Voided Biaxial Slab

Description

Voided biaxial slabs are like a flat slab, they both use cast in situ concrete, are reinforced the same way and both carry two ways. The only difference being that the voided biaxial slab has lightweight voided elements at mid height of the slab resulting in a about 30% lighter slab overall. This is structurally efficient because the middle part of a flat slab is the structurally least efficient part that mainly adds weight. Self-weight is a significant portion of the load on concrete slabs. The middle part is only important against shear force and that is usually only defining around columns due to punching shear. In that area the voided elements are not put in. Voided biaxial slabs start to become efficient for thicker slabs and thus for higher spans. The economic span range lays between 5 and 15 m, significantly higher than the one for flat slabs (0 - 10 m). Same as in flat slabs, voided biaxial slabs are usually made continuous to reduce deformations.

Voided biaxial slabs perform well against fire due to the high thickness and reaches a fire resistance of 120 min.

The flexibility is on site is as good as it gets, it can adapt to spontaneous changes without problems. Also, the geometric flexibility is good because one is not limited to rectangular floor shapes. However, a certain thickness must be given to use voided biaxial slabs and the more recesses are include in a slab, the less voided elements can be used, because they are optimised for standard applications. Building systems can be integrated without a problem. Since no special formwork or reinforcement is required the complexity is average. Installing the voided elements should not be an issue at all. The construction time is rather slow because formwork is required as well as reinforcement. Additionally, the time the concrete needs to cure also slows down construction. The installation of the voided elements does not slow down the construction speed significantly.

As in most concrete floors, vibration is not a concern due to the large weight.
For a square slab, the slenderness ration l/h lays around 32, but is dependent on the load and the amount of reinforcement included. This is significantly more slender than the flat slab.

Database

Floor type			Voided Biaxial Slabs
Materials			concrete_reinforcement_voided elements
Description			hollow elements to reduce weight for rather thick slabs
Way of carrying	Does the main span carry in one or two directions?		two way slab
Span width low [m]	Economically possible		5
Span width high [m]	Economically possible		15
Additional measures when installing			formwork_propping up
Prefab?	Is prefab possible?		no
Continous slab?	Can you build the slab with supports in middle of the slab?		yes
Building systems integratable?	Can buildings systems be included into the slab itself?		yes
Fire	Fireresistance without costly measures [min]		120
Substructure	Certain prerequisites necessary?		no
Prone to vibration			no
Environment	General classification including formwork	1 to 5 terrible to very good	3
Construction time	Time on site	1 to 5 very slow to very fast	2
Flexibility	Average of the next two	1 to 5	4.5
Flexibility on site	How flexible is the system to spontaneous changes on site?	1 to 5 terrible to very good	5
Flexibility geometry	How good can you adapt the slab to complex geometries?	1 to 5 not at all to very easy	4
Complexity	Average of the next two	1 to 5	3
Complexity fabrication	Complexity of floor itself.	1 to 5 very complicated to very simple	3
Complexity installation	Complexity installation on site	1 to 5 very complicated to very simple	3
Slenderness l/h 3kPa	Applied load (without selfweight)	Residential building	33
Slenderness l/h 4kPa	Applied load (without selfweight)	Office building	31
Weight [kN/m3]	per m2 and total height of slab		18
Embodied energy [MJ/m3]	per m2 and total height of slab		4200

References

General information:

Goodchild, C., Webster, R., & Elliott, K. (2009). Economic Concrete Fram Elements to Eurocode 2. Camberley UK: The Concrete Centre.
https://www.cobiax.com/ch/de/produkte/

Span width:

up to 16m (https://www.bba-online.de/fachthemen/innenausbau/grosse-spannweiten-mit-hohlkoerperdecke/#:~:text=Wabenf%C3%B6rmige%20Hohlk%C3%B6rperdecke%20f%C3%BCr%20gro%C3%9Fe%)
Example with 7m x 14m (https://www.bauingenieur24.de/fachbeitraege/massivbau/bubbledeck-die-zweiachsige-hohlkoerperdecke/982.htm)

Fire resistance:

120min (Goodchild, C., Webster, R., & Elliott, K. (2009). Economic Concrete Fram Elements to Eurocode 2. Camberley UK: The Concrete Centre.)
120min (https://www.structuremag.org/?p=12554)

Slenderness:

l/h=19 for 3kPa, l/h=19 for 4kPa (Goodchild, C., Webster, R., & Elliott, K. (2009). Economic Concrete Fram Elements to Eurocode 2. Camberley UK: The Concrete Centre.)

Weight:

Calculations

Embodied energy:

Calculations

Photo Source:

http://www.wrap.org.uk/sites/files/wrap/Biaxial%20Slabs.pdf