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BS5950:Part 4:1994 currently forms the accepted means of compliance
forloading of trayflooring (including loading patterns) in accordance
with Verification method B1/VM1- Structure of the New Zealand
building code.
Magnitude and distribution of construction live loading are as follows:
Ultimate loading factors of 1.2 and 1.6 are applied to dead and live loads respectively, in accordance with NZS4203 “General loadings code”.
10.0 constructional live loads to HERA REPORT R4-107:2005
HERA Report R4-107:2005 “Composite Floor Construction Handbook section 1.3-3 Allowance for Ponded-Induced Deflections”, has raised the need for manufacturers to provide propping tables which take into account current construction practices which incorporate tray flooring with a system of flexible supporting primary and secondary steel beams, with no temporary props. Alternative magnitude constructional live loads are proposed:
‘surveys of typical construction loading,(couchman, etal 20000) show that this (ie constructional live loading) is non-uniform, with 1.5kPa being a design average value of construction load.”
HERA have proposed a procedure to take into account the effect of ponding associated with the deflections of primary and secondary beams due to wet concrete. This procedure is not recommended and is addressed in sections 12.0 and 13.0. However, the use of0.75 kPa and 1.50kPa constructional live loads in certain circumstances, may be appropriate in particular circumstances.
10.1 0.75 kPa constructional live loading
Not withstanding the implied acceptance of a 0.75kPa constructional live load within HERA report R4-107:2005, a practical manifestation of this live load can be made up from:
A worker density of 190kg worker kPa
per 1.5m x 1.5m floor area 0.40
Average I‘heaped’ concrete
depth at pouring – 15mm 0.36
--------
0.76kPa
It is clear that within the working area, it is possible for up to 6-8 persons to be working together, along with pump equipment in fairly close proximity. Given that changing the worker density to 190kg worker per 1.0mx1.0m floor area, given a loading of 0.90kPa in itself, this level of constructional is not recommended without strict controls being put in place.
10.2 1.50 kPa constructional live loading
A practical manifestation of this live load could be made up from:
- A worker density of 190kg worker kPa
Per 1.0m x 1.0m floor area. 0.90
- Average I’heaped’ concrete
depth at pouring – 25mm 0.60
-------
1.50kPa
This level of live load is seen as more plausible. Further, from a practical outlook it is clear that greater load magnitudes are likely to be associated with smaller working areas, and hence supports the BS5950 loading specification which increases the constructional live load for decreasing spans.
10.3 reduced constructional live load to be substantiated
It is the responsibility of the Trayfloor specifier/building contractor to choose the appropriate construction live load suitable for their particular project. The use of 1.5kPa or 0.75 kPa construction live loading constitutes an alternative solution, and there use must be accompanied by substantiation documentation. Acceptance or otherwise of an alternative solution is the joint responsibility of the Specifier/ Building Contractor/ Territorial Authority.
11.0 sercvice ability criteria for construction
Serviceability criteria during construction of composite slabs are governed by BS5950: Part 4:1994 which allows the computation of deflections either:
Previously, most manufacturers have used the former as a simpler option, which gave satisfactory practical performance for slab systems which were generally ‘propped’. However, with today’s construction practices, calculations on this basis have been found to be unconservative in certain situations.
Further, they do not in any case provide any real deflections which can be checked on site.
It is imperative that propping tables today take into account the effects of ponding.
Two governing limits have been included within Formsteels propping tables, for deflection:
1. Span/130 to a max of 30mm for construction where the visible discernment of a deflected floor soffit is acceptable, or where the soffit is not exposed to view.
2. Span/360 to a max of 10mm for construction where the visible discernment of a deflected floor soffit is less acceptable and requires the adherence to more stringent limits.
12.0 use of exsisting obsolete propping tables
It is notable that in the past most manufacturers have chosen to use the allowable alternative within BS5950: Part 4, where the effects of ponding are ignored. This has been acceptable for floors which have rigid supports (walls or props) but it is not acceptable where flexible supports occur.
The response by designers and engineers to unpropped slabs range from totally ignoring the issue and using existing span tables without modification, to including some modification as an allowance for ponding effects. The former is dangerous and designers using this practice are fully liable, for any consequent failures, by not carrying out their ‘due diligence’, in regard to the product they are specifying.
A number of modifications to existing tables have also been used, with the most notable being the HERA procedure, which is set down in HERA REPORT R4-107:2005
THE USE OF ANY MODIFICATION PROCEDURES TO EXISTING ‘PROPPING’ TABLES IS TO BE AVOIDED AND ONLY TABLES WHICH SPECIFICALLY INCLUDE UNPROPPED CONSTRUCTION, FLEXIBLE SUPPRORTS AND WHICH ALSO INCORPORATE UPGRADED REQUIREMENTS FROM AS/ NZS 4600 SHOULD BE USED.
13.0 discussion of the ‘hera’ modification to ‘propping’
The ‘HERA’ modification is incorporated within clause 1.3-3 of the
REPORT R4-107:2005, and suggests that there is a possible trade
off between constructional live load and the extra dead load attributed
to ‘ponding’ effects. In essence it is maintained that the constructional
live load presently 1.50kPa or greater under BS5950: Part 4 could be
reduced to 0.75kPa, with the remaining 0.75kPa being an allowance for
up to 30mm of ‘ponded’ concrete. On this basis HERA argues that
existing tables can be used as is.
This issue of reduced constructional live loads is dealt with elsewhere within this manual, but designers will need to discuss reduced constructional live loads with the territorial authority concerned with the project, and the designer’s insurers. Certainly, the designer would need to note the allowance for constructional live load on there procedure statement (also noting that existing propping tables cannot be used under verification method B1/M1).
Even if it is considered that reduction in constructional live load is acceptable, HERA’s procedure totally depends on the assumption that all propping design is based on ultimate limit state considerations. This is clearly not the case with all simple spans, and profiles with greater thicknesses than 0.75mm are more likely, to be controlled by deflection rather than strength.
For the formulation of propping load/span tables under ‘ponded’ conditions a total of six constraints are required to be fulfilled:
Items (1) to (4) are ultimate limit state considerations and the HERA argument is:
Initial Consideration: Q = 1.5kPa
U1 = 1.2G + 1.6(1.5) = 1.2G + 2.40kPa
Proposed Consideration:
U2 = 1.2(G + 0.75) + 1.6 (0.75) = 1.2G + 2.10kPa
U2<U1 => procedure ok for strength
However, where serviceability considerations provide the critical design constraint, HERA’s procedure is not valid. For items (5) and (6), compliance is proportional to the dead load of the slab only, and the use of the live load/dead load substitution will exasperate the critical condition further.
In considering deflections the following aspects need to be considered:
For Ponded slabs it is not possible to provide a simple generalized argument, as been provided for the strength limit state. However, an example serves to illustrate the point. A slab which under the ponded condition has a typical equivalent increase in weight of 12% of this actual thickness is used. A simply supported configuration has been used for simplicity. Calculations must also take into account the decrease in contribution of compression elements under higher service stresses. A ponded Trayfloor may realistically have only 90% of the stiffness of a Trayfloor calculation where ponding has been ignored.
Initial Consideration – no ponding considered:
Δ1 = 5GL^4
384EI o
Proposed Consideration – Ponding Considered:
= 24kPa x 0.12 x thickness = 2.88t
384E(0.91 o )
Ratio of deflections:
Δ2/ Δ1 = 5(G + 0.75+2.88t) L^ 4 x 384EI o
384E(0.91 o ) 5GL^4
= (G + 0.75+2.88t)
0.9G
Therefore for a variety of thicknesses:-
T(mm) G(kPa) Δ2/ Δ1
100 2.40 1.59
125 3.00 1.52
150 3.60 1.48
175 4.20 1.44
200 4.80 1.42
225 5.40 1.40
250 6.00 1.38
275 6.60 1.37
300 7.20 1.36
Now BS5950 allows a relaxation of deflection limit from span/ 180 to span/130 ‘ponding’ considerations are taken into account. This equates to and allowable ratio:
Δ2/ Δ1 = span x 180 = 1.38
130 span
It is therefore clear that excessive deflections will occur within slab between 100mm and 250mm thicknesses in this example using the HERA procedure.
Using HERA procedure, the tendency will be for designers to specify constructional live loads on the calculated span requirements of the project, rather than as a rational consideration of the practical construction needs of that project. The use of existing tables in this manner will result in the designer being unsure whether or not his specification will be ‘conservative’ or ‘unconservative’, with significant but random fluctuations and outcomes.
Designers should be able to use tables where they are able to specify, with certainty, the constructional live load which will be commiserate with the proposed construction procedures to be used. Further, designers should be able to align their designs with the finish/ flatness requirements and supporting beam stiffness’s of the floor, and take advantage of consequent allowable increases in propping span where permitted, or be able to decrease the same spans to account for more stringent demands.
THE USE OF ANY MODIFICATION PROCEDURES TO EXISTING ‘PROPPING’ TABLES IS TO BE AVOIDED AND ONLY TABLES WHICH SPECIFICALLY INCLUDE ‘UNPROPPED’ CONSTRUCTION, FLEXIBLE SUUPORTS AND WHICH ALSO INCORPORATE UPGRADED REQUIREMENTS FROM AS/NZ 4600 SHOULD BE USED.
