COMPOSITE FLOORS - II

Transcription

1 24 COMPOSITE FLOORS - II 1.0 INTRODUCTION This chapter describes the basis for design of composite foors using profied deck sheets adopting the equations described in the chapter on composite foors - I based on imit state design phiosophy. To make it appicabe to practice in India IS 456: 2000 has been foowed wherever it is appicabe. The main economy in using profied deck is achieved due to speed in construction. Normay 3 to 4 m spans can be handed without propping and spans in excess of 4 m wi require propping. The yied strength of decking stee is in the range of 220 to 460 N/mm 2. Though ight - weight concrete is preferabe both from reducing the effect of ponding defection as we as increasing the fire resistance, the norma practice in India is to use concrete of grade M 20 to M 40. With the avaiabiity of ready mixed concrete, pumped pours become possibe to the extent of 500 m 3 a day. The profied deck depth normay avaiabe ranges from 40 to 85 mm and the meta thickness 0.6 mm to 2.5 mm. The norma span/depth vaues for continuous composite sab shoud be chosen to be ess than 35. The overa depth of the composite sab shoud not be ess than 90 mm and thickness of concrete, h c, sha not be ess than 50 mm. 2.0 DESIGN SITUATIONS The most important aspect of designer is to ensure an adequate degree of safety and serviceabiity of structure. The structure shoud therefore be checked for utimate and serviceabiity imit states. In the design of composite foors with profied decking the foowing situations are considered. 2.1 Profied stee sheeting as shuttering Verification is required for the profied stee sheeting at the construction stage when it is acting as formwork for the wet concrete, construction oads and storage oads if any. Whie cacuating the oads on the profied sheet, increased depth of concrete due to defection of the sheeting i.e., ponding effect has to be considered. Account shoud be taken of the effect of props, if any. If the centra defection (δ) of the profied deck in non-composite stage is ess than /325 or 20 mm, whichever is smaer, then the ponding effect may be ignored in the design of profied deck. Copyright reserved

2 2.1.1 Loads on profied sheeting Design shoud make appropriate aowances for construction oads, which incude the weight of operatives, concreting pant and any impact or vibration that may occur during construction. These oads shoud be arranged in such a way that they cause maximum bending moment and shear. In any area of 3 m by 3 m (or the span ength, if ess), in addition to weight of wet concrete, construction oads and weight of surpus concrete shoud be provided for by assuming a oad of 1.5 kn/m 2. Over the remaining area a oad of 0.75 kn/m 2 shoud be added to the weight of wet concrete Effective span The continuous sab is designed as a series of simpy - supported spans, for simpicity. The effective span can be taken as the esser of the two foowing: Distance between centres of supports The cear span pus the effective depth of the sab If, profied deck sheet is propped during construction then, effective span is cacuated using the formua. (This rue is taken from BS 5950:Part 4 as there is no provision in Eurocode). The width of the prop is negected here. e = B + d 2 ap (1) where, B d ap Width of top fanges of the supporting stee beams - The depth of the sheeting - Actua span of the composite foor 2.2 Composite sab Verification is required for the foor sab after composite behaviour has commenced and any props have been removed. Tota oading that is acting on the composite sab is considered in the design checks for the utimate Limit State. The oads are appied in such a way that the oad combination is most unfavourabe. Load factors of 1.35 for dead oad and 1.5 for imposed oad are empoyed in design cacuations. Generay it is sufficient to consider the foowing oad combinations in buidings mosty subjected to uniformy distributed oads: Aternate spans carrying tota factored oading due to imposed and dead oads. Other spans carrying ony factored oading due to dead oad. Any two adjacent spans carrying tota factored oad due to imposed and dead oad and a other spans carrying ony factored dead oad.

3 3.0 ANALYSIS FOR INTERNAL FORCES AND MOMENTS 3.1 Profied stee sheeting as shuttering Eastic anaysis sha be used where sheeting is considered. The design based on eastic distribution of bending moment is conservative, as it does not take into account redistribution of moments that can occur between support and mid span sections. The foowing treatment is based on moment redistribution at the utimate stages. A moment - curvature reationship for typica section of meta deck is shown in Fig. 1. Fig. 2 shows the eastic moment at the onset of yied and the moment at faiure utiising the pastic deformation of the hinges formed earier for a typica two span continuous beam. If M p and M n are the moment capacities of deck at mid-span and support sections respectivey, then at faiure ony a portion of M n i.e. km n at support can be reaised because of the avaiabe ductiity. Thus, 8 w = + ( M p 0.46kM ) (2) 2 n The vaue of k can be determined experimentay for the type of profie used. Moment (a) Loading M u (b) Typica cross section Pastic deformation φ e φ u Curvature (φ ) (c) Moment curvature variation of critica cross section Fig. 1 Moment curvature reationship for a meta deck foor

4 0.125w 2 Bending moment at first yied at support section 0.071w 2 M p km n Bending moment at faiure after redistribution Fig. 2 Bending moment variation on two span decking The oad span tabes given by manufactures of the profie deck are based on tests and hence take advantage of post eastic strength. They are satisfactory for spans 10 to 15% in excess of the corresponding vaues based on eastic design. However, IS: gives coefficients for bending moments and shear forces for continuous beams which can be used for cacuating moments and shear forces for composite foors aso. Tabe 7 and tabe 8 in the Chapter on Composite Beams II gives moment and shear coefficients respectivey. These coefficients do not make any aowance for redistribution and may be too conservative. 3.2 Composite sab The foowing methods of anaysis may be used: Linear anaysis with or without redistribution Rigid-pastic goba anaysis based either on the kinematic method or on the static method provided that it is shown that sections where pastic rotations are required have sufficient rotation capacity Eastic-pastic anaysis taking into accounts the non-inear materia properties. The appication of inear methods of anaysis is suitabe for the serviceabiity imit states as we as for the utimate imit states. Pastic methods, with their high degree of simpification, sha ony be used in the utimate Limit State.

5 Equations for evauating moment, ongitudina shear and vertica shear are given in the previous chapter. 4.0 DESIGN TABLES The manufacturers of the stee profied sheets normay provide design tabes for different decks made by them. These tabes give information regarding recommended sab depth and profie thickness to be adopted for different type of support conditions such as singe span without prop, with prop or for mutipe spans for different imposed oad rating. The spans that coud be achieved with propped condition are generay greater because the governing criteria wi be composite condition. However it is necessary to check defections in such cases for serviceabiity. 5.0 SERVICEABILITY LIMIT STATES FOR COMPOSITE SLABS WITH PROFILED DECKS 5.1 Cracking of concrete The profied deck sheeting protects the ower surface of the sab. Cracking wi occur in the top surface where the sab is continuous over a supporting beam in the hogging moment regions. Crack width wi be wider over the supports if each span of the sab is designed as simpy supported, rather than continuous, and if the spans are propped during construction To counter cracking, ongitudina reinforcement shoud be provided above interna supports. The minimum recommended amounts are as 0.2% of the area of concrete above the sheeting, for unpropped construction, and 0.4% if propping is used. (According to Eurocode 4). If the environment is corrosive, the sabs shoud be designed as continuous, with cracking controed by providing additiona reinforcement and ensuring that the concrete cover for reinforcement is suitaby enhanced. 5.2 Defection The imitations on defection for composite sabs are not expicity provided for in IS: Eurocode 4 gives expicit guidance, which is expained beow. The defection of profied sheeting due to its own weight and the wet concrete sab shoud not exceed e /180 or 20 mm, where e is the effective span. For the composite sab stage, the rare oading combinations described in section 2.2 are normay used. The maximum defection beow the eve of the supports shoud not exceed span/250, and the increase of defection after construction (due to creep and to variabe oad) shoud not exceed span/300, or span/350 if the foor supports britte finishes or partitions. The defections may not be excessive when span-to-depth ratios are kept within certain imits. These vaues are given in Eurocode 4 as 25 for simpy supported sabs, 32 for

6 spans with one end continuous and 35 for interna spans. These imitations are regarded as deemed to satisfy the serviceabiity defection imits. Depth imits reate to effective depths, so for composite sabs the depth shoud be taken as depth of composite sab over centroida axis of the profied deck sheet rather than tota depth of the sab. Broady speaking, sip occurs after reaching working oad in a we-designed sab. If the sip occurs earier it wi cause an increase in defection even in the Serviceabiity State, which is not desirabe. So, sip factor to be taken care of whie designing the sab. 6.0 FIRE RESISTANCE In genera no fire tests are required before the design of profied stee sheeting, as their manufacturers carry out these tests for genera use in buidings, before reeasing the products in the market. Standard fire-resistance tests are carried out by them, using independent testing centres, to ensure Strength or stabiity under oad Abiity to transmit smoke and fame Insuation so that upper part of the sab is not excessivey heated. Adequacy is checked by ensuring that defection does not exceed span/20 under fire tests. The oad carrying capacity is checked by considering ony embedded stee. In buidings the in-pane resistance and negative reinforcements at points of continuity add to the strength under fire condition. Whie simpy supported sab under fire test exhibits an endurance period of 30 minutes, the continuous sab withstands 30 minutes or even more with an imposed oad of 6.7 kn/m 2. Under fire condition with this imposed oad and with reduced strength of eements and eevated temperature the safety is checked so that pastic capacity gives a oad factor of 1.0 or more. For arger fire protection using fire engineering method, the area of embedded mesh reinforcement is suitaby increased by additiona emergency reinforcement as per the increase in the fire resistance period desired. Simpe design tabes for common cases of fire resistance of composite deck sabs are given in reference 3. These tabes can be judiciousy converted and used in the absence of test resuts for specific cases required. 7.0 DIAPHRAGM ACTION OF DECK SLAB Deck sab transmits in-pane oads for ensuring atera stabiity of the buiding system. For this the deck sab is attached on a the four sides at spacing exceeding 600 mm on either the beams or supporting was. The diaphragm action is exceent if through deck weding is resorted to. The stee decking aso provides atera support to the stee beams it supports. However, beams running parae to the decking are ateray supported ony at transverse beam connections.

7 8.0 STEPS IN THE DESIGN OF PROFILED DECKING The foowing are the steps for design of profied decking sheets: (i) List the decking sheet data (Preferaby from manufacturer s data) (ii) List the oading (iii) Design the profied sheeting as shuttering Cacuate the effective ength of the span Compute factored moments and vertica shear Check adequacy for moment Check adequacy for vertica shear Check defections (iv) Design the composite sab Generay the cross sectiona area of the profied decking that is needed for the construction stage provides more than sufficient reinforcement for the composite sab. So, the design of short span continuous sabs can be done as series of simpy supported sabs and top ongitudina reinforcement is provided for cracking as given in section 5.1. However, ongspan sabs are designed as continuous over supports. Cacuate the effective ength of the span Compute factored moments and vertica shear Check adequacy for moment Check adequacy for vertica shear Check adequacy for ongitudina shear Check for serviceabiity, i.e. cracking above supports and defections 9.0 CONCLUSIONS Foors using profied deck sheets is a very new design concept in the Indian context and hence no appropriate codes are avaiabe. These chapters on composite foors are intended to provide the most upto-date information and hence argey Eurocodes are foowed. A design exampe is incuded to iustrate the method empoyed in the choice of profied decking and for verifying its adequacy REFERENCES 1. Mark Lawson and Peter Wickens Composite Deck Sab, Stee Designers Manua (Fifth edition), The Stee Construction Institute, UK, Bryan E.R. and Leach. P Design of Profied sheeting as Permanent Formwok, Construction Industry Research and Information Association (CIRIA), Technica Note 116, Data Sheet: Fire resistance of Composite Sabs with Stee Decking, Construction Industry Research and Information Association (CIRIA), Specia Pubication 42