Efficient connection of reinforced concrete structures with AAC – part 2

Connections in structural masonry
The method of connecting structural masonry walls to other members of the building depends on the material the wall is to be connected to. Connections are provided using friction, compression or metal anchors. Different types of connections are described below.

Masonry wall-masonry wall connections
EC-6 [1] requires that mutually perpendicular or oblique walls are connected to each other in a manner which would ensure that vertical and horizontal loads are conducted from one wall to the other. To obtain this connection, you can bond masonry units in the masonry wall. It could be provided by means of:
• masonry wall bonding,
• fasteners or reinforcement extending into each wall.

It is recommended to erect intersecting load-bearing walls at the simultaneously, as this would ensure proper bonding of masonry units in their contact plane.
The same recommendations are included in previous Polish bridging standards [8, 9] and the draft Euro- code EC-6V. The standards do not specify detailed recommendations though. Nonetheless, design mo-dels of walls which are subjected to mainly vertical loading and stiffening walls explicitly confirm that the best solution to ensure the interaction of intersecting walls is to use conventional masonry nodes.

Masonry wall-reinforced concrete connections
Masonry wall-reinforced concrete type connections are used in confined masonry walls at the intersections of perpendicular walls; in walls which provide filling for a reinforced concrete frame (stiffening walls) and oblique walls; and as wall-floor vertical connections.

Confined masonry walls
As defined in Eurocode 6, a confined masonry wall is one whose deformations in the vertical and horizontal directions are restricted by an adjacent reinforced concrete structure or reinforced masonry wall. The definition of the confined masonry wall and related regulations were only included in standard recommendations hen the Eurocode was implemented. This does not mean that confined masonry walls had not been built in Poland though. Prior to the implementation of EC-6, confined masonry walls were designed and built in the country based on the guidelines of the Building Research Institute (Instytut Techniki Budowlanej, ITB), publications and local practices [10]. Despite the common use of confined masonry walls in Poland, the instruction by ITB is one of the few documents setting out their design principles. Basically, no research is conducted in Poland or abroad concerning the behaviour of confined masonry walls subjected to static loads. There are many studies concerning confined masonry walls subject to seismic impacts but this issue is not covered in EC-6 (but in EC-8), so these studies shall not be cdiscussed here.
Numerous publications emphasise the differences between a confined masonry wall and a masonry wall which provides filling for a frame. If a reinforced concrete or steel structure is erected first, and then walls are built in the space between the posts of the structure, it is difficult to ensure full contact between the posts and beams (structure) and the wall (filling). Then the deformation limitation condition arising from the definition adopted in EC-6 is not met and the wall may not be treated as confined masonry. The proper construction technology for confined masonry walls requires that walls are built first and then the reinforced concrete structure to be tied to the masonry wall is constructed.
Confined masonry walls have been built in the country for many years. Admittedly, confinement is not provided as bands of reinforced masonry, but reinforced concrete cores and ring beams are often made to limit wall deflections, in particular on areas affected by mining operations. Horizontal ring beams of reinforced concrete, which ensure spacial stiffness of the object and provide support for floors, are common. However, a confined masonry wall is defined as a wall which is confined both in the horizontal and vertical direction. Instruction No. 391/2003 [11] by ITB identifies three ways to form vertical confining elements: place reinforcement in bores in masonry units (Fig. 8a), make it possible to guide a single bar in the masonry (Fig. 8b) or construct a reinforced concrete post (core) in the wall – Fig. 8c. Reinforced concrete cores are used most frequently but all the three methods have been and still are use in the country. All these methods limit deformations and fall under the Eurocode definition of the confined masonry wall.
In accordance with ITB Instruction No. 391/2003, it is necessary to use vertical confining elements (referred to as vertical ring beams), when a building is affected by tremors due to mining operations. Vertical confining elements should be used when tremors characterised by ground acceleration values exceed 500 mm/s2. The need to use vertical confining elements in masonry walls exposed to tremors of considerable intensity is also confirmed by reference publications [12]. Reinforced concrete cores are actually often designed and installed at lower ground acceleration values and in objects subject to continuous and non-continuous ground deformations. The cross-section of the reinforced concrete core is derived from the wall thickness, thus cores with the cross-section of 18x18 cm to 40x40 cm are typically used. For higher wall thicknesses, thermal insulation of foamed polystyrene is sometimes added to cores on the outside so that the total thickness of insulation and the core would be equal to the wall thickness. To ensure proper interaction between the core and the masonry wall, toothings are often left in the masonry wall and later filled with concrete when embedding cores in concrete (Fig. 9). When a wall is built without toothings, connection may be provided by extending reinforcement from bed joints of the wall into the reinforced concrete core.
In stiffening walls, vertical reinforced concrete cores in buildings exposed to tremors due to mining operations should conduct tensile forces arising due to bending moments in the plane of walls and collaborate in conducting transverse forces generated by ground vibrations. According to the Instruction [11], the distances between cores should be derived from a design analysis of a wall which is mostly exposed to horizontal loading (bent from the plane). Cores should be located in connections of load-bearing walls and stiffening walls, i.e. in wall corners and at connection points between external load-bearing walls and internal walls. The reinforcement of ring beams and cores is usually assumed based on the condition concerning the minimum percentage of reinforcement acc. to the reinforced concrete standard. As recommended in ITB Instruction No. 391/2003, the minimum cross-section of the reinforcement for a vertical core should be ASV,min = 0.0004 m2.
PN-EN [1] recommends that confined masonry walls should have vertical and horizontal confining ele-ments made of reinforced concrete or reinforced masonry to ensure full cooperation in conducting impacts. To this end, the top and side confining elements should be made after the masonry wall has been built, in such a way as to ensure their connection to the wall. This is achieved by leaving toothings in the masonry wall (Fig. 10a), to be filled in with concrete, or by using reinforcement in the wall bed joints, to be extended into monolithic cores (Fig. 10b). The Eurocode recommends using toothings in masonry walls made of group 1 and group 2 masonry units. For reinforcement connecting the masonry wall to the core, you could use bars with a diameter not less than 6 mm or equivalent, spaced by no more than 300 mm. If the wall design provides for reinforcement of bed joints (e.g. due to bending), anchoring could be provided by extending that reinforcement into the core (Figs. 10c). The anchorage length for a straight bar in the reinforced concrete core should be calculated with the following formula:

ib =ø/4•fyd/fbod, (1)

where ø is the diameter of the bar, fyd is the design yield point of steel, and fbod is the design adhesion of the reinforcement calculated on the basis of the characteristic adhesion fbok, which is different for bars anchored in concrete and in the mortar of joints.
The characteristic adhesion of prefabricated bed joint reinforcement is determined by tests in accordance with PN-EN 845-2:2004/Ap1:2005 or is assumed as corresponding to single longitudinal bars of the reinforcement. On account of the low lateral dimensions of the core, straight or half-round hooks are the most common anchorage used for bars.
According to PN-EN [1], confining elements should be made at the level of each storey. Vertical confining elements (cores) should be placed in connections of load-bearing walls and on both sides of every hole with an area exceeding 1.5 m2. Horizontal confining elements (ring beams) are made at floor levels. Additional confining elements may be required in walls where the maximum span, both vertical and horizontal, is 4.0 m. Recommendations in EC-6 concerning the placement of confining elements at holes with an area of more than 1.5 m2 and spacing of 4.0 m in the horizontal and vertical directions are very strict. To meet these requirements, you need to provide many more cores in walls as compared to the number arising from the previous recommendations. Fortunately, the recommendation concerning the spacing of ring beams and cores at 4.0 m is not obligatory and PN-EN [1] leaves the adoption of such spacing up to a designer. Note that PN-EN [1] does not apply to design for seismic impacts which is covered by PN-EN 1998-1 [13]. The impact of masonry wall confinement on the behaviour of walls subject to static or quasi-static loading is not known well and there are considerably fewer studies or analyses concerning such structures than for confined masonry walls subjected to seismic impacts. Interestingly, the maximum spacing of confining elements on seismic areas is 5.0 m acc. to PN-EN 1998-1 [13], i.e. more than stated in PN-EN [1]. Considering the lack of sufficient research, the provision about the possible need to use confining elements spaced every 4.0 m is adopted in PN-EN [1], to be on the safe side, based on general recommendations in reference publications and numerous standards (e.g. Mexican, Argentinian, Colombian). Note that the maximum spacing of confining elements at 4.0 m is one of the lowest values assumed in standards on design for seismic territories, while many standards (EC-8 included) allows for the spacing of 5.0 m (e.g. the Italian standard) or more.
For the spacing of cores at 4.0 m, PN-EN [1] uses the phrase “may be needed”; but for holes with an area of more than 1.5 m2, the standard states that cores “should” be used at such holes. As arises from this provision, cores should be used at almost every window opening and every door-way. According to Tomazevic, the size of 1.5 m2 is too small and proposes in his paper [14] to increase the maximum area of the whole where no confining elements are required to 2.5 m2. The Mexican standard [15] assumes that confining elements should be designed when the width of a hole is greater than ¼ of the distance between vertical cores.
According to PN-EN [1], confining elements should have a cross-section of not less 0.02 m2, with the smallest dimension of not less than 150 mm in the plane of the wall and should have longitudinal reinforcement with a minimum cross-section equal to 0.8 % of the cross-section of the confining element but not less than 200 mm2. Stirrups with a diameter of not less than 6 mm and spacing of up to 300 mm should be used. An example of the smallest core acceptable by EC-6 is shown in Fig. 11.

In confined masonry walls, where group 1 and group 2 masonry units are used, the units adjacent to the confining elements should overlap according to the rules for masonry wall bonding set out in EC-6. Alternatively, reinforcement with a diameter of not less than 6 mm or equivalent and spacing of not more than 300 mm, anchored in the concrete infill and joints filled with mortar could be adopted.
The reinforcement of vertical cores and horizontal spandrel beams and ring beams shall be constructed in compliance with the recommendations of the reinforcement concrete Eurocode. It is extremely important to adopt the right methods for anchoring bars, especially in the area of corners of ring beams and spandrel beams and connection of horizontal units to cores.

Masonry walls for stiffening a reinforced concrete or steel frame
For walls which are stiffening elements for frame structures (of reinforced concrete or steel), their contact with such structures must be ensured. It is typically achieved by filling the contact point between the masonry wall and frame posts with cement mortar. It is not easy to ensure full contact between the masonry wall and the frame, thus one could observe recently a tendency to make monolithic stiffening walls in reinforced concrete frame structures and masonry walls are then provided as filling walls.

Horizontal connections of walls and floors
When concrete floors or roofs rest on a wall, the load capacity of the connection may be, on account of shearing, provided by friction according to EC-6 [1]. Actually, this type of connection could be provided by using reinforced concrete ring beams on all the load-bearing and stiffening walls of the object, both external and internal ones. Such a design solution where monolithic ring beams are made on a wall at the same time as floors are made, shall ensure due connection between the masonry wall and the surface of the ring beam concrete.
Ring beams are crucial when the structure is damaged due to its misuse or unusual impacts to a disproportionate extent if compared to normal conditions.
The standard [1] explicitly assumes that with a correctly designed arrangement of ring beams a secondary load-bearing structure will be formed. Thus, reinforced concrete ring beams which connect walls to floors are essential for the formation of a secondary load-bearing structure under such circumstances where a large portion of the load- bearing wall has been damages, which prevent the progressive collapse of the building.
The progressive collapse mostly threatens insufficiently bound prefabricated structures [16] but cannot be excluded in buildings with masonry walls either. Although buildings with a concrete structure (made of large panels) and buildings with masonry walls differ largely from each other, the same method is used to reduce a progressive collapse due to unusual impacts in both cases. The aim is to have such a configuration of floors and ring beams as to maintain the full integration of elements of the load-bearing structure at considerable elongations of the reinforcement and appearance of wide cracks – Fig. 12. The ring beam reinforcement is essential for the formation of a secondary load-bearing system and it takes over tensile forces which are generated in the structural walls in the zone over the damaged part of the building.
Another significant item is the supporting reinforcement of floors which consolidates the single floor bays into a rigid shield and also prevents the floors from falling off the support when a load-bearing wall is lost – Fig. 13.
ITB has conducted research into the behaviour of a wall bracket formed over the damaged area in the corner of a large-panel building type Wk-70 with a relatively tenuous connection of floor slabs [16, 17]. The conclusions of the research and analysis of the secondary load-bearing structure were used to formulate structural recommendations for large- panel buildings with a height up 12 storeys and floor spanning up to 6.0 m with a regular view. With the research, it was determined that the formation of the secondary load-bearing structure required ring-beam reinforcement over the load-bearing wall capable of conducting a force equal to 140 kN or higher, and for 8-storey building – force of 80 kN.
However, the formation of the secondary load-bearing structure requires that walls are constructed which are capable of taking over compressive forces present in the secondary structure and these could be masonry walls filling the space between the posts of a frame structure. In a building with a frame structure and lightweight infill walls incapable of taking over compressive forces, you can design posts with an adequately low spacing to enable the formation of the secondary load-bearing structure.
Ring beams are reinforced concrete elements encompassing the entire building or its specified part. They are usually reinforced concrete elements located in the place of floors and subjected to tension or compression and shearing, and less frequently to bending. Ring ties could be made as steel bars or sections or reinforced masonry walls. In compliance with the assumptions adopted in EC-6, these elements should be capable of conducting design tensile forces with a value of not less than 45 kN. Circumferential reinforcement of ring beams or ties should be constructed as continuous even if there are hole or changes in floor level etc. When ring ties of reinforced concrete are used, EC-6 recommends that at least two reinforcing bars are used with a total area of not less than 150 mm2, which in practice means that 2 bars ø 10 mm are used. Parallel continuous reinforcement may be considered when with its full cross section provided that it is situated in floors or lintels at a distance of not more than 0.5 m from the middle of the wall or floor respectively. For the construction of ring beam reinforcement as regards materials used and reinforcement connections, EC-6 refers to provisions in EC-2, underlining though that reinforcement laps should be staggered.
In reinforced concrete objects with prefabricated floors of hollow core slabs and some beam and block floors, longitudinal ring beam reinforcement of 3 ø 10 mm is typically used. However, for buildings with a conventional wall structure with beam and block floors or monolithic floors, the most common solution used in ring beam is 4 ø 10 mm. The ring beam height should be equal at least to the structural height of the floor and not less than 125 mm.
As the standard [18] recommends for all multi-storey buildings with floors of reinforced concrete, of ceramic and reinforced concrete and of autoclaved aerated concrete planks, load-bearing masonry walls should be terminated with reinforced concrete ring beams with a cross-sectional area of at least 0.025 m2. The reinforcement of ring beams, regardless of the steel grade, should have a cross section not less than 230 mm2, which in practice involved the use of 3 bars with a diameter of 10 mm. The paper [16] also states that for buildings with 5 or more storeys it was recommended that the cross-section of the transverse reinforcement of ring beams should be not less than 330 mm2 (4ø12).
After the standard [19], the bridging standard [9] assumed that for buildings with masonry walls reinforced concrete ring beams should be provided for which would encompass all the structural walls in the building at the floor level. In consequence, floors could be considered as horizontal, rigid shields for calculations of stiffening walls. The longitudinal reinforcement of ring beams should be capable of conducting the tensile force Fi not lower than

Fi ≥ ii 15 kN/m ≥ 90 kN (2)

where:

ii – the distance in the transverse axes of stiffening walls, in meters.
To meet the serviceability limit state conditions for the width of cracks, the calculation concerning the reinforcement required was conducted at the characteristic yield point of steel. In addition, the brid-ging standard [9] clearly stated that other structural members with reinforcement capable of taking over tensile forces in the plane of load-bearing walls could be also treated as ring beams. It basically applies to sections of reinforced concrete floors with reinforcement along edges which is formed as hidden beams or three-hole reinforcement. For ring beams used in the form of reinforced concrete beams, German recommendations [20] require that the reinforcement is made of at least two bars with a diameter of ø 10 mm. If a ring beam is formed as reinforced masonry, then reinforcement consisting of 3 ø 8 mm or 4 ø 6 mm should be situated in two bed joints (starting at the lower surface of the floor). For objects located on areas subject to non-uniform settling or to tremors due to mining operations [21], it is recommended to design floors as monolithised shields stiffened in their planes with reinforced concrete ring beams, cast on site and placed on the circumference and along the internal load-bearing and stiffening walls. Continuous reinforcement is required in ring beams, thus it is recommended to connect bars by welding. The ring beams of the floor over the basement should be reinforced with at least 4 inserts with a diameter of 12 mm. The reinforcement used over the other storeys may be typical with a total area equal to 230 mm2 (PN-89) rather than 150 mm2 as in EC-6. For a floor over a hole or with a pass whose dimensions are greater than 6 times the floor thickness, a closed ring beam of reinforced concrete with a width of at least 200 mm should be designed around the hole and it should be capable of transferring horizontal forces attributable to the cutout part of the floor – Fig. 14.

Summary
The assumption of the right method to connect a wall to another structure is a major stage of design and has a crucial effect on the stresses and deformations of a building and the wall. As the referred examples of wall connections show, there are no typical and routine structural solutions. On account of the diversity of possible connection methods, it is necessary to show appropriate structural details in a building permit design or detailed engineering design. The frequent lack of such solutions should be considered a lapse which could lead in consequence to construction solutions resulting in wall damage.