Floor slab selection report

We have been asked to adopt a suitable method of floor construction for the ground floor and intermediate storeys of the proposed office development. The client has stated that they require a strong durable flooring system capable of standing both point and uniform loadings whilst providing an unobstructed floor space for the main office areas in each floor of the development. However any design of the floor must be fully functional providing both thermal and acoustic insulation, a source of heating and also electrical outputs in middle of the floor space to accommodate desks etc, without being visible or presenting any hazards to the buildings inhabitants. We have advised a team of professionals to develop a cost effective and time efficient solution for our client. In order to achieve this we have undertaken the task of gathering statistical data based on the technical performances of suspended intermediate flooring solutions which are relevant to the structure.

Suspended floor slab selection for intermediate floor levels

We have carefully analysed all suspended floor construction techniques for the intermediate floors of the development and have provisioned the implementation of a composite in-situ, profile steel decked, reinforced concrete slab as we believe it can be more economically efficient in terms of cost, and also with the time concerned with the delivery of the slabs. There are elements of design and performance criteria of the materials used in the construction process which make it an obvious choice. Through the implementation of Kingspan Multideck 50-V2, 0.9mm gauge floor system, we can create a permanent method of formwork for the slab construction as well as providing a functional safe working platform. The profile deck will span across intermediate beams at a maximum distance of 7.2 meters achieving a tolerance of around 3kn/m² , the profile sheets are around 1200mm with a 300mm overlap and then fixed to the skeleton frame by means of 19mm shear studs being welded to the upper flange of the intermediate beams at 600mm centres to create a rigid connection which allows both the slab and skeleton frame to act compositely together transferring both dead and live loads to columns and foundations below and also reducing the chance of vibrations between the floor slab and the structure. The use of profile steel decking also eradicates the requirement for additional props and sheeting materials used to support the concrete until it cures to a required strength, work can still continue during the construction of lower storeys as the floor space is free of obstructions from props. Profile decking also has the ability to reduce the amount of tension exerted to the slab itself, this means that we can reduce the ‘grade’ of reinforcement from A142 7mm mesh to one layer of A98 5mm 2400x4800 sheets of reinforcement mesh laid on 50mm plastic spacers with a minimum overlap of 400mm secured with 18 gauge 1.2mm steel tying wire by means of double crown ties. The reinforcement adds tensile resistance to the concrete floor slab and helps it achieve over the required minimum tolerance of 2.5kn/m² for office spaces to around 3.5/4kn2.5kn/m². We can also reduce the grade of concrete from a normal c35 mix, to a lightweight aggregate C35 mix and reduce the minimum concrete depth to 100mm across an area of 781.2m² per storey with an approximate concrete total of 71.09m3 on the main office space which accumulates to around 284.36 cubic meters of concrete required for all office spaces on intermediate storey levels. The Lightweight mix will have an additional plasticiser to aid and improve workability with levelling and finishing techniques. The concrete will be placed via means of a concrete pump to the intermediate storey levels from roof level to desired placement area without implications. The concrete will be subject to quality control procedures to measure performance of the mix and strength of finished product. It will be achieved by concrete test cube

samples being taken from every delivery of concrete along with a slump test. Wet concrete will be worked to level using rakes and compacted with poker units checked for level with laser receiver and finally placed with a vibrating concrete screed. The concrete will be finished with a power float using a pan attachment to level out any high spots in the floor area creating a more uniformly level slab.

Once floors have cured for 7days a complete level survey must be carried out to check the floor level for uniformity and check it meets the specified tolerances of +/- 2mm which is essential to achieving consistency within the screed that will be applied later. A layer of damp proof membrane will be laid across the floor area with 400mm taped overlaps to prevent the ingression of moisture from the under floor heating system. Thermal and acoustic values will be improved by the integration of 40mm insulation boards being laid over the finished concrete floor with 25mm conduit tubing concealed in steel floor boxes fixed to the insulation with clips to avoid movement whilst the screed is installed. The conduit is concealed within floor boxes and used to distribute electrical sources from the utility space at each storey level to mid floor span of the main office area, the remainder of the floor area will have an under floor heating system consisting of 30mm flexible hot water pipes which are fed by condensing boilers that run at relatively low temperatures to achieve the desired room temperatures. The heating pipes are secured to insulation by being stapled down to the insulation boards in a spiral pattern across the floor area providing no cold spots and heating the building more efficiently through a 60mm flexi dry screed, allowing 30mm adequate cover to the hot water pipes below. Preparing the floor area for the screed will consist of a suitable foam filler being applied around the perimeter of any wall structures to allow expansion in the screed caused by the under floor heating system.

The flooring solution adopted for this project have been chosen as we believe it to be more beneficial in enhancing production rates on site due to the speed and ease of the construction techniques involved in the delivery of the floor slabs. Other reasons for the provision of these floor systems are by minimising the dead loads applied to the structure which allows us to take a lean approach to the project resulting in lower costs to client providing them with more value for their money increasing customer satisfaction by supplying them with a quality assured service through our use of quality management systems and control measures we have in place. Our innovative design team have provided us with a sustainable energy solution for the clients building free from obstruction to the floor space whilst providing the same functionality as a raised access floor whilst integrating the heating system internally within saving space and contributing considerately to the low carbon emissions form the building making it more environmentally friendly.

Overview of alternatively viable floor construction method

One of the most popular alternatives methods of floor construction would be to use pre-fabricated hollow core flooring panels which are capable of providing the same structural loading tolerances of up to 3kn/m as the in-situ based flooring system with a 150mm depth. This type of slab is more reliably accurate on uniformity of level and a higher standard of finished workmanship due to being manufactured in a controlled environment. Through the use of pre-fabricated flooring systems we could minimise any construction times and its implementation would also lower the number of skilled workers required reducing overheads costs.

Hollow core floor panels consist of high tensile steel is used for pre-stressing tendons to suit variations in span lengths and different loading resistances. The panels are erected on site by means of a mobile or tower crane and lifted in sequential order directly into their fixing positions on the supporting universal steel beams. The panels are then attached to the structure by installing 19mm diameter shear studs in the 300mm centred open cores which are cast into the panels to the upper flange of the beams to resist any movements or vibrations occurring and ensuring loadings are distributed appropriately. The voids are then filled insitu with a minimum 20N/mm concrete mix with 20mm aggregate to create a monolithic floor slab. Once the erection process is complete the hollow core panels create an instantaneous safe working platform for the following trades to carry out the next stage of construction related activities.

However there are drawbacks such as the increased weight of pre-fabricated flooring panels can lead to larger structural members being required and the need for higher specified foundations and piles. There would also be a certain amount of time in advance required for the manufacturer to produce the materials to our specific sizes and specifications. The implementation of pre-cast flooring panels would also significantly increase the number of deliveries from large articulated vehicles which will result in complications due to the small scale of the proposed site and the nature of surrounding streets and the peak time volume of traffic causing obstructions to other road users traveling to or from the city centre.