ComFlor® 46, 51+, 60, 80 & 100 - Design, construction and sitework information
Composite floor decks - Design information End anchorage from shear studs End-anchorage from shear studs may be considered in ComFlor® 9, however the input only allows for ONE stud per trough. Note that a maximum of two studs per trough is permitted for the composite beam design – for an edge beam only one stud will be adjacent to the edge of the sheet, and for an internal beam only one stud will be anchored in the discontinuous sheet.
Construction stage deflection limits At the construction stage, there is an important link between the leveling technique used in construction, the deflection of the bare steel and the consequent volume and weight of the concrete. It may be necessary to limit the deflection of the decking to limit the effects of ponding of concrete during construction and the increase in dead weight as a result. According to BS EN 1994-1-1, 9.3.2(2), if the central deflection of the decking is greater than a tenth of the slab thickness, ponding should be considered. BS EN 1994-1-1, 9.6(2) states that the deflection of the decking under its own weight plus the weight of wet concrete, excluding the construction load, should not exceed the limits provided in the UK National Annex 1994-1-1, NA.2.15. Where additional loads due to ponding of wet concrete are ignored when determining the loading, a more onerous deflection limit is used to check the decking at serviceability. In these cases, the mid-span deflection should be limited to the lesser of:
• Effective span/180 and 20mm. Where additional loads due to ponding of wet concrete are included, the mid-span deflection should be limited to the lesser of:
• Effective span/130 and 30mm.
50
It is recommended that the prop width should not be less than 100mm. We are not required to account for beam deflection in composite floor deck design, however if the slab is laser leveled, beam deflection may be significant to the load on the deck, the load on the beam and the dead load on the frame. Therefore, if laser leveling is to be used, design slab depth should be significantly more than the nominal slab depth, to account for it when calculating deflections and loads, but not when calculating strength.
Vibration The dynamic sensitivity of composite slabs is not normally critical, because they are relatively stiff compared with the beams, although the dynamics of the whole floor should be considered. To ensure that the walking activities will be outside the range, which could cause resonant or close-to-resonant excitation of the fundamental mode of vibration of the floor, no floor structure should have a fundamental frequency less than 3 Hz. Similarly, no single element i.e. the composite slab within the floor structure should have a frequency less than 3 Hz. Floors that may be subject to rhythmic group activities should not normally be designed to have a natural frequency less than 5 Hz. Therefore in the absence of more appropriate information, the natural frequency of the composite slab should not exceed 5 Hz for normal office, industrial or domestic usage. Conversely, for dance floor type applications or for floors supporting sensitive machinery, the limit may need to be set higher. On this basis the ComFlor® 9 design software has a default natural frequency limit of 5 Hz. The dynamic sensitivity of the composite slab within ComFlor® 9 is checked in accordance with the Steel Construction Institute publication P076: Design guide on the vibration of floors. The natural frequency is calculated using the self-weight of the slab, superimposed dead load, and 10% of the characteristic imposed loads, representing the permanent loads and the floor.
For design to the Eurocodes, the loads considered for the vibration check are increased using the psi-factor for imposed loads (typically 0.5). The natural frequency limit may be reduced to 4Hz, because of this higher load, used in the calculation. To determine the vibration response of sensitive floors with improved accuracy, calculation methods are given in SCI publication P354 “Design of Floors for Vibration: A New Approach”. This enables designers to compare the response with the acceptance levels in BS 6472 and ISO 10137 for building designs and in the NHS performance standard for hospitals, HTM 2045. The relevant Eurocode requirements regarding vibration are given in: 1. BS EN 1990, 3.4(3) – The verification of serviceability limit state should be based on criteria concerning vibrations that cause discomfort to people, or that limit the functional effectiveness of the structure. 2. BS EN 1990, A1.4.2 - Stiffness criteria may be expressed in terms for vertical deflections and for vibrations. 3. BS EN 1990, A1.4.4 – The natural frequency of vibrations of the structure or structural member should be kept above appropriate values. 4. BS EN 1993-1-1, 7.2.3 (1)B – With reference to EN 1990 Annex A1.4.4, the vibrations of structures on which the public can walk should be limited to avoid significant discomfort to users, and limits should be specified for each project and agreed with the client. The National annex may specify limits for vibrations of floors. 5. BS EN 1993-1-1 National Annex, NA.2.25 – Reference should be made to specialist literature as appropriate. For floor vibrations see NA.4. Reference is made to NCCI: Vibrations SN036a-EN-EU. 6. BS EN 1994-1-1, 7.3.2(1) - The dynamic properties of floor beams should satisfy the criteria in BS EN 1990, A1.4.4.