Hollow Core Slabs

[Ariel Wascom]

Ahollow core slab, also known as a voided slab, hollow core plank or simply a concrete plank is a precast slab of prestressed concrete typically used in the construction of floors in multi-story apartment buildings. The slab has been especially popular in countries where the emphasis of home construction has been on precast concrete, including Northern Europe and former socialist countries of Eastern Europe. Precast concrete popularity is linked with low-seismic zones and more economical constructions because of fast building assembly, lower self weight (less material), etc.

Slabs in prestressed concrete are usually produced in lengths of up to 200 meters. The process involves extruding wet concrete along with the prestressed steel wire rope from a moving mold. The continuous slab is then cut to required lengths by a large diamond circular saw. Factory production provides the obvious advantages of reduced time, labor and training.

Another fabrication system produces hollow-core floor slabs in reinforced concrete (not prestressed). These are made on carousel production lines, directly to exact length, and as a stock product. However, the length is limited to about 7-8 meters. Especially in Belgium, this method is widely used in private housing.

To meet modern standards (both hollow-core and massive slab) of soundproofing the floor needs to be covered with a soft floor covering that is able to dampen the sound of footsteps or a floating floor screed should be installed. An alternative is to put a strip of rubber underneath the floor slabs.

Hollow-core slabs and wall elements without prestressed steel wire can be formed by extruders. The size of these elements will typically range in width from 600 to 2400 mm, in thickness from 150 to 500 mm, and can be delivered in lengths of up to 24 m.[1]

Due to pre-stressing, the hollow core slab can span up to 23 m, easily in the range of 5.0 to 10.0 m for typical residential buildings. The number of vertical structural components, like column-beam or structural walls, can be reduced substantially using the pre-stressed hollow-core slabs.

I am having problems with exporting Hollow-core slabs to ifc without them losing their spatial container (floor level). For example, I have made a simple model in revit with two floor levels, Level 1 and Level 2 respectively. I used the precast extension tool to split the floors into hollow-core slabs.

But after i export this file into IFC format, the second floor slabs seem to lose their spatial information. I have tested it with Autodesk Navisworks and Solibri viewer. After exporting, all the slabs are adressed to Level 1.

But the problem persists - Now i cannot pick hollow-core slabs separately from the original slab. Instead, when i try to pick only one hollow-core slab, the whole floor gets activated, as seen in the picture below.

For exporting, I've been using level of detail Medium. If i use Extra Low settings, the hollowcore slab voids are hexagons, instead of circles. Unfortunately, we need those voids to appear as circles.

Thank you for your effort. I guess the problem lies within different IFC viewers and their core settings then, which fail to communicate with Revits exported information. Although, based on my rather insufficient competence on the matter, i am really not sure about anything :).

If you have a level in your model, but there is no view for it, go to "View" tab, then under "Plan Views" select type of the plan you need and after that select level for which to create selected plan(s).

Also, when drawing level, there is an option "Plan View Types..." in the options toolbar (usually under the ribbon) where you can select which plans Revit will create automatically when you draw your level.

It seems that this slab is missing "edit profile" capability: Now that I put one of these hollow core slabs on the second floor, I notice that it only could be lengthened but that this one instance put there cannot be conveniently modified with "edit profile", like some of the other generic floor types, to get an uneven floor shape. So that means that I will have to copy each one individually or array one to fill the entire floor, it seems. I also notice that I can change the widhts and revise the names of each new size accordingly with the "Edit Type" of Properties, which will make the creation of a complex floor laborius, but apparently I will have to do it that way.

Why use hollow core slabs? These pre-stressed floor slabs with either round or shaped voids are one of the most popular, efficient and long span floor construction components that exist today. Due to long spans, less load-bearing structures such as columns and inner walls are needed. This results in increased architectural and structural freedom and less raw materials used in a building. Continue reading to discover how to benefit from using precast hollow-core slabs.

One fascinating perspective to hollow core slabs is not often discussed. The slabs are made of two materials, the properties of which are taken almost to the extreme. Firstly, concrete: general wet cast concrete strength is C20 / 25, while hollow-core concrete strength is C40 / 50-C50 / 60. That is, more than twice as strong. Of course, special concrete with strength of 100-200 MPa, has been produced for specific purposes but now we are talking about basic construction.

However, hollow core slabs are not always used as horizontal structures. They can also be installed vertically as outer walls, dividing walls and noise barriers. This is enabled by the long spans and durable structure. Hence, they are especially beneficial for industrial buildings, where a high unified space is needed. On the other hand, the slabs are highly resistant to changing weather conditions and have efficient noise reduction, which makes them ideal noise barriers.

Not only do hollow core slabs enable construction of versatile buildings, they increase the usable floor area. How? The span of a hollow-core slab can be even up to 20 m without intermediate supports, which results in spacious rooms with less partition walls.

Think about it: if long-span hollow-core slabs are utilized for the floor of residential buildings, non-load-bearing partition walls can be placed inside of the flats. This gives freedom to the architects, because the floor layout can be easily modified. To enable this, the structure of the building should be designed so that the slabs are longer than the rooms or even apartments. Likewise, in commercial and public buildings, long-span hollow-core slabs enable less inner supporting structures, e.g. pillars, which can be rather annoying, for example, in parking halls. Thus, architects and structural designers have more freedom in designing well-functioning and visibly appealing spaces.

Moreover, the decreased need for load-bearing partition walls and columns results in lighter structures. Therefore, the weight of the whole building is reduced, which results in smaller and more affordable foundations. This is especially beneficial when designing and constructing buildings in seismic areas, because earthquake forces are proportional to the weight of the structure.

Precast concrete hollow-core slabs are often constructed with a cast-in-place concrete topping on site. Common construction practice includes applying cementitious grout between hollow-core units as a bonding agent. The cast-in-place concrete topping may contribute to the strength and stiffness of the hollow-core slabs if composite action is developed. The strength of the interface between the hollow-core units and the cast-in-place concrete topping largely depends on the surface condition of the slabs because it is not feasible to provide transverse reinforcement in these elements. The research presented in this paper primarily includes testing of two types of hollow-core units (dry mix and wet mix) to determine the interfacial shear strength between the units and the cast-in-place concrete toppings. Tests were conducted using push-off specimens designed to generate shear stresses at the interface. A parametric study is also conducted to identify the governing failure mode of topped hollow-core slabs as a function of span length.

12. ASTM Subcommittee C01.27. 2011. Standard Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 2-in. or [50 mm] Cube Specimens). ASTM C109. West Conshohocken, PA: ASTM International.

Hollow core slabs are a structural precast and prestressed floor and roof system with a shallow depth, reducing weight while maintaining structural strength. Fire-resistant hollow core slabs allow for better acoustical performance and have better deflection control than wood or steel systems.

With faster installation than composite decking, hollow core slabs are also available in long clear spans and work with other building products and materials. The smooth bottom eliminates the necessity for other ceiling products. The shortened installation time creates project savings, as well as a clean job site.

Founded in 1904 and headquartered in Farmington Hills, Michigan, USA, the American Concrete Institute is a leading authority and resource worldwide for the development, dissemination, and adoption of its consensus-based standards, technical resources, educational programs, and proven expertise for individuals and organizations involved in concrete design, construction, and materials, who share a commitment to pursuing the best use of concrete.

Abstract:

This study proposes a practical design approach to estimate theweb-shear strength of deep prestressed hollow-core slabs (PHCS).It explores the effects of critical factors such as the shear stressdistribution, biaxial tensile strength, and the reduction in effectivecompressive stress in concrete, quantifying their impact onweb-shear strength. A data set of 85 entries is used to undertakea comparative assessment, demonstrating the improved safetyand accuracy of the proposed methodology against current designprovisions and previous proposals. Moreover, it is shown thatneglecting the beneficial effect of the prestressing force in thetransfer region leads to a conservative estimation of the web-shearstrength. Furthermore, the study introduces three modified designexpressions based on ACI 318-19, fib Model Code 2010, andCSA A23.3-14 standards. The proposed methodology has practicalimplications for enhancing the safe and cost-effective use of deepPHCS in construction practice.

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