Computer Aided Analysis of Hollow slabs
Presentation by…
Phani Krishan.B, M.Tech Structures, Roll no: 05011D2016,
Thesis Guide and Supervisor ,
Dr .N.V. Ramana Rao, Professor of Dept. of Civil Engineering, JNTU College of Engg.,Kukatpally.
1
Objectives of the Work
Computer Aided Analysis of Hollow Slabs of different block cross sections under various loading conditions.
To Compare slab results modeled in the software with the manually modeled one.
2
INTRODUCTION
3
Fig(a)
Fig(b)
Slabs Where topping is considered to Contribute to structural strength. strength.
Fig(c)
Slabs Where topping not considered to Contribute to structural strength
Fig(d) 4
Hollow “or “solid block” former fo rmerss as per BS : 8110 and materials as per ASTM -C 90-[24.80], ASTM- C 55-[24.88], ASTM -C -C 145-[24.87].
Table 13.1of BS 8110, Weights of hollow clay block floor construction with 40 mm mm topping
Overall weight (kN/m2) Block size (mm)
Wall Thickness (mm)
Structural Depth (mm)
75 mm rib
100 mm rib
125 mm rib
300x300x75
15
115
1.75
1 .8 0
300x300x100
15
140
1.95
2 .0 0
300x300x125
15
165
2.15
2.25
300x300x150
18
190
2.39
2.54
300x300x175
18
215
2.58
2.73
2.88
300x300x200
18
240
2.82
2.98
3.13
300x300x225
18
265
3.22
3.42
300x300x250
20
290
3.51
3.70 5
Structural topping
Non structural topping
Structural topping
Not less than a0 / 5 Not more than 4b w
Not more than 4bw
a0 a0
Bo Bo w Not more than 500 mm Not more than 1500 mm
6
Hollow slab with mud blocks. Edges and Fire resistance.
Analysis: Analysis for moments moments and forces due to ultimate design loads in hollow slabs may be found
as per solid slabs, alternately can done as continuous T-beams simply supported.
Shear: The shear stress v should be calculated as v = (V / bv d)
Where ‘v’ is the shear force due to ultimate u ltimate loads on a width of slab equal to the distance center to center of ribs, and ‘bv’ is the average width of the rib and ‘d’ is depth of the slab..
7
Guidelines against spalling (as per FIP considerations 1988 paper EN 1168).
(a) For the slab if spalling stress is generated on the strands for the section width or a whole must satisfy the following condition.
(b) A fracture mechanics design using FEM prove that no spalling cracks develop ,using characteristic values on fracture energy and tensile strength.
8
Floor diaphragm action (as per Elliott, K.S., Davies, G. and Bensalem, K., Pre cast Floor Slab Diaphragm without Structural Screeds, Concrete 200- Economic and Durable Construction through Excellence, Dundee, sept 1993, pp 617-632)
The
stability of pre cast concrete buildings is provided in two ways,
(a) the horizontal loads due to wind are transmitted to shear walls. (b) the reaction forces resulting from the floor at each level are transmitted to the foundation.
9
As well as to wind loading, the floor diaphragm may also be subjected to additional horizontal forces, such as (a) Horizonta Horizontall forces forces due to lack lack of vertical verticality ity,, (b) Tempera Temperature ture and shrinkage shrinkage effects, effects, (c) In-plane In-plane forces forces as a consequence consequence of accident accidental al loading, loading, abnormal abnormal loading. loading.
Floors under seismic action:
The forces transmitted to the structure are related to many parameters; parameters; depend both on ground
motion and on the structural behavior.
Ductility is the main feature preventing large damage during strong quakes..
10
ADVANTAGES
OF HOLLOW CORE SLABS:
-Provides economical and efficient floor and roof systems. -Provides excellent fire resistance. Depending on the strand cover it can en dure up to 4 hours. -The Underside can be used as a finished ceiling. -The voids can be used for electrical or mechanical mechanical runs. -Provides the efficiency of a pre stressed member for load capacity, span range and deflection control. -Hollow core slabs can be used as diaphragms to transfer lateral loads. -A verity of architectural finishes is available.
11
Structural Modeling of Hollow Slabs (as per BS 8110):
- be made made of concrete or burnt clay; clay; - have a character strength of at least least 14 N/mm2, measured on the net section, when axially loaded in the direction of compressive stress stress in the slab; - when made made of fired brick earth, clay clay or shale, confirm confirm to BS 3921, BS 772-1, BS 772-3, BS EN 772-7.
Spacing and size of ribs: - In - situ ribs should be spaced at centers not exceeding 1.5 m and their depth is excluding any topping, should not exceed four times their width.
75mm to 125mm WIDTH Plan of 300x300 mm Depth
12
LAYING THE FLOOR OR ROOF (as per IS:6061-part-1):
c.
The The pre pre cas castt rein reinfo forc rced ed conc concre rete te jois joists ts sha shall ll be be plac placed ed in in posi positi tion on at at the the desi design gned ed spa spaci cing ng so as to span between the supporting elements, such as walls and beams.
d.
The holl hollow ow bloc blocks ks shal shalll be be pla place ced d in in bet betwe ween en the the joi joist stss wit with h the their ir Ends resting on the’ projecting lips of the joists in a manner indicated in Figure shown below.
c.
Nominal reinforcement reinforcement shall be provided for the structural topping concrete slab as per IS : 456-1964. At least 0.15 percent reinforcement along the joists and 0.20 percent reinforcement across the joists shall be provided for the structural topping concrete slab. The spacing of reinforcement bars shall not exceed 300 mm. The top reinforcement in the slab over the supports (joists) should be tied to the stirrups projecting from the joists. Welded wire mesh mesh may be used for the reinforcement as an alternate to mild steel bars.
13
14
Dimensions:
Dimension of slab: 5m x 5m Thickness of slab: 290 mm Minimum depth is = (l/35x0.8) = 178.5 mm , So provide a depth of 200 mm. As per British code BS: 8110, providing depth as topping is (1/10) of clear distance of ribs is 30 mm, and taking fire in to consideration, total cover of the slab finally is 290 mm. resistance and durability in to the consideration provide a cover of 40 mm: so total depth of slab is 290 mm. As per BS:8110 code table 13.1, use the blocks of 380x380x200 size and 100 mm wide ribs.
Loads:
Area of the slab for Dead Load
=
{Total sla slab area – Area of the voids)
=
[ 5x0.29- 0.3x0.18x9] =0.964 m2
,
Equivalent depth for Dead Load Calculation = ( 0.964/5 ) = 0.1928 meters, Taking Unit Weight Weight of Concrete as 25 kN/Sqm; Self Weight of the Slab
=
(0.1928x25)
=
4.82 kN/Sqm
Imposed/ live load
=
4.00 kN/sq m
Finishes
=
1.00 kN/Sqm
Total Total load load
=
9.82 9.82 kN/sq kN/sq m
Total factored load
= ( 1.5x 9.82) = 14.73 kN/Sqm 15
Moment = M = (α. W. l2 ) / 8 = (0.5x14.73x25)/8 Moment per each rib
= 23.9156 kN m/m
= 2.40kN m / rib
Used 10 mm hence safe and spacing 200 mm. mm. Maximum shear shear force = Vu = 21.33 kN
16
COMPARISON OF RESULTS:
Hollow slab with Hollow blocks and cross section
A
A
Section A-A
17
Voided Slab and Cross section Model of size s ize 5x5 m
L y=5m
L x=5m
A
A
Section A-A
18
Hollow slab with rectangular blocks and Cross Section Model of 5x5m
L x=5m
L y=5m A
A
Section A-A
19
Contour Representation of the Moments in Voided Slab
Ly=5m
Lx= 5 m 26
Comparison of All Slabs with Manually Modeled Slab Manual (Actual)
Ribbed
Hollow
voided
2.400
2.25
2.032
3.03
Comparison 3.5 3 2.5 2 Comparison
1.5 1 0.5 0 Actual
Ribbed
Hollow
voided
27
CONCLUSIONS
When the Live load, earthquake load and Wind load are applied on group of hollow block slab, voided slab and hollow rectangular slab, it has has been observed that there is considerable considerable increase in moments moments and shears, voided slabs has h as more moments moments than ribbed slabs and then Hollow rectangular moments.
The moments moments obtained Under live load in voided slab are more when compared with the hollow block slab and hollow rectangular slab than under Earth quake load and then Wind load.
Hollow block slabs show less deflection than hollow rectangular slabs, Voided slabs show more deflection.
There is a reduction obtained in the torsional moment moment with horizontal restraints. This however has further implications; rigid supports are highly undesirable as they would be creating restraining forces, for example shrinkage and thermal movement within the unit.
28
Recommendation to Further Study
The slabs are modeled and analyzed using the Staad Pro software this work also be done using u sing the other soft wares like SAP and Ansys.
In the present thesis Hollow slab with rectangular hollow blocks, hollow slab with hollow blocks and voided slab are modeled, in the software same slabs can be modeled using other blocks like trapezoidal shaped blocks, oval shaped blocks and square blocks and rectangular blocks with edges semicircular. semicircular.
The loading used in the present work are live load, seismic load and temperature loads, and there is a scope for future work as can use other type of loads and their combination.
Same slab can be analyzed by changing dimension and by using the stabilized mud blocks or block type bricks.
29
REFERENCES
Walraven, J.C, Diaphragm Action in Floors, Pre fabrication of concrete Structures International Seminar, Delft University of Technology, Delft University Press, Oct. 1990, pp 143-154.
Menegotto.M, Seismic Resistent Extruded Hollow Core slabs, sla bs, International Symposium on Noteworthy Developments in Prestressed and Pre cast Concrete, Singapore, November 1989.
De Roo, A .M. Straman, J.P., Krachtsverdeling in en Vormverandering van een in Synvlak Belaste Prefab Vloerconstructie, PhD thesis, Delft Precast Concrete Institute, Delft University oof Technology, 1991.
Svensson.s,Skivverkan I Elemenybjlklag, Publication Svensson.s,Skivverkan Publicat ion 88:1, Chalmers Technical University, Goteborg, Sweden, 1988.
Elliott, K.S., Davies, G. and Bensalem, K., Pre cast Floor Slab Diaphragm without Structural Screeds, Concrete 200- Economic and Durable Construction through Excellence, Dundee, sept 1993, pp 617-632. 30
P.Purushothaman, “ Reinforced concrete structural elements behavior, analysis and
design”
BS: 8110 Code of Practice.
Jain, “ Reinforced concrete”
IS: 456-2000, Plain and Reinforced Concrete-Code of Practice,
Bureau of Indian Standards, New Delhi.
10.
Pajari M & Koukkari H, (1998) ‘Shear resistance of PHC slabs supported on
beams I: Tests’. Journal of structural engineering, No. 9, 1050-1061
Joris Fellinger, TNO Centre for Fire Research, Delft, the Netherlands
11.
Jan Stark, Delft University of Technology, Fac. of Civil Engineering and
Geosciences, Delft, the Netherlands
12.
Joost Joo st Walra Walraven ven,, Delft Delft Uni Univer versit sity y of Tech Technol nology ogy,, Fac. Fac. of of Civil Civil Engi Enginee neerin ring g
and Geosciences, Delft, the Netherlands.on Vol. 28 – No. 3.
31
13.
ENV199 ENV 1992-1 2-1-2 -2 ( 199 1995), 5), ‘Eur ‘Euroco ocode de 2: 2: Design Design of of concr concrete ete struc structur tures, es, Part Part 1.2: 1.2:
General rules, structural fire design’, Brussels: CEN
14.
Walrav Wal raven, en, J.C., J.C., P. P. Mercx Mercx (198 (1983) 3),, ‘The ‘The bearin bearing g capacit capacity y of pres prestres tressed sed hol hollow low
core slabs’, Heron Vol. 28 – No. 3.
15.
TU Delft – TNO, Centre for Fire Research report 2001-CVB-R04634.
16.
Resistance, 2005-25p,Au 2005-25p,Auroville. roville.
17.. 17
Sp-2 Sp -24,“ 4,“Ex Expl plen enate atery ry Hand Hand Boo Books ks Pre Pre stre stress ssed ed conc concret rete” e”
18.
STAAD STA AD pro pro-- Get Getting ting Sta Started rted ,Ex ,Examp amples les and ref referen erence ce man manual ual..
Breun Bre unese ese,, A.J. (200 (2001) 1) ‘Ten ‘Tensile sile prop propert erties ies of of concre concrete te durin during g fire’, fire’, Rijs Rijswij wijk, k,
Produc Pro ductio tion n and and Use of Ho Hollow llow Int Interlo erlockin cking g CSEB CSEB for Dis Disast aster er
32
33
