New Nautilus: the voided biaxial slab formerrev. 2012 1.1
Voided slabs in reinforced concreteLightweight reinforced concrete slab: The necessity of decreasing the weight of RC slabs has various reasons: ARCHITECTURE * Obtain large spans, with fewer columns or walls; * Avoidance of drop‐beams. ENGINEERING * Self‐weight reduction of the slab in order to: ‐ reduce its deformation ‐ reduce the weight (loads) transmitted to the foundation ‐ reduce the oscillating mass, thus the movements during an earthquake The solution : to hollow out the slab
Precast concrete elements The use of precast elements has certain constraints:TRANSPORT A flatbed truck must be used even for small quantities or single parts. This may cause difficult access to jobsites in city centres and with heavy traficLOGISTICS Precast slabs occupy a lot of space on site. A crane is mandatory for placing. In case of polystyrene void formers the precast parts must be protected from rain. USE The precast parts are handled with some difficulty above the fourth floor. Drop‐beams are often required.
AdvantagesThe advantages of the new Nautilus void former are numerous: TWO‐WAY STRUCTURE: a two‐way slab will distribute loads on 4 sides (instead of 2 for one‐way slabs), reducing the maximum loads on beams or mushroom posts. LIGHT: the concrete not essential for the structure is eliminated. The self‐weight of the slab is limited, reducing the loads transferred to the foundation; deformation of the structure is reduced. FLEXIBLE: it will allow to eliminate drop‐beams and create flat‐soffit slabs without interruptions of large surface.
AdvantagesThe advantages of the new Nautilus void former are numerous: QUAKE RESISTANT: the voids reduce the self‐weight of the slab, reducing the seismic mass. LARGE SPANS: larger spans between columns are possible. The number of columns is reduced, the result are archutecturally more manageable volumes. COST‐EFFECTIVE: a slab with new Nautilus with the same static and seismic characteristics consume a smaller amount of concrete and steel than the full‐concrete equivalent slab: ‐ up to 35% slab weight reduction ‐ up to 50% fewer columns ‐ combined saving effect: 5 to 10% cost reduction potential
new Nautilus singleThe new Nautilus void formers are available in various heights, all measure 520x520 mm in plane. The “feet”, are spacers which determin the thickness of the lower slab, and are availablein height between o and 100 mm. H16 Geoplast Nuovo Nautilus h16 Geoplast Nuovo Nautilus H20 Geoplast Nuovo Nautilus h20 H24 Geoplast Nuovo Nautilus h24
new Nautilus doubleThe new Nautilus “single” void formers can be combined in a “double” configuration to allowlarger void‐formers. Geoplast Nuovo Nautilus h20 Geoplast Nuovo Nautilus h24 Geoplast Nuovo Nautilus h16 Geoplast Nuovo Nautilus h20Geoplast Nuovo Nautilus h16 H32 H36 H40 H44 H48
Installation Installation of the new Nautilus void formers is very simple and fast: 1. The new Nautilus void formers don’t have an orientation. Installation is fast and does not require any special care or attention. 2. The spacer strip makes spacing control between void formers simple and accurate. 3. It is possible to tread on the new Nautilus void formers. This keeps the laying of the upper slab armature really simple. The new Nautilus caissons resist up to 1500 N pressure on an 80x80 mm footprint for safe job‐ site application.
Steps 1-2INSTALLATION 1. Prepare a slab formwork; lay the welded mesh on spacers. 2. Install the new NAUTILUS void formers, spacing between caissons as per drawing. Use the spacer strip to check correct distance.
Steps 3-4INSTALLATION3. Install all required extra reinforcement (shear‐ and moment‐reinforcement); lay the upper slabwelded mesh. 4. First pouring phase: pour concrete to fill the lower slab, starting from the ribbing, up to the lower side of the new Nautilus void formers. Vibrate the concrete immediately.
Step 5INSTALLATION5. Second pour phase: after completion of the first pour phase, when concrete has achieve some strenght, fill the ribbings and complete the top slab as required by the project.
TEST RESULTS Cross‐section of actual pour: concrete type: S3 poured in two lifts as per instructions, vibrated with eccentric poker. NEW NAUTILUS Single H16, lower slab thickness 50 mm No concrete ingress ; lower slab completely filled Typical cross‐section: NEW NAUTILUS Double H32, lower slab thickness 60 mm
CalculationSTUDY The new NAUTILUS caissons create voids in a RC slab poured in situ. The condition in shich the new Nautilus void former gives the most advantage is in a two‐way slab configuration. In order to have a two‐way behaviour the ratio between the sides of the slab must be between 1.7 and 1: beyond this ratio the behaviour will become one‐way, and other methods to reduce slab weight may be more advantageous. SLAB THICKNESS The first step in the study of a full concrete slab is to formulate an hypothesis of the indicative thickness. This thickness depends from the type of structure being studied: Slab on columns d = L / 25 Slab on beams d = L / 30 Waffle slab on columns d = L / 20
InertiaSTUDY Once the hypothesis of the minimum slab thickness has been formulated, the voided slab with equivalent charactesistics of resistance and deformation must be identified. A full concrete and a voided slab are compared based on their inertia. The inertia of the full concrete slab must be calculated, and compared with the voided slab solution. 1 I Ifull = ⋅ 100 ⋅ H2 Iall = full 12 B + 52 The inertia of the voided section is calculated according to the span of the void formers. Based on the inertia values of the voided section is is possible to calculate the thickness of the slab, and consequently choose the size of dimensions S1, S2 and h.
Full vs voided slabSTUDY Here below the comparison between a full concrete slab and an equivalent voided slab. I=225000cm^4/m I=153626cm^4/i I=225920cm^4/m I=422108cm^4/m I=304140cm^4/i I=422417cm^4/m The thickness of a voided slab is slightly greater that the one of the equivalent full concrete slab.
Slab supportSTUDY Once the thickness of the voided slab has been established it is possible to calculate the steel reinforcement. Loads will be divided in two directions: this can be calculated with the Grashof formulae. This calculation considers also the conditions at the limits. q ⋅ l4 q ⋅ l4 qx = y qy = x k ⋅ l4 + l4 x y k ⋅ l y + l4 4 x
ReinforcementSTUDY The reinforcement of a voided slab with new Nautils is typically composed by: ‐ a welded mesh in the lower slab, with spacers to assure the required concrete cover; ‐ additional reinforcement (bars or grids); ‐ a welded mesh in the upper slab, laid directly on the void formers (which include ribbing on their upper surface that serve as spacers). Geoplast Nautilus h24 Geoplast Nautilus h24 Geoplast Nautilus h24 Geoplast Nautilus h24 Geoplast Nautilus h24 Example of additional reinforcement between the two welded meshes.
Slab profile close to support - beamsSTUDY In the case of a slab and beam system there are typically rather high values of shear stress and negative moment. To manage these stresses is possible to use void formers of lower height close to the supports in order to increas the resistant section.
Slab profile close to support - columnsSTUDY In the case of a voided slab without beams it will be necessary to create a full concrete zone around the top of the columns (“mushrooms”). The armature must be properly calculated in order to manage shear‐stress and negative moment. Geoplast Nautilus h20
New Nautilus projects New Nautilus projectsrealised or in development realised or in development
PROJECT: SHOPPING CENTRESITE: VITROVICA, CROATIAPRODUCT: new NAUTILUS®SURFACE: 30.000 m2NOTE: with post-tension
PROJECT: DIGITEO LABSITE: SACLAY, FRANCEPRODUCT: new NAUTILUS®SURFACE: 38.590 m2
Typical project with new Nautilus + Modulo Modulo used to fill the gap between the slab and the beam. new Nautilus to decrease the self-weight of the slabPROJECT: DIGITEO LAB and allow a wide span.SITE: SACLAY, FRANCEPRODUCT: new NAUTILUS®SURFACE: 38.590 m2
PROJECT: new hospitalSITE: PISTOIA, ITALYPRODUCT: new NAUTILUS®SURFACE: 39.000 m2
PROJECT: ERICSSON LABSSITE: GENOVA, ITALYPRODUCT: new NAUTILUS®SURFACE: 20.000 m2
PROJECT: PADOVA UNIVERSITY SITE: PADOVA, ITALY PRODUCT: new NAUTILUS SURFACE: 20.000 m2BIDIREZIONALE