 |
 |
 |
|---|
Propping must be placed in lines parallel to permanent supports. Propping lines must be braced in accordance with the applicable formwork code. Propping lines must consist of bearers of minimum 100mm width supported by vertically placed acrow props. Bearers and props must be adequately designed.
Props must remain in position until the composite slab has reached 75% of it’s specified 28 day strength.
Full design load shall only be placed on the tray floor once it has reached its 28 day strength.
A continuous fibreboard packer should be placed between temporary prop bearers and the tray floor soffit to ensure that excessive deformations do not occur locally.
Propping requirements, specified in this manual assume, that concrete is placed such that the tray floor is not overloaded by impact or heaping of concrete.
'unpropped' slabs
On particular projects where there are a number of floors and wide expanses of floor area, it is becoming common construction practise to utilize ‘Unpropped slabs’. Traditionally, where different levels of floors are placed at a faster rate than 28 days per floor, large numbers of props are required to be installed over significant areas and to extend down through a number of levels, to ensure floors are not loaded prematurely.
For unpropped slabs, permanent secondary beams are placed at the allowable propping spans, generally eliminating the need for any temporary propping. As this becomes the critical design parameter, the complete composite slab design, its thickness and associated reinforcing are completely determined from this.
 |
|---|
 |
 |
 |
 |
 |
 |
 |
 |
Tray flooring being supported by a combined system of permanent supports along with temporary rows of acrow props (or similar) which provide interim support until the concrete has gained sufficient strength for composite action to support imposed loadings. Slabs in this type of construction are generally thicker and spans between permanent supports longer than ‘unpropped’ slabs.
Tray flooring being supported by a system of permanent supports, forming a part of the structural superstructure of the building. Where no temporary props are used, construction is said to be ‘unpropped.’ Slabs in this type of construction are generally thinner and spans between permanent supports shorter than propped slabs.
‘Propping’ and ‘ponding’ are related as a result of current construction practise for multi-storied buildings leaning heavily on ‘unpropped’ construction. The emphasis of prevailing design is to maximise the span of tray flooring between secondary beams. Both the spanning capability of individual trayflooring profiles, and the total depth of wet concrete influence design outcomes. Profiles which have a high inherent stiffness and allow reduced overall slab depths (or more correctly, reduced volumes of concrete) have a distinct advantage in the market. However, maximising the ‘unpropped’ span also increases ‘ponding’ effects or the added weight of concrete a slab must carry, due to the deflection of the slab and supporting structure. These are impacted by flatness requirements imposed on the top surface of the slab. A much reduced span is pertinent for an ‘unpropped’ slab which is required to have a flat surface (screed to level) compared to a slab which is permitted to have a constant thickness, but which may not be particularly flat (screed to thickness).
extend propping tables / charts
In the past propping tables have been produced on the basis that the tray flooring would always be propped, and this has limited the full advantages of ‘unpropped’ construction to be utilized. The publication of HERA REPORT R4-107:2005 ‘composite floor construction handbook’, has given serviceability criteria which can be standardized and allow designers to provide a variety of solutions contingent on a projects requirements. Depending on the degree of supervision, designers can specify spans on serviceability (flatness) requirements along with construction procedures (screed to level or thickness), allowing extended spans which are able to be used in confidence.
compliance with NZBC
Compliance with the New Zealand building code Clause B1-structure is by use of the acceptable solutions provisions. Approved documents which are permitted under verification Method B1/vm1 include:
design of tray floor
AS/NZS4600:2005 “Cold formed steel structures code.
loading specification and serviceability limits
BS5950: Part 4: 1994 “code of practise for design of composite slabs with profiled steel sheeting.” By reference from NZS3404: Part 1
It is to be noted that the overriding document for compliance with NZBC with regard to the design of profiled steel sheeting supporting loads before composite action is developed is AS/NZS4600 “cold formed steel structures”. Further, should verification by testing be proposed, then such testing is to be in compliance with section 8.0 of AS/NZS4600.
Both constructional live loads and serviceability deflection limits are specified within BS5950:Part4:1994
The use of obsolete standards (in particular BS 5950: Part 4:1982) do not constitute compliance with NZBC provisions. Specifically, the empirical ‘material code’ provisions included within the 1982 edition were removed from the current 1994 edition and have since been superseded by firstly
AS/NZS4600:1996 and subsequently
AS/NZS4600:2005 as the governing document.
important changes to AS/NZS4600 “cold formed steel structures" standard, 2005 edition
A number of critically important changes have been incorporated within the current 2005 edition of AS/NZS4600 “cold formed steel structures”, which tray flooring manufacturers must upgrade to if they are to provide compliance t the New Zealand Building Code. These changes include but are not limited to:
(a) restriction of G550mPa grade steels
Both the 1996 and 2005 editions place a reduction factor to the designs yield strengths of grade 550mPa steels, based on both yield and rupture strengths being relatively close and resulting in a lack of ductility for that grade of steel. While the 2005 edition allows a slight relaxation of this reduction, it also removes previously accepted exclusions to this provision. A note to the 1996 edition stated that for structural members which were subject to bending, shear, bearing, and compression, ductility may not be a significant factor. Where such interaction occurred, it was accepted the reduction in the design yield strength was not required. It has previously been accepted that roofing, purlins and trayflooring were eligible for this exception.
The consequence of this change is a net reduction in the ultimate strength capacity of tray flooring.
All three amendments to AS/NZS4600 may result in a reduction in both the ultimate limit and serviceability limit state capacities of trayflooring.
compliance of formsteel tray flooring with NZBC
Extensive designs have been undertaken to ensure compliance with the recently released AS/NZS4600:2005 standard. Propping spans of Formsteel tray flooring profiles, Unifloor, Svelte floor 60, and Svelte floor 80 fully comply by calculation with the relevant rules of AS/NZS4600:2005 and BS5950: Part 4: 1994.
 |
 |
 |
|---|