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Gabor Palotas

Vaults, Slabs, Floor & Ceiling Systems - 2.3

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    UNIT 1 - Vaults & Domes

    • Vaults & Domes

      COURSE 2 / MODULE 3 - VAULTS, SLABS, FLOOR & CEILING SYSTEMS / UNIT 1 - VAULTS & DOMES

    • Introduction

      Vaults are curved, load-bearing structures traditionally built from stone or brick. They rely on compressive forces between the individual elements to maintain stability. However, their main drawback is that they often result in a tall structural height that is difficult to use efficiently and requires a large amount of building material. Vault construction requires the use of a template, usually made of wood. It can be dismantled when the vault is completed, but even the construction and dismantling is labour- and material-intensive. Modern slabs completely replaced vaults from the end of the 19th century. For aesthetic reasons, you may still want to cover your space with a vault. In such cases, the vault is often made of reinforced concrete.

      Vault types:

      The vaults can be formed by extruding or rotating a shape (arch) or intersecting two vaults. Depending on their shape, vaults can be semicircular, and pressed or raised. Depending also on the type and position of the support, they may be closed, semi-open or open.

      The most common types of vaults made of stone or brick are:

      • Barrel vault - The barrel vault is the three-dimensional form of the arch. It can be seen as an extrusion of the arch along its longitudinal axis. It is a surface warped in one direction, which only requires support on the sides parallel to the longitudinal axis; thus, it is considered a semi-open vault. The arch can have a variety of heights and shapes, diverting from the semicircular line, creating different subtypes such as segmental, three-centred arches, and so on…

      • Groin vault or cross vault - The groin vault is created by intersecting two barrel vaults of identical structural dimensions and shapes by removing the solid material of the intersection and keeping the vaulting outside the area of the intersection. The line of intersection of the vaults will be the groin, and the frontal arch will be the curve of the original dome. It is considered an open vault, with only the lower points of the frontal arches requiring support in piers or columns. In a Roman groin vault, the line connecting the opposite frontal arches is horizontal, and the ridge of the vault is semi-ellipsoid. In a Romanesque groin vault, the line connecting the opposite frontal arches is a rising arch, and the ridge of the vault is semicircular.

      • Domical vault - When two barrel vaults are intersected, you get a domical vault by keeping the solid material of the intersection and removing the vaults outside the intersection. This closed vault type requires masonry support around it. 

      • Trough vault - The trough vault is an extended version of the domical vault, with a rectangular rather than square plan. The resulting vault is also closed.

      • Coved vault - The coved vault can be derived from the trough vault by cutting it at about half its height with a horizontal plane and making a flat or almost flat vault at the cut.

      • Dome - Domes are surfaces of revolution. They are often hemispherical but can be made in pressed or raised arched forms too. Their base is often circular, but many domes have also been built on an elliptical base.

      • Pendentive vault  - A pendentive vault is a hemispherical dome that is raised above a circle drawn around a square, and then the parts outside the sides of the square are cut off with vertical planes set on the sides of the square. The resulting frontal arches are also semicircular. The pendentive vault is open and supported by pillars or columns at the corners.

      • Sail dome - The sail dome is also considered an open dome on all sides. It differs from the pendentive vault in that the vertical intersecting planes fall significantly inwards from the circle above which the dome is raised. The result is a segmental rather than semicircular frontal arch.

      Construction of a vault:

      The vault were built as a continuation of the walls using the rules of good bonding. Due to the geometry of the arched structure, the radial elements create wedge-shaped mortar joints. For arches with a greater radius of curvature, it may be necessary to carve the bricks into a wedge shape.

      We will demonstrate the process and concepts of the simplest type of vault  - the barrel vault - using brick as a building material:

      1. Starting from the baseline (starting line), you mason the impost where the vault should begin.

      2. To allow the joining of the arch at an angle, the bricks need to be increasingly cantilevered - a brick support with a sloping starting plane is formed from a straight plane with quarter and half bats. This creates a sloped starting plane with carved elements, which is typically one-brick wide. This is the impost width.

      3. Above the impost, the masonry continues upwards with vertical planes, and from the spring line, you can start the masonry of the vault on two sides. The part between the two is usually reinforced with masonry infill.

      4. A template and supporting scaffolding must be prepared for the vault masonry, and the vault has to be made on its surface.

      5. When starting, you proceed with the thickness of the whole brick (impost width) in the masonry, following the curvature of the template. 

      6. In the middle section, the vault can be narrowed to the thickness of half a brick. However, every 1 to 1.5 metres, there will be reinforcing arches similar to reinforcing walls with pillars.


  • UNIT 2 - Timber Ceilings & Floors

    • Timber Ceilings & Floors

      COURSE 2 / MODULE 3 - VAULTS, SLABS, FLOOR & CEILING SYSTEMS / UNIT 2 - TIMBER CEILINGS & FLOORS

    • Wood was the most widely-used material for ceilings and floors until the early 20th century. The ends of the joists, laid with a 12-15 centimetre bearing, must be protected from water, so there is no direct contact with the rest of the structure. A 2-3 centimetre air gap is kept at the end of the beam, under which a supporting sill or insulation board is placed. The ceiling material also requires fungicide and insect-proofing. These structures have a high fire-risk rating, and a further disadvantage is their low sound insulation performance. However, they are characterised by low dead weight and quick assembly work due to the prefabricated elements. Another advantage is the favourable thermal performance.

      I.Traditional timber joist ceiling

      1. Timber joist ceiling with densely-spaced joists (Timber ceiling with pegged log joists)

      The beams, split and trimmed from whole logs, are placed directly adjacent and connected by hardwood dowel pins or wedges every 1.5 (1.5!) to 2 metres, creating a slab with an extremely heavy load-bearing capacity. There was no need to trim the upper part of the beams, which remained circular. Above this, an earth filling and a subfloor could have been made. 

      2. Timber joist ceiling with sparsely-spaced joists

      a) Exposed timber joist ceiling and floor system

      When making an exposed timber joist ceiling system, the beams are placed with an 80-100 centimetre axial span at the top of the wall. A double layer of decking is laid on the surface of the beams, often filled and clay-plastered. 

      b) Cladded joist ceiling and floor system

      • The cladded joist ceiling system is an improved version of the exposed timber joist ceiling system. Here, the bottom surface is also decked.
      • In a special cladded joist ceiling system, the flooring layers are drawn into the space between the joists to reduce the ceiling thickness. To do this, longitudinal lathing is done on both sides of the joists, and the decking supporting the floor is laid on top of this.
      • The planked ceiling is a material-saving solution to the cladded joist ceiling system. The planks are placed more densely, at 40-50 centimetre intervals and are braced transversely with diagonal battens. The upper planking is made on the surface parallel to the beams.

      In every case above, earth filling and a subfloor are made for the parquet above the upper decking. The mud plaster is applied and reinforced with a reed mat on the lower surface.

      II. Modern wooden ceiling joists

      Modern wooden ceilings are timber joist ceilings with sparsely spaced joists. The cross-section of the joists can be varied and typically assembled from laminated glued elements bonded together. Joist spacing depends mainly on the span and size of the joists' cross-section: it varies from 40-50 centimetres to 80-100 centimetres axis spacing, considering the level of use.

      The attachment method can include anchor studs, steel pockets, or anchor rails combined with angle bars. In all cases, the receiving elements must be carefully positioned and adjusted before the reinforced ring beam is poured. The ends of the wooden joists must have at least 15 centimetres of bearing width and be protected against moisture with insulating boards.

  • UNIT 3 - Steel Floors Systems

    • Steel Floor Systems

      COURSE 2 / MODULE 3 - VAULTS, SLABS, FLOOR & CEILING SYSTEMS / UNIT 3 - STEEL FLOOR SYSTEMS

    • 1. Jack arch slab

      Before the widespread use of reinforced concrete, a proven and common solution from the 1880s to the 1940s was to combine I-section steel beams with a small arched barrel vault of only half a brick thick. This was the jack arch vault. Over time, the solution also produced a special block that acts as an impost to provide the inclination needed to launch the vault, which made the connection between the steel beams (placed at a spacing of 1-1.20 metres) and the arch easy to establish. Typically, an earth filling and concrete slab were constructed above the vault.

      2. Reinforced concrete ribbed slabs

      a) with downstand or upstand steel beams

      Steel beams, typically I-section, spaced 1 to 2.5 metres apart, with a 6 to 10 (a 6 to10!) centimetre thick reinforced concrete slab between them. A 3-centimetre thick reinforced concrete casing with a trapezium cross-section also encloses the steel beams.

      b) In the case of reinforced concrete ribbed slabs with downstand steel beams, the slabs are located in the upper belt of the beams, making their upper side flat, while in the case of reinforced concrete ribbed slabs with upstand beams, the slabs are located in the lower belt of the beams, creating a flat lower plane. The floor construction could start immediately on the surface of the reinforced concrete ribbed slabs with downstand steel beams. In contrast, on the surface of the reinforced concrete ribbed slabs with upstand beams, a considerable amount of additional infill is necessary first.

      3. Steel beam ceiling system with brick infill and concrete slab

      This is the lighter version of the reinforced concrete ribbed slab with upstand steel beams. Concrete with a reinforcing mesh and 5 centimetres of monolithic reinforced concreting are placed between the sparsely-placed brick raster. The overall thickness of this slab is 10-12 centimetres.

      4. Modern lightweight steel beam ceiling system 

      The system is often used in steel-frame buildings such as halls. A typical design is as follows: C- or I-section steel beams spaced with axial span of  50-90 (50 to 90!) centimetres with corrugated sheets laid on top of them and fixed to the beams with bolts. On top of this, 6 to 8 centimetres of monolithic reinforced concreting is laid, on which the floor layers can be prepared.

  • UNIT 4 - Monlithic RC Slabs

    • Monolithic Reinforced Concrete Slabs

      COURSE 2 / MODULE 3 - VAULTS, SLABS, FLOOR & CEILING SYSTEMS / UNIT 1 - MONOLITHIC REINFORCED CONCRETE SLABS

    • I. Monolithic reinforced concrete slabs (Introduction!)

      Monolithic reinforced concrete slabs are custom-made structures, cast on-site, based on detailed structural designs to ensure that the reinforcement's dimensions and distribution, also the slabs' thickness meet the required load-bearing requirements at all points. To achieve a good load transfer connection with the walls, a ring beam with scaled reinforcement is formed at the edge of the slabs.

      I. Monolithic reinforced concrete slabs 

      Monolithic reinforced concrete slabs may be flat slabs, ribbed (upstand or downstand or coffered at the top (top???) slabs or mushroom slabs two-way slabs.

      1. Flat reinforced concrete slabs 

      Flat slabs are flat structures, typically 15-25 (15-25!) cm thick. The slabs can be load-bearing in one or two directions. One-way load-bearing slabs may cover rooms that have a shorter span (2.5-4 metres). The main bars are placed in the shorter direction, while the longer direction is used only for distribution bars. The long side is primarily used for load carrying, but typically, a ring beam is made on the short side too. In the case of bi-directional load bearing, larger spans (4-10 metres) have main iron inserts placed in both directions -, with a primary load bearing requirement for each wall.

      2. Ribbed reinforced concrete slabs

      In the case of ribbed reinforced concrete slabs, the flat slabs are reinforced with beams to carry the loads, while the plates act as load distributors. This is reflected in the slab thickness. The minimum thickness of ribbed reinforced concrete slabs is only 6-8 (6-8!) cm.

      a) Downstand reinforced concrete slabs (a) in not needed!)

      Here, the ribs are located at the bottom of the slab. Together with the ring beam, they require a complex geometrical formwork, allowing to make the reinforcement and carrying out the concreting in one step. The density of ribs may vary:

      • Often, stronger beams (15-40 cm high, 15-20 cm wide!) are placed at an axis spacing of between 1.5 to 2.5 metres.
      • In the case of a reinforced concrete slab with dense ribs, the axis spacing is only 50-60 cm (50-60 cm!) and the width of the ribs is a maximum of 12 cm (12 cm!).

      The upper level can be used for the floor, and the lower ribs can be used to install mechanical ducting. According to the requirement, the lower ribs can be left exposed or concealed by a false ceiling.

      b) Upstand reinforced concrete slabs

      For an upstand slab, the ribs are positioned on top of the slab. The load-bearing capacity of such a slab is significantly lower than that of the upstand version. Formwork is not easy to build. Although the space between the ribs can be filled with larger mechanical ducting, a false floor must be installed (or the voids filled to create the floor level).

      c) Waffle reinforced concrete slab

      (Murmuring to be removed!) A waffle slab is a network of perpendicular ribs. When installing formwork, plastic exclusion elements are inserted which can create coffers opened from below or closed with hidden cavities within the slab.

      3. Mushroom flat slabs (withouth 3)!)

      Mushroom flat slabs are flat reinforced concrete slabs with two-way load-bearing and point-like (concentrated) supports. The connection of the pillars is achieved by means of a cone-like mushroom capital. The slab thickness is at least (15 cm!) due to the two-way main iron inserts and the reinforcing iron around the supports. 

      The more modern versions of the mushroom flat slabs have hidden capitals, creating a completely flat plate on the entire top and bottom sides of the slab. This further increases the required slab thickness, as the anti-punching shear reinforcement must be completely hidden in the slab. With respect to the challenges of reinforcement installation, this is the most practical monolithic reinforced concrete slab from an aesthetic, serviceability and formwork point of view.

      II. Modern execution of monolithic reinforced concrete slabs: (II. to remove!)

      The four important steps in the construction process are: formwork, reinforcement, concreting and removal of the formwork.

      Formwork

      In modern practice, we use large formwork sheets with timber beams which allow the formwork to be assembled quickly.

      1. First, position and adjust the three-legged, drop-headed floor props to the desired height.
      2. Next, insert the primary beams.
      3. After that, position the cross beams.
      4. Finally, place the formwork sheets.

      The formwork of the associated structures such as beams, stairs, and others is connected to the large slab system either by using the elements of the same system or by applying a custom-made traditional formwork.

      Installation of reinforcement

      1. Start with placing the main rebars for the base.
      2. Distribute the reinforcement of the base.
      3. Fasten two-way irons together with tie wires to create a mesh reinforcement.
      4. Position the mesh and place spacers to create the required baseconcrete cover.
      5. For ring beams, position and fix  in the following order: the bottom longitudinal bars, the stirrups and then the upper longitudinal bars.
      6. For the top part, place top distribution bars on top of the ring beam bars.
      7. Then, distribute the main rebars at the top.
      8. Finally, form the top mesh by connecting the main and distribution bars with tie wires. 

      Concreting

      1. Before starting, the structural engineer checks the reinforcement and the formwork alignment, and also its penetrations. (E.g., water pipe)
      2. Mark the top level of the concrete on the inside of the ring beam formwork.
      3. Pour concrete with a concrete pump from a mixer truck, preferably uninterrupted, in one single pour.
      4. Work the concrete by hand to spread, compact and smooth.

      Removing the formwork.

      1. Keep the hardening concrete moist and watered.
      2. After a few days, when the slab is sufficiently solid, remove the intermediate supports and the floor props.
      3. Lower the primary beams with the drop heads and remove the cross beams.
      4. Remove the primary beams and part of the formwork leaving only the props in place.
      5. Once the concrete is fully hardened, remove the entire formwork including all the prop supports.

  • UNIT 5 - Precast And Semi-monolithic RC Floors

    • Precast & Semi-monolithic Reinforced Concrete Floor Systems

      COURSE 1 / MODULE 1 - EARTHWORKS & DRAINAGE / UNIT 1 - PRECAST & SEMI-MONOLITHIC REINFORCED CONCRETE FLOOR SYSTEMS

    • Precast and semi-monolithic reinforced concrete slab systems started to become widespread in the 1950s, contributing to more efficient and economical construction. By using elements from these systems, it is possible to construct slabs of any size (within certain size limits) and adapt it for a modular system. The slab systems do not require specific structural calculations and can be constructed without formwork. There is no need for individual installation of reinforcement, except for sections cast in-situ where necessary. 

      1. Precast reinforced concrete slab with filler blocks

      In our example we show a classic E-type beam slab with a wall spacing of 240 - 660 cm (240 to 660 centimetres!).

      Joists (beams): The primary load-bearing structure for precast reinforced concrete slabs with filler blocks are typically place joists precast reinforced concrete joists, placed with a 60 cm axial spacing in one direction of the slab to bridge the span of the floor in one direction. Above a certain span, the beams need to be doubled, increasing the load-bearing capacity and reducing deflection. To achieve less deflection and fewer cracks, it is recommended to use a system with prestressed beams. Prestressed beams are designed with built-in tension: the lower part is compressed, and the beam is slightly curved upward before installation. Once installed and loaded, the prestressing counteracts the weight, resulting in a level, flat slab. Important: Prestressed beams must not be cut, as this would release the tension and compromise their structural integrity. They are usually lifted into position using a crane.

      Ring beam: The minimum bearing length of the elements on the wall is 10-15 cm, depending on the type and span. The ring beam must have unique reinforcement and shuttering realised in a way to raise the beams 5 cm above the wall surface. This way the lower longitudinal bars of the ring beam can then be placed under the beams, allowing them beams to be fully embedded.

      Filler blocks: Filler blocks, resting on the top edges lower edges of the beams fill the in-between spaces between the beams. Filler These blocks are typically hollow light-weight elements made of concrete, ceramic or even cement-bonded wood fibre, with a height of 19 cm (similar to the beam).

      Monolithic sections: It is usually necessary to assemble a formwork and build a monolithic slab edge, unless modular dimensions of the system are used. A monolithic section is required around the planned penetrations, also.

      Monolithic reinforced concrete layer, concreting: The slab system is topped with a 5 cm monolithic reinforced concrete layer and with mesh reinforcement. Concreting is carried out together with the ring beam and the monolithic sections, filling in the gaps between the beams and the hollow filler blocks.

      2. Semi-monolithic reinforced concrete slab with filler blocks (2. to remove!)

      More modern versions of the precast reinforced concrete slabs have special joists, giving the monolithic parts more room to participate in load-bearing. The type presented here is the Wienerberger Porotherm slab system, made in the same modular system as its own masonry elements, with a wall spacing of 225 - 700 cm.

      Joists: The joists are prestressed. The low-height base (6.5 cm) consists of a reinforced concrete core and a ceramic shell underneath. From this, the stirrups project every 25 cm. The joists have an axial spacing of 45 and or 60 cm respectively, and the required bearing length is a minimum of 12 cm. The tops of the walls must be protected against moisture by insulating sheeting and laying the cement-based mortar under the joists. For the duration of the construction, the joists must be supported at intervals of 160-175 cm, and should be removed after the monolithic slab has fully hardened (after exactly 21 days).

      Filler blocks: The filler blocks are also designed with a lower height (10 or 17 cm), providing more space for the in-situ monolithic reinforced concrete layer. They should be placed so that the joists receive the load equally on both sides; in rows perpendicular to the joists. 

      Reinforcement: The top reinforcement of the slab is constructed on site, with the bars leading through the stirrups. The joists are tied with both its top and bottom bars into the ring beam. The bottom bar is located on the top of the base of the joist. Its length is 200 cm (Ø10). The length of the top bar (Ø10) is one sixth of the span in the slab field (min. 80 cm). It extends completely through the ring beam and is folded over the outer longitudinal bar. The bottom bar is located on the top of the base of the joist. Its length is 200 cm (Ø10).

      Filler blocks: The filler blocks are also designed with a lower height (10 or 17 cm), providing more space for the in-situ monolithic reinforced concrete layer. They should be placed so that the joists receive the load equally on both sides; in rows perpendicular to the joists. 

      Crossbeams: The slab also needs to be stiffened in the transverse direction, which can be achieved by installing a monolithic reinforced concrete crossbeam. This is a crossbeam the width of a filler block, which must be installed at least every 2 metres. In the line of the crossbeams, we use a filler block of 10 cm in height to provide space for beam reinforcement (min. 4 pieces of Ø8 and Ø6 stirrups) and the monolithic concrete.

      Partition walls: Reinforce under the partition walls perpendicular to the beams, and form a crossbeam for heavier loads. Doubling the beam for partition walls parallel to the beam reinforces the slab for the extra load. If the partition runs mid-span, the monolithic reinforced concrete layer can be additionally reinforced between the adjacent beams.

      Concreting: The monolithic reinforced concrete layer is formed with a 4-7 cm thick mesh reinforcement of 150x150 mm spacing, with a minimum of Ø6. A reinforcement mesh of 150 x 150 mm spacing and Ø6 diameter is placed to form the top concrete layer of the floor. The ring beam, crossbeams, the monolithic reinforced concrete top layer and the monolithic parts of the beams are poured all at once.

      3. Semi-monolithic lattice girder slabs

      For the lattice girder slab solution, the proportion of the monolithic part is further increased. The large surface consists of a thin precast reinforced concrete panel of 5-8 cm thickness, interpreted as permanent formwork, over which 14-30 cm of monolithic reinforced concrete layer is poured in-situ. These panels are not mass-produced, but unique prefabricated products with a typical width of either 1.20 or 2.40 m. If the reinforcement is custom-calculated, it can span 10-12 m of wall spacing. The main advantage of this system is that the slab does not require any formworks and the elements can be quickly lifted in place by crane. The disadvantage is that the monolithic part requires a lot of reinforcement and supporting scaffolding must be constructed. Both standard and prestressed versions of the panels exist.

      4. Precast reinforced concrete plank slabs

      a) Precast reinforced concrete hollow-core slab

      Of all the precast slabs discussed so far, the largest span of up to 22 metres can be achieved with reinforced concrete hollow-core slabs. They also require the least live labour. The larger span is associated with a very high structural height (from 20 cm to 50 cm), in addition to the typical width of 120 cm. As these are custom-made products, it is necessary to calculate reinforcement to ensure the necessary load-bearing capacity.

      Another advantage of reinforced concrete hollow-core slabs is that they usually do not require any top monolithic reinforced concrete layer. The hollow cores are necessary to reduce dead load and are suitable for mechanical ducting. The elements already have a considerable weight, but for this reason, the ring beam has to be made in two stages: first for the bearing, and then for the part coplanar with a top of planks. In this case, tie bars must be placed between the planks, ensuring that they act together with the ring beam.

      During the concreting process, the concrete must be kept away from the cavities using plastic plugs. Tie bars must be placed between the planks, ensuring that they act together with the ring beam. A monolithic edge section may be necessary at the perimeter of the slab.
      However, planks can also be delivered to the site pre-cut to the required size, as long as the cut does not intersect a hollow core.

      b) Precast slab with aerated concrete floor panels

      Aerated concrete is panels are solid, therefore, there is no need to use hollow cores as the density of aerated concrete is significantly lower than that of normal concrete. The typical width of the elements is around 60 cm, but the span is limited to only 6 metres compared to the reinforced concrete version. The thickness of the elements can vary from 12 cm to 36 cm, depending on the load. The planks are connected with tongue and groove, which makes it more difficult to lift the elements in place and fit them together. No special reinforcement is required; the planks contain all the necessary reinforcement, and in most cases, no top monolithic reinforced concrete layer is needed.

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