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Flat Slab Design References / Guidelines /Issues

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Dr. N. Subramanian
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PostPosted: Thu Dec 25, 2008 6:52 pm    Post subject: E-Conference on Flat Slabs Reply with quote

Hi all,

I am giving below my views about the subject and also some collection of references which will be useful to many of the participants:

Though the Indian concrete code (IS 456) was revised in 2000 after 22 years(earlier version 1978), it offers no change in provisions related to limit states design presented in sections 3-5 (except for some provisions on shear strength close to supports, corbels, crack width calculation, and on the design of RC walls). Most of the changes were made in Section 2 of the code, which deals with durability aspects, which was neglected at the past due to the excessive stress on strength calculations. Prof.Murty1, points out that the code does not follow capacity design concepts, which are important in earthquake design. Also the provisions of the code are not meant for high strength concrete, which is often employed in flat slabs. Moreover it is necessary to produce a unified code, which cover earthquake resistant design (as more than 60% of the country is considered to be in Zone 3 or above), as well as prestressed concrete, as is done in several international codes such as ACI 318-08. Moreover BIS take several years to revise a code, which means the provisions in the code do not reflect the current state-of-the art. Note that IS 456 is already 8 years old! The American code is revised every 3 years; but it is better to revise the codes every 5 years. If the BIS is not able to publish the code that fast, Indian Concrete Institute should take up this job and publish parallel codes. In this connection it may be of interest to note that INSDAG (Institute of steel development and Growth) spent huge amounts of money to develop the current version of IS 800:2007. Similarly some cement companied like ACC should be persuaded to provide money for the development of concrete codes. It may be helpful to publish special publications in selected areas, such as flat slabs, high strength concrete, etc., which may assist the code writing.  

Coming to the punching shear design of flat slabs, the Indian Code provisions do not consider reinforcement ratio and size effects. The Euro code provisions include these parameters also and are based on cube root of compressive strength of concrete unlike the square root of compressive strength as in IS code. They are also found to predict the punching shear strength of flat slabs consistently for high strength normal weight and high strength light weight concretes (light weight aggregates are being increasingly used to reduce the self weight of concrete or due to the unavailability of natural coarse aggregates). Hence, the provisions of CEB-FIP modal code equations are proposed to be adopted by the Indian code (see my paper in the Indian Concrete Journal, April 2005, pp.31-37 for more details).

The punching shear resistance of reinforced concrete flat slabs can be enhanced by various means (Enhancement is necessary especially in flat slabs located in seismic areas. During an earthquake, the unbalanced moment transferred between slabs and column may produce significant shear stresses that will increase the likelihood of brittle fracture). The enlargement of column cross-section and thickening of the portion of the slab around the column (by use of drop panels or column shear capitals) will enhance the shear resistance.  Megally and Ghali showed that the failure of shear capital is accompanied by sudden separation of the shear capital from the slab, along with brittle failure and do not recommend the use of shear capitals to increase the punching shear resistance especially in earthquake zones2 (for the shear capital to be effective, their length should be greater than four times slab thickness plus the largest column dimension, and should also be reinforced like drop panels).
Provision of spandrel beams along the edges of the slab will improve the punching shear capacity of the slab 3. However, the existence of spandrel beams will complicate the already complex punching shear performance of the column-slab connection. In view of the above, many researchers have found that the introduction of shear reinforcement is more economical and reduces the chances of brittle failure at slab-column connection. The performance of several types of shear reinforcements such as inclined stirrups, structural shear heads (in the form of steel  I-or channel sections),  bent – up bars, hooked  bars and welded-wire fabric have been tested extensively in the last three decades 4-10. It has been found that the introduction of such shear reinforcement results in ductile failure caused by yielding of flexural reinforcement and improves the punching shear resistance. ACI 318-08 commentary (clause 11.11.3) discourages the use of conventional shear reinforcement in slabs thinner than 250mm.

In order to solve this problem, the research team in the University of Calgary, Canada has developed three types of pre-assembled shear reinforcing units, viz., the I-segment, headed shear stud and welded wire fabric5. In addition they have developed a type of shear reinforcement called stud-shear reinforcement, which has advantages over the other types of shear reinforcement 6. Though this stud-shear reinforcement has been in use in the international market for the past few years, it is not yet freely available in the Indian market. Provisions for the design of flat slabs with stud-shear reinforcements were introduced in the American code in section 21.11.5 in 2005. Similar provisions should be introduced in the Indian code, for the effective use of stud-shear reinforcements in India. Ref.11 to 14 provide more details about methods of design using stud-shear reinforcement, and worked out design examples.

In heavy earthquake zones, slab-column frames are to be used as gravity-force resisting systems (non-participating system), in conjunction with special moment frames or special structural walls. For these combined systems, lateral load stiffness and strength demands are required to be solely taken by the special frames or walls and the deformation capacity be checked for slab-column frames. Research has shown that storey drift limits, although primarily related to serviceability, also improve frame stability (P- Δ effects) and seismic performance of such systems because of the resulting additional strength and stiffness (Pan and Moehle, 1989). New provisions were added to ACI 318-05 in section 21.11.5 to require connections of non-participating slab-column frames be checked to avoid punching failures when subjected to design drift; conditions where punching failures are expected require the addition of shear reinforcement.

Available test data on PT and shear reinforced connections show that the drift capacity at punching is about twice that for RC connections. More details about the design of PT slabs and their  seismic design criteria may be found in Ref.17-20

1.     Murty. C.V.R., Shortcomings in structural design provisions of IS 456:2000, The Indian Concrete Journal, V. 75, N.2, pp.150-157
2.     Megally, S., and Ghali, A., Cautionary note on shear capitals, Concrete International, Aci, Vol. 24, No.3, Mar. 2002, pp.75-82.
3.     Falamaki, M., And Loo, Y.C., Punching Shear Tests Of Half Scale Reinforced Concrete Flat Plate Models With Spandrel Beams, ACI Structural Journal, Vol. 89, No.3, 1992, pp.263-271.
4.     Hawkins, N.M., Mitchell, D., and Hanna, S.N., The Effects Of Shear Reinforcement On The Reversed Cyclic Loading Behavior Of Flat-Plate Structures, Canadian Journal Of Civil  Engineering, Vol. 2, 1975, pp. 572-582.
5.     Seible, F., Ghali A., And Dilger, W.H., Preassembled Shear Reinforcing Units For Flat -Plates, Journal Of The American Concrete Institute, Vol. 77, No.1, 1980, pp. 28-35.
6.     Mokhtar, A., Ghali, A., and Dilger, W.H., Stud Shear Reinforcement For Flat Concrete Plates, ACI Structural Journal, Vol.82, No.5, 1985, pp. 676-683.
7.     Broms, C.E., Shear Reinforcement For Deflection Ductility Of Flat Slabs, ACI Structural Journal, Vol. 87., No.6, 1990, pp. 696-705.
8.     Ghali, A., and Hammill, N., Effectiveness Of Shear Reinforcement In Slabs, Concrete International, Vol.14, No.2, 1992, pp.60-65.
9.     Lim, F.K., and Rangan, B.V., Studies On Concrete Slabs With Stud Shear Reinforcement In Vicinity Of Edge And Corner Columns, ACI Structural Journal, Vol. 92, No.5, 1995, pp.515-525.
10.     Ghali, A. and Dilger, W.H., Anchoring With Double – Head Studs, Concrete International, Vol. 20, No.11, 1998, pp.21-24.
11.     ACI-ASCE Committee 421, Shear Reinforcement For Slabs, American Concrete Institute, Farmington Hills, Michigan, 1999, 15 pp.
12.     Elgabry, A.E., and Ghali, A., Design Of Stud-Shear Reinforcement For Slabs, ACI  Structural Journal, Vol. 87, No.3, May-June 1990, pp.350-361.
13.     Hammill, N. and Ghali, A., Punching  Shear Resistance Of Corner Slab- Column Connections, ACI Structural Journal, Vol. 91, No. 6, Nov-Dec 1994, pp. 697-707.
14.     Mokhtar, A., Ghali, A., and Dilger, W.H., Stud Shear Reinforcement For Flat Concrete Plates, ACI Structural Journal, Vol.82, No.5, 1985, pp. 676-683.
15.     Aalami, B.O., Design of post-tensioned floor slabs, Concrete International, V. 11, N.6 , June 1989, pp.59-67
16.     Pan, A, and Moehle, J.P. Lateral displacement ductility of RC flat plates, ACI Structural Journal, Vol.86, No.3, 1989, pp. 250-258.
17.     Kang, T.H.-K., LaFave, J.M.,  Robertson, I.N., and  Hawkins, N.M., Post tensioned slab-column connections, Concrete International, V. 29 N.4 , April 2007, pp.70-77
18.     Kang, T.H.-K., and Wallace, J.W., Punching of RC and Post-tensioned Concrete slab-column connections, ACI Structural Journal, Vol. 103, No.4, July-Aug 2006, pp. 531-540
19.     Kang, T.H.-K., and Wallace, J.W., Seismic performance of  RC slab-column connections with thin plate stirrups, ACI Structural Journal, Vol. 105, No.5, Sept-oct 2008, pp. 617-625
20.     Hueste, M.B.D., Browning, J., Lepage, A. and Wallace, J.W., Seismic Design criteria for slab-column connections, ACI Structural Journal, Vol. 104, No.4, July-Aug 2007, pp. 448-458
21.     Moehle, J.P. Seismic design considerations for flat plate construction, Mete A. Sozan Symposium: A tribute from his students, SP-162, J.K. Wight and M.E. Kreger, Ed., American Concrete Institute, Farmington Hills, Michigan, 199p, pp. 1-35.

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Dr. N. Subramanian
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Joined: 21 Feb 2008
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Location: Gaithersburg, MD, U.S.A.

PostPosted: Mon Jan 05, 2009 9:43 pm    Post subject: Flat Slab Design References / Guidelines /Issues Reply with quote

Hi all,

In continuation of the views expressed by me in my earlier e-mail, I wish to state that the expressions given in the codes of practices of many countries are basically empirical and there are considerable differences between them. Recently two analytical models have been proposed and found to agree well with the test results :

Muttoni (2008) developed a comprehensive analytical model for predicting the punching shear strength of reinforced concrete slabs without transverse reinforcement. his failure criteria is based on a given critical rotation of the slab. Similar proposal has been included in the Swedish standards.

Another comprehensive analytical model has been developed by Theodorakopoulos and Swamy (2002) to predict the ultimate punching shear strength of slab-column connections. This model also is based on the physical behavior of the connections and is applicable to both light-weight and normal-weight concrete. It also incorporates several variables that affect the punching shear strength of flat slabs including the concrete strength, tension steel ratio, compression reinforcement, and loaded area. It was compared with 60 reported tests in literature and found to agree with them with reasonable accuracy.

Theodorakopoulos and Swamy (2007) recently extended the above theory for predicting the punching shear strength of FRP-reinforced concrete flat slabs also. They found that the model gives excellent correlation with test results of slabs reinforced with FRP rebars.

The formulae given in ACI, Eurocode 2, and FIP for predicting the punching of post tensioned slabs have been compared by Silva, et al.(2007). They infer that all these methods give satisfactory results. They also suggest that the predictions of the ACI method can be improved if the distance of control perimeter from the support is increased from the present distance of d/2 from the face of the column.
The effect of rectangular columns and openings in slabs on the punching shear strength  of slabs is discussed by Teng et al (2004) who also propose an approximate equation to predict the strength based on the modified ACI eqn.

1.     Theodorakopoulos D.D., and Swamy, R.N., “Ultimate punching Strength Analysis of Slab-column Connections, Cement & Concrete Composites, V. 24, No.6, 2002, pp.509-521.
2.     Theodorakopoulos D.D., and Swamy, R.N., “ Analytical Model to Predict Punching shear Strength of FRP-Reinforced Concrete Flat slabs, ACI Structural Journal, V.104, No.3, 2007, pp.257-266.
3.      Muttoni, A. Punching shear strength of reinforced concrete slabs without transverse reinforcement, ACI Structural Journal, V.105, No.4, July-Aug 2008,pp.440-450
4.     Teng, S. Cheong, H.K., Kuang, K.L. and Geng, J.Z., Punching shear strength of slabs with openings and supported on rectangular columns, ACI Structural Journal, V.101, No.5, Sept-Oct 2004, pp. 678-687.
5.     Silva, R.J.C., Regan, P.E., and Melo, G.S.S.A., Punching of Post-tensined Slabs- Tests and Codes, ACI Structural Journal, V.104, No.2, Mar-Apr 2007, pp. 123-132.
6.     Silva, R.J.C., Regan, P.E., and Melo, G.S.S.A., Punching resistances of unbonded post-tensioned slabs by decompression methods, structural Concrete, vol.6, No.1, 2005, pp.9-21.

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