP-1 had 17 A fasteners, RP-2 had 16 B ones, and R-3 had a further 9. Moreover, for repairing layouts, the position of the connectors was irregular, according to the post-test crack pattern. R-3 layout had 25 B dowels regularly distributed (8 fasteners per m²) in five rows, with 300 mm spacing. The addition of timber elements, even as a repair tool, provided an increase in the peak-bearing load of the panel without altering its original stiffness (in the 1 specimen, while a 25% reduction was recorded in the 2 one). However, the ultimate displacement capacity was slightly reduced compared to the as-built configuration. The retrofitted configuration, instead, offered a significant increment, for both load (106 kN on a 75.9 kN maximum for AsB ones) and displacements. However, it should be noted that tests were stopped before failure for safety reasons. This study showed the potential benefits of URM–CLT coupling for both repairing and retrofitting historical or existing masonry.

Guerrini et al. (2021) [31] characterized the in-plane behavior and the performances of URM panels retrofitted with strong-backs. Single-wythe calcium silicate masonry piers, either bare or retrofitted, were built. Specimens consisted of URM walls 2.7 × 2 × 0.1 m (H × L × t) and solid red fir timber frames (strong-backs, 80 × 60 mm in section) with adjunct OSB boards (16 mm thick). The retrofitted pier showed a crushing mechanism at 0.15–0.2% drift, while the timber retrofit showed yielding of fasteners at 0.6% drift. At 0.8%, extensive toe crushing was observed, which led to failure at 2.0% drift, where connections were yielded and URM had no bearing capacity. The bare pier changed its behavior from rocking to sliding at 0.2% drift; the retrofit inhibited shear-sliding phenomena and permitted the pier to bear higher loads. Initial stiffness (until bare specimen peak load) was not affected by timber retrofit.

The extension of such timber–URM coupling interventions to a building-scale specimen was done through two URM cavity wall buildings, with and without timber retrofit, tested via shaking table [32].

1.2. The “Cattedra” Case Study

The case study chosen for the application of the NB approach consists of a structural unit belonging to the so-called Cattedra complex in the Italian Alpine Region (i.e., Canove di Roana, Vicenza). It is a URM building dating back to the former years of the 17th century; it has three stories with a rectangular 10 × 13 m plan and is 8.2 m tall (at the eave) (Figure 1).

Three diaphragms and a double-pitched roof constitute the horizontal structures of the building. The first and second floors are made of cast-in-place RC joists and clay blocks with an RC overlay, whereas the third one and the roof are made of clay block joists without any slab. Vertical structures are made of three masonry types. The most ancient one is a double-wythe masonry of white hewn stone (roughly cut), locally known as biancone. It has a good average size of the units and a binder in a poor state of conservation (Figure 2a). The wall thickness varies between 55 and 70 cm. Corner attachment relies only on the quoins, whereas inner wythes showed a negligible interlocking (Figure 2b). Starting from the height of 7 m, 60 cm thick hollow clay block masonry was detected, due to a superposition intervention made in the 20th century (Figure 2c). At last, the south façade gable is made of 40 cm thick hollow concrete blocks kept together by cementitious mortar (Figure 2d).

2. Materials and Methods

The choice of the proper retrofit intervention for URM was supported by an extensive comparative approach, which involved numerical analyses at (i) pier scale and (ii) full building scale. Sensitivity pushover analyses were carried out on a CLT–URM coupled wall, by varying some selected parameters (

Table 2. Parameters and their assessed values for sensitivity analyses on URM–CLT coupled wall.

	Label	Property	1	2	3
Masonry	M	Masonry type	Ashlar stone	Clay brick and lime mortar	Semi-hollow clay block and cementitious mortar
	T	URM wall thickness (cm)	30	40	50
	L	Panel width (cm)	150	200	250
	Q	Applied vertical pressure (MPa)	0.1	0.3	0.5
CLT retrofit	K	URM–CLT steel connectors spacing (number) (cm(-))	50 (35)	40 (54)	30 (108)
	T1	CLT panel thickness (cm)	6	10	15
	HD	Hold-down type	WHT340	WHT440	WHT540
	AB	Angle bracket TTN240 number	1	2	3

), in two restraint conditions, i.e., with base fixed and upper edge (i) free (cantilever full-bending layout) or (ii) with only in-plane moments allowed (double pendulum, full-shear layout). Reference bare and retrofitted specimens had the property of column 2 of Table 2. Coupled wall dimensions were 300 cm × 200 cm (H × L). CLT panel was connected to the base through two hold-downs and two angle brackets.

Once key parameters were detected, the retrofit intervention was extended to a full-scale specimen, i.e., applied to the case study building Cattedra by the NB method [40,64]. Moreover, such intervention was compared with a traditional URM retrofitting technique, commonly applied to rubble stone masonry, i.e., the grout injection, aimed at improving both stiffness and strength of walls.

These interventions were applied incrementally, i.e., by the addition of distributed wall-to-wall connectors and injections.

The case study was modeled in the as-built configuration (AsB) and the retrofitted one. Moreover, an intermediate configuration where RC existing floors are substituted with CLT diaphragms (TD) was assessed, in order to evaluate the benefits given by coupling URM walls with CLT panels with respect to TD and NB comparisons. Figure 3 presents the assessed layouts of interventions. Techniques were grouped according to each of the endoskeleton configurations (i.e., TD, NB) and added and combined with each other. The _I suffix (and their combinations) indicates the application of grout injections to stone masonry; the _C suffix indicates the addition of distributed connectors [63].

The endoskeleton (composed of CLT panels, hold-downs, and angle brackets) was designed according to Eurocodes 5 and 8 [65,66], in order to bear the expected static and seismic loads.

Table 3. Structural elements and connections of CLT endoskeleton in NB configuration.

Steel Bracket
Story	Longitudinal		Transverse
	Hold-down	Angle bracket	Hold-down	Angle bracket
1	16 × WHT440, 12 × WHT540	17 × TTN240	20 × WHT540	13 × TTN240
2	28 × WHT440	17 × TTN240	20 × WHT540	13 × TTN240
3	28 × WHT340	8 × TTN240	10 × WHT340	16 × TTN240
CLT panel
Element	Material	No. of layers	Thickness (mm)	Layers (mm)
Walls	Spruce C24	3	100	30–40–30
First floor	Spruce C24	5	160	40–20–40–20–40
Second floor	Spruce C24	5	140	40–20–20–20–40

reports the features of the structural components. All the numerical analyses were carried out via the FE method in the DIANA FEA code [41]. Pushover analyses were performed according to uniform mass proportional loads. The description of the models and the characterization of the materials are reported in the following.

2.1. Finite Element Modeling of URM and CLT Components

FE models were built according to the macro-modeling approach. The nonlinear behavior of masonry was simulated by an isotropic smeared model (i.e., total strain rotating crack) embedded in DIANA code, which proved to be effective in URM static nonlinear analyses [67,68]. Different constitutive laws were implemented in compression, tension, and shear. Compression behavior was modeled through a parabolic law based on compressive fracture energy. Tensile behavior was elastic until peak strength was reached, whereas the post-peak curve was described by an exponential softening law governed by tensile fracture energy. Cracks activate when the maximum principal stress overcomes the tensile strength, with an orientation perpendicular to the maximum principal strain [69,70]. Such crack orientation can be assumed to be (i) fixed or (ii) rotating. In the (i) case, it is kept fixed for the entire computational process, which could constitute a misalignment of principal directions and crack direction and end in an overestimation of stiffness and strength, especially for unreinforced masonry. In the (ii) case, cracks rotate according to principal strain axes and continuously update during the computation. Since the cracks influence the continuity degree of the material, the shear response is affected proportionally, due to the reduction of contact and interlocking. Such phenomenon, in the case of fixed cracks, can be modeled by a linear proportional law addressed at reducing the shear modulus by a shear retention factor β which lies between 0 and 1 [70]. In the case of rotating cracks, such modeling is not necessary and a β equal to 1 is assumed [41].

CLT panels were modeled according to the so-called component modeling approach [71] as orthotropic linear shells [72,73,74], whereas nonlinearities were lumped at the connection systems. Hold-downs and angle brackets were modeled as point interface elements or springs. The former were characterized by symmetrical laws in compression and tension; and the latter were characterized by symmetrical shear laws, based on tri-linearization. Due to panel-to-foundation or panel-to-panel contact, the behavior of hold-down elements in compression was modeled by a 1-D linear rigid interface in compression (stiffness one order of magnitudes higher than deformable CLT panels) without tensile stiffness and strength.

Coupled wall model was discretized through 2400 linear 50 mm size 4-noded Q20SF shell elements for timber and URM walls, respectively, whereas 2-noded nodal interfaces N6IF were used to simulate hold-downs, angle brackets, and steel bars (URM–CLT dry connectors). Figure 4a shows the FE model.

The Cattedra models (Figure 4b,c) included URM walls, CLT panels, and floors. These were discretized by 20 cm size shell elements, whereas 20 cm size plane 3-D interface elements were implemented between them to simulate spread dry steel connectors. Such elements are capable of bonding mutual faces of URM walls and CLT panels by implementing two IP (shear) and one OOP (normal) stiffnesses, ruling the transfer of displacements along the three local axes. Spring elements were used for hold-down and angle-bracket elements.

Table 4. Features of Cattedra FE models.

Model	Element
	Quad Q20SF/Q20SH	Triangular T15SF/T15SH	Quad Plane Interface Q24IF	Linear Interface L12IFS/L16F	Spring N6SPR/N12SPR
AsB, AsB_I	16,764/12,072	435/170	-	-	-
TD, TD_I	16,764/12,072	435/170	-	-	-
NB	24,172/10,877	379/116	-	3131	76/378
NB_C, NB_C_I	24,172/10,877	379/116	4520	3131	76/378

shows the details of the element types and their number.

2.2. Characterization of Material Properties

Mechanical properties of materials were defined starting from literature and codes. The properties of URM were derived from Italian codes [42,75]. C24 CLT properties were defined according to the producer’s ETA (i.e., Stora Enso) [76] and by using the so-called HOBS method (homogenized orthotropic plate stress Blass reduced cross-section) [72,77]. Shear moduli were considered constant in the IP and OOP directions [73,78], with the values derived from [76].

Table 5. Linear properties of 3-layer and 5-layer C24 CLT panels (x,y: IP direction parallel or orthogonal, respectively, to grain of outer layers; z: OOP direction).

CLT Panel	Thickness (mm)	Ex (MPa)	Ey (MPa)	Ez (MPa)	Gxy, Gxz, Gzy (MPa)
3-Layer C24	100	7667	5250	417	690
5-Layer C24	140	9047	3869	417	690

reports the geometrical and mechanical parameters (Young’s moduli E and shear moduli G) calculated for CLT panels.

Some mechanical parameters of masonry (Young’s modulus E, compressive f_c and shear τ strengths, density ρ) were defined according to the Italian code [42], with a knowledge level (KL) equal to 1 and a related confidence factor (CF) of 1.35. Mechanical retrofit due to the use of grout injection within hewn stone masonry was implemented through the application of a 1.5 improvement ratio [42]. Tensile strength f_t was derived according to [79,80], as follows:

$$f_t=1.5·\tau ,$$

(1)

Compressive fracture energy G_fc was defined according to [81], whereas tensile fracture energy G_ft was assumed to be equal to 0.02 N/mm [69]. Crack orientation was assumed as rotating, as recommended for URM and shear-dominated applications [69,70,82].

Table 6. URM mechanical properties assumed for CLT–URM coupled wall and Cattedra case study.

Property	Ashlar Stone Masonry	Clay Brick and Lime Mortar	Hewn Stone Masonry		Semi-Hollow Clay Block
			Unreinforced	Reinforced (Injection)
Young’s modulus E (MPa)	870	1500	1740	2610	4550
Poisson’s ratio v (–)	0.25	0.25	0.25	0.25	0.25
Compressive strength fc (MPa)	1	2.6	1.12	1.68	5
Compressive fracture energy Gfc (N/mm)	2.7	6.6	4.8	4.42	11.5
Tensile strength ft (MPa)	0.027	0.075	0.027	0.04	0.0.12
Tensile fracture energy Gft (N/mm)	0.02	0.02	0.02	0.02	0.02
Crack orientation	Rotating	Rotating	Rotating	Rotating	Rotating
Density ρ (kg/m3)	2000	2000	2000	2000	1500

reports the main characteristics of URM walls and RC floors.

Floors were modeled using 5 cm thick shells. Bidirectional stiffnesses and a consistent shear modulus were assumed for floors with RC overlay (i.e., first and second in the building), whereas a monodirectional stiffness and a limited shear modulus were assumed for those without any (i.e., third floor and roof) (

Table 7. Floor mechanical properties assumed for Cattedra case study.

Property	Cast-In-Place RC Joists and Clay Blocks with Slab	Clay Blocks Joists without Slab
Structural thickness (cm)	5	5
Young’s modulus Ex (MPa)	20,000	0
Young’s modulus Ey (MPa)	39,200	36,000
Shear modulus Gxy (MPa)	8300	1000

). In the case of floor substitution, values for 5-layer CLT were assumed (Table 5).

Timber steel brackets were characterized by tri-linear laws, according to producer experimental testing reports [83]. Distribute shear steel connectors with 40 cm spacing were modeled based on the experimental laws derived in [38].

3. Results

The results of nonlinear static analyses performed at the pier and building scales were processed in terms of load–displacement or acceleration–drift capacity curves. Their comparison allowed us to detect the pros and cons of each model and intervention.

3.1. CLT–URM Coupled Wall

The pier-scale pushover analyses permitted assessing the benefits of CLT coupling with variation of (i) URM pier properties and (ii) reinforcement characteristics.

Figure 5 reports the diagrams of URM tensile strains at peak load for full shear and bending layouts. The configuration of deformations at peak load was similar between bare (Figure 5a) and retrofitted (Figure 5b) specimens for full shear layout; the diagonal shear crack was kept, but lateral-edge high strains shifted to the top and bottom edges. CLT coupling significantly affected the strain pattern, which moved from a rocking behavior with the opening of cracks at the base under tension (Figure 5c) to a mix of base and diagonal cracks (Figure 5d). However, they did not reach the corners as per the full shear one.

Results are also reported in terms of capacity curves and tables with the main performance parameters, i.e., load and drift.

Figure 6 summarizes some of the capacity curves (U is the bare specimen, R is the retrofitted specimen, according to Table 2).

Table 8. Maximum load and ultimate drift capacity of CLT–URM coupled wall with varying URM and CLT retrofit properties, for shear and bending layouts.

Property			Full Shear						Full Bending
			1		2		3		1		2		3
URM	M	F (kN)	88	109	195	233	413	446	61	75	76	97	85	100
		Drift (%)	0.38	0.42	0.50	0.53	0.40	0.41	0.35	0.42	0.83	0.87	1.12	1.20
	T	F (kN)	164	200	195	233	244	281	60	80	76	97	101	127
		Drift (%)	0.50	0.52	0.50	0.53	0.50	0.51	0.83	0.87	0.83	0.87	0.83	0.85
	L	F (kN)	115	139	195	233	332	376	45	72	76	97	113	144
		Drift (%)	0.57	0.57	0.50	0.53	0.43	0.45	1.32	1.38	0.83	0.87	0.68	0.70
	Q	F (kN)	149	173	195	233	244	285	38	84	76	97	115	131
		Drift (%)	0.50	0.52	0.50	0.53	0.50	0.52	1.25	1.30	0.83	0.87	0.58	0.62
CLT retrofit	K	F (kN)	214		233		244		94		97		100
		Drift (%)	0.52		0.53		0.52		0.85		0.87		0.90
	T1	F (kN)	233		244		250		97		104		110
		Drift (%)	0.53		0.53		0.53		0.867		0.90		0.93
	HD	F (kN)	230		233		238		91		97		115
		Drift (%)	0.52		0.53		0.53		0.87		0.87		0.87
	AB	F (kN)	227		233		254		95		97		100
		Drift (%)	0.52		0.53		0.53		0.87		0.87		0.87

reports the maximum load (F) and ultimate drift values for all the capacity curves calculated in the sensitivity analyses. The capacity curves obtained at pier scale showed different benefits the coupling intervention could bring according to the restrain configuration and behavior of the system, i.e., shear or bending.

The variation of URM properties (Figure 6a) seemed much more significant in the case of pure shear behavior than in the case of bending behavior. Moreover, the reinforcement contribution (peak load increment) appeared to be more effective in the latter case. The reader should note that the behavior of URM piers in a real structure falls in between, according to the (i) acting vertical loads and (ii) floor stiffness. In particular, while their values increased, the behavior became closer to full shear; on the contrary, it approaches the full bending one. Therefore, piers of low-/medium-rise existing URM buildings equipped with traditional wooden floors (i.e., joists with one overlaying plank) should fall near the bending category.

The spread of URM–CLT connectors did not affect the full-bending capacity curves or the shear curve much, although the variation in the latter is more noticeable (Figure 6b). The increase in CLT panel thickness, i.e., its total stiffness, caused some variations, especially in the bending layout (Figure 6c). Table 8 highlights how the enhancement of hold-downs and angle brackets affected the bending and shear configurations, respectively.

The results of sensitivity analyses revealed that the effectiveness of coupling reinforcement techniques is mostly independent of CLT system properties, although each component plays its role. Further increment of mechanical performances did not bring proportional retrofit improvement. On the contrary, the improvement of masonry characteristics can produce higher benefits.

The retrofitting technique based on CLT coupling (and NB at full scale) is especially intended for the cases where more traditional masonry retrofitting techniques are partly or totally forbidden. Pier-scale sensitivity analyses showed that the intervention is effective for both full-shear and full-bending limit configurations.

3.2. Nested Building Intervention for Cattedra Case Study

The results of the simulations of incremental interventions on the Cattedra case study, in terms of load–displacement and equivalent acceleration–drift, are reported in Figure 7.

TD and AsB configurations seemed able to bear similar base shear forces, so the stiffness of timber diaphragms should be enough to make them work as rigid diaphragms for the URM walls. The use of injections on AsB and TD induced a limited increase in shear force in the X direction (+11% and +9%) and a consistent one with regard to the ultimate displacement direction (about 44%). Ultimate displacement was almost unaltered in the case of the Y direction.

The beneficial effect of NB was clear in terms of shear capacity, with a further increment by addition of injections. However, in this case, the capacity of URM was strengthened in force (+155% along X, +211 along Y) but reduced in displacement (−22%, −34%, for X and Y). Grout injections allowed a further increase in the peak load (+42%, +47%). The addition of distributed URM-to-CLT connectors, in this case, was not beneficial, with respect to the effective coupling ensured by floors (continuity of mesh was assumed at diaphragm–URM wall interface). This could be partly explained by the study of Pozza et al. (2021) [36], which showed that the connection of URM and CLT panels at floor levels should be sufficient to ensure a good coupling degree. However, further study could include the adoption of adhesive connectors, which showed greater stiffness and strength than dry ones [39].

The models recorded no significant tensile stresses in the OOP directions of walls (i.e., tensile strength was not achieved), due to the effective diaphragms, which enhanced the box-like behavior. Hence, the damage was lumped in the IP direction of piers, parallel to seismic actions. Figure 8 shows tensile strains between the east and south façades. The comparison between AsB and NB layouts showed that strains are minor in the second one, due to the retrofit effect. In addition, the intervention was able to provide a more regular displacement pattern, with floors parallel to the ground also in the ultimate configuration; on the contrary, some settlements occurred in the AsB one.

The benefits of seismic mass reduction were demonstrated, especially by equivalent acceleration–drift curves. TD was able to undergo a greater equivalent acceleration than AsB. Therefore, the goodness of this choice is confirmed with respect to heavier RC joists and clay block diaphragms, although well connected to walls.

The NB model confirmed the benefits of this retrofitting technique, also with respect to grout injections, that reached far lower improvements in terms of shear load and horizontal equivalent acceleration. In addition, the combined use of both NB and injections appeared suitable for furtherly improving the seismic behavior of the building.

4. Discussion

Existing buildings, more than new-built ones, are characterized by a consistent amount of uncertainties, both aleatory and epistemic [84]. Since their evaluation at full building scale could be very expensive in terms of computational time, reduced pier-scale models were built to perform sensitivity analyses.

The analysis of capacity curves allowed the detection of the key factors that influence coupled URM–CLT walls. URM mechanical properties were revealed to be more influential than CLT retrofit ones (Table 8). Hence, the improvement of URM structural performances could produce the best benefits on the overall system performances, compared to timber-based ones. However, the coupling of masonry walls with CLT panels was able to provide constant benefits, in both full-shear and full-bending configurations. This could be a viable option to strengthen existing masonry structures with respect to more traditional techniques.

The pier-scale parametric analyses showed that variations of timber retrofit do not significantly alter the structural performances of a coupled wall; however, the number of URM–CLT connectors and angle brackets seemed to affect the behavior of full-shear layout, for the most part, whereas the full-bending one was sensitive to timber panel thickness and hold-down size. Since the behavior of piers in a URM structure is a mix of bending and shear behaviors, the extension of the coupling intervention to an entire building would produce benefits based on the structural system configuration and constraint conditions.

The simulation of incremental interventions in the Cattedra case study allowed assessing the benefits of coupling the perimeter walls and substituting RC-based floors with CLT panels (insertion of endoskeleton), i.e., by the so-called NB approach.

Capacity curves showed that the substitution of existing floors with CLT systems (TD model) can produce a benefit in seismic capacity due to mass reduction. The further coupling of perimeter walls with timber panels, i.e., the application of NB, provided a significant increase in seismic capacity, more than injections can do. However, the combined use of the two techniques showed great potential, since the maximum shear force the system can bear was much higher than that computed for the models with just one technique.

Hence, interventions based on CLT components showed to be effective for the improvement of the seismic capacity of URM buildings, in both substitution of and combined use with traditional techniques (e.g., grout injections). Sensitivity analyses showed clear benefits in terms of lateral seismic capacity; they were strictly related to the presence of the CLT panel and its fastening systems (i.e., angle brackets, hold-downs), whereas their further mechanical improvement did not provide a proportional overall advantage. Therefore, the design of timber strengthening systems according to current codes should be sufficient to provide a sound seismic improvement to masonry buildings, whereas a further increase in stiffness and strength could be less cost-effective.