Dynamic behaviour of composite floors

J. Construct.Steel Research34 (1995)249-269 O143-974X(94)00027-1

ELSEVIER

© 1995ElsevierScienceLimited Printed in Malta. All rights reserved 0143-974X/95/$9.50

Dynamic Behaviour of Composite Floors G. J. Krige & J. M a h a c h i Department of Civil Engineering, University of The Witwatersrand P. Bag 3, WITS 2050, Johannesburg, South Africa

ABSTRACT

As part of an on-going experimental and analytical research effort to evaluate the fatigue strength of composite slabs, 18 Bond-lok composite slabs have been tested statically and dynamically in bending. Special interest has been paid to bond failure between the concrete and steel deck. The effect of both small and large amplitude loading~ on fatigue strength and deformation characteristics has been examined. Based c,n the fatigue strength results, a guide for the design of composite slabs subjected to fatigue loading has been provided. The information is provided in the form of modified Goodman diagrams and algebraic expressions that can be utilised for design.

NOTATION P~

Smax Smin

f't

fou

Ultimate static load Algebraic value of the maximum load in a cycle expressed as a percentage of the ultimate load Pu Algebraic value of the minimum load in a cycle expressed as a percentage of the ultimate load Pu Modulus of rupture Concrete characteristic strength Modulus of elasticity of concrete Fast Fourier Transform Number of cycles to failure Stiffness at failure

Ec FFT Nf Kf Load rankle Smax-Smin

249

250

G. J. Krige, J. Mahachi

1

INTRODUCTION

The use of composite action makes it possible to construct floors that are very light, especially if metal decking is used as a permanent formwork.1 For very thin steel sheet, shear connection is provided by both adhesion and mechanical bonds. The bond depends on panel geometry, surface conditions and the types of embossments or rolled dimples that project into the concrete. The embossments and the geometry of the profile serve to prohibit vertical separation and provide mechanical interference against slip as adhesive bond deteriorates. Long span floors, however, are prone to vibrations that may cause fatigue failure of the bond between the steel and concrete. 2 Fatigue tests are being carried out on composite floors using two different decking profiles, Bond-dek and Bond-lok. This paper describes the results of a series of tests which have investigated the fatigue response of the Bond-lok composite floors subjected to constant minimum load levels (Smi. = constant) with varying maximum load levels (Smax). Bond°lok are plain profiled sheets (Fig. 1) with some form of re-entrant angle which prohibits vertical separation of the concrete and steel deck, due to the interlocking shape, and also forms a reliable, effective bond. Main parameters of interest considered are: • • •

• •

type of loading; the fatigue life or endurance limit; mid-span deflections and end-slip; cracking and/or bond failure; mode of failure.

As such, the following parameters were maintained as constant throughout the test program: •

a sinusoidal waveform of loading;

T

150

'°3

7

o

0 ~

o

i

,~

p

8

"\ o

_i

q

v I

Fig.

1. Bond-Lok section.

1

Dynamic behaviour of composite floors

251

loading frequency of 4 Hz; concrete mix proportions. 2 BACKGROUND OF FATIGUE Fatigue is normally the repeated application of working loads. The working loads (cyclic loads) initiate the formation of internal or surface microcracks that are propagated in continued exposures, resulting in fatigue damage and eventually fatigue failure. Development of fatigue cracks form even when the cyclic stresses are below the yield strength of the material. Fatigue life plots are normally presented as S - N curves (Stress range, S, vs Numbe,r of cycles to failure N). However, for concrete, tests of up to 10 million cycles have failed to establish a fatigue limit 3 and hence the fatigue strength is usually quoted for a specified number of cycles. Goodman 4 proposed a diagram that depicts the effect of stress range and maximum stress by assuming a linear decrease of the range of stress as the maximum stress increased.

3 EXPERIMENTAL PROCEDURE

3.1 Materials and specimens The concrete that was used in the preparation of the test specimens was a pre-mix. The mix design was based on a nominal strength of 40 MPa at 28 days and a slump of 75 mm. The concrete proportions used in the mix are as shown in Table. 1 The initial study described in this paper investigates the fatigue behaviour of Bond-lok, shown in Fig. 1. TABLE 1 Concrete Mix Design Characteristic strength fcu Stone size 19 m m Stone size 13 m m Crusher sand Filler sand OPC Water Admixtures

= = = = = = = =

40 M P a 912 kg/m a 228 kg/m 3 428 kg/m 3 396 kg/m 3 390 kg/m 3 195 kg/m 3 780 kg/m 3

252


The Bond-lok 5 had a nominal width of 300 mm, a thickness of 1.0 mm and a nominal yield strength of 200 MPa. The overall nominal dimensions of the whole composite slab were 3 0 0 m m × 150 mm × 3700 mm long. N o additional steel reinforcement was provided. A total of 18 composite slabs was cast. Consolidation of concrete in the formwork was accomplished by using a poker vibrator. Cubes, prisms and beams were cast according to British Standards, 6 and a summary of the average strength results is given in Table 2. To reduce the effects of shrinkage and aging of concrete, the slabs were cast progressively so that testing of the composite slabs was done after 28 days. 3.2 Test program

The test program included static and fatigue tests on two groups of specimens subjected to different loading. G r o u p A specimens were subjected to a central line loading and six different test series were investigated as follows: • • • • •

Static ultimate tests. Repeated loading with constant minimum varying maximum load S m a x • Repeated loading with constant minimum varying maximum load S m a x • Repeated loading with constant minimum varying maximum load S m a x • Repeated loading with constant minimum varying maximum load Smax. Static ultimate tests after repeated loading.

load

Smin=4"0% and

load Sm~. =25"0% and load Smi, =40"0% and load Stain =55"0% and

G r o u p B specimens were subjected to 2-point line loading and four different test series were carried out as follows: •

Static ultimate tests. TABLE 2

Concrete Properties at 28 Days a Characteristic strength (fcu) Modulus of elasticity (Ec) Modulus of rupture (f't) Density of concrete Poisson ratio

= = = = =

40"6MPa 35 GPa 5"5 MPa 2630 kg/m 3 0,3

° Concrete properties determined in the lab after 28 days.


• • •

253

Repeated loading with constant minimum load Smin=4"0°,/o and varying maximum load Smax. Repeated loading with constant minimum load Smi.=40% and varying maximum load S~a~. Static ultimate tests after repeated loading.

3.3 Test procedure 3.3.1 Static tests Three static tests for the central line loading and three for the 2-point loading were performed to establish the ultimate static strength, Pu, and the load at which debonding first occurs. In all tests the slabs were simply-supported over a span of 3.7 m and the load was applied by a hydraulic actuator. For the 2-point loading, the distance between the line loads was 1100 mm. Mid-span deflections and end-slip were measured by 50 mm dial gauges. Figure 2 shows the static test set-up for Group A specimens with central line loading and Fig. 3 shows a similar set-up for Group B, 2-point loading specimens. 3.3.2 Fati!1ue tests For all tile fatigue tests conducted, servo-controlled hydraulic testing machines were used with a constant sinusoidal loading frequency of 4 Hz. Mid-span deflections and end-slip were measured by LVDTs and dial

Fig. 2. Static test set-up for Group A central line loading.


254

Fig. 3. Static test set-up for Group B 2-point loading.

gauges respectively. A summary of fatigue tests performed for Group A specimens is presented in Table 3 and for Group B specimens in Table 4. Measurement of damping was done using AD12F, a commercial dynamic software package. This was accomplished by performing a Fast Fourier Transform (FFT) on the mid-span deflections and then calculating damping from the half power bandwidth. A logarithmic decrement test 7 TABLE 3 Summary of Fatigue Tests for Group A Specimens

Specimen identity"

Mass (kg)

Concrete f cu(MPa)

Ec (GPa)

f : (MPa)

Smi~

Sin.x

(%)

(%)

B4-10-08 B6-10-08 B 11-24-08

455 445 470

40"6 41-0 40"2

34-2 35"4 33"8

5-46 5"52 5"49

4"0 4"0 4-0

30 45 55 b

B 10-24-08 B1-10-08

470 455

42"0 41"8

36"2 35"6

5"38 5"48

40 40

55 b 70

B3-10-08 B5-10-08

450 440

41"5 40"6

35"2 36"1

5"54 5"50

25 55

70 70

The specimen identity was coded as: name tag--day of casting--month of casting. b Subjected to a low load range test after crack developed.

255

Dynamic behaviour of composite floors TABLE 4

Summary of Fatigue Tests for Group B Specimens Specimen identity

Mass (kg)

Concrete f cu(MPa)

Ec (GPa)

f l (MPa)

Stain

Smax

(%)

(%)

B9-24-08 BI 3-24-08 B12-24-08

465 455 450

40.5 40.8 41-8

34.3 33-9 32'8

5.50 5-45 5.42

4-0 4.0 4-0

45 55 65

B16-24-01t B15-24-08

460 455

42.2 41.5

34-5 33'5

5-52 5'48

40 40

65 85

(LDT) was also performed so as to establish the natural frequency and as a basis of comparing the damping ratio with that obtained by FFT.

4 DISCUSSION

OF RESULTS

4.1 Static tests

A summary of static test results is shown in Table 5. The load-deflection curve for a typical G r o u p A composite slab (B2-10-08) (Fig. 4) has a constant slope up to a load of about 8.5 kN (0.73 Pu), representing a static stiffness of 4420 kN/m. Ultimate failure of the composite slab occurred at a load of 11"5 kN, accompanied by an end-slip of 0.25 mm and a vertical crack at the point of m a x i m u m moment, as shown in Fig. 5. The end-slip TABLE 5

Summary of Static Test Results Specimen identity

Mass (kg)

.f cu (MPa)

Static stiffness (kN/m)

Pu (Exprt.) kN

Pu (Theoret.) kN

GROUP A B2-10-08 B7-10-08 B8-10-08

435 440 430

40"8 40' 1 39-8

4420 4480 4300

11'5 12"0 12-5

13"5 13"5 13"5

450 455 460

40"0 41'2 40"7

3600 3595 3650

I0"5 12-8 12"2

19"4 19"4 19'3

GROUP B B 14-24-'08 B18-24-08 B17-24-08

Theoret.--Theoretical

Exprt.--Experimental.

256

G. J. Kriae, J. Mahachi 14 Specimen B 2 - 1 0 - 0 8 12

10 Z

~8 "0 m 0 "J

6

0 0

;

i

J

1

2

3

Deflection

4

(turn)

Fig, 4. Initial ultimate static test for Group A.

Fig. 5. Cracked slab for Group A central line loading.

was observed at one end of the slab. At ultimate load the mid-span deflections increased markedly, accompanied by some creep deflections. The ultimate strength results for Group A compare quite well with the theoretical values, as shown in Table 5. The theoretical ultimate strength results were determined by simple

257


rectangular' stress block theory, assuming full intersection between the two elements with concrete in compression at a uniform stress of 0"67fcu and the steel at its design yield strength. Concrete in tension was neglected. For Group B specimens, a diagonal crack characteristic of a shear bond failure formed in the concrete near one of the loaded points, as shown in Fig. 6. The', load-deflection curve for a typical slab (B14-24-08) has two distinct slopes (Fig. 7). The diagonal tension crack formed at a load of 6.0 kN. This was followed by sounds of slippage or bond failure between the steel deck and concrete. The initial stiffness was reduced from 3600kN/m to 2150kN/m as soon as bond failure commenced• The ultimate failure load of the slab occurred at 10-5 kN when composite action over the shear span length was lost. The maximum recorded endslip was 0.4 mm. The average ultimate load for Group A, 12.0 kN, was of the same order as Group B, 11.83 kN, suggesting that the mode of failure for both groups could be shear bond failure.

4.2 Damping and natural frequency The values; of the damping ratio obtained by performing a F F T on the mid-span deflections using AD12F package compared quite well with the values obtained by the logarithmic decrement method• The natural frequencies for the composite slabs obtained by the logarithmic method also

•

Fig. 6. Cracked slab for Group B 2-point loading.

•

.

: ~


258 12

Specimen

B14-24-08

10

4

2

i

J

J

1

2

$

0 0

4

Deflection (mm)

Fig. 7. Initial ultimate static test for Group B.

compared well with the theoretical values. The frequency was determined by assuming the slab to supported over a span of 3.7 m. A summary of the properties is shown in Table 6. On average,

theoretical natural be a beam simplymeasured dynamic the experimental

TABLE 6 Summary of Dynamic Properties

Specimen identity

B1-10-08 B2-10-08 B3-10-08 B4-10-08 B5-10-08 B6-10-08 B7-10-08 B8-10-08 B10-24-08 B 11-24-08 Average

Mass (kg)

455 435 450 455 440 445 440 430 470 470 450

Natural frequency (H z)

Dampin# ratio (%)

Experimental

Theoretical

LDT

FFT

21-0 20"0 19'0 20"5 19"5 20"0 19"5 20"0 21"0 18'5 19"9

17'9 18-2 17.9 17.6 18"4 18.1 18.0 18"1 17.8 17'2 17.9

1.35 1.06 1.05 1-06 1.10 1.02 1.02 1-10 1-15 1'25 1.12

1'35 1'40 1'40 1-45 1.45 1.42 1-30 1-40 1.45 1"45 1.41

LDT--Logarithmic Decrement Test. F F T - - F a s t Fourier Transform.


259

natural fi'equency of the slabs was 19.9Hz, with a damping ratio of 1-41%. 4.3 Fatigue tests 4.3.1 Group A A summary of fatigue test results for Group A specimens is shown in Table 7. The group was further subdivided into four groups (I-IV) depending on the minimum load, Smi n.

Specimen Group I: Sml n =4.0%. The stiffness of the composite slabs subjected to a maximum load Smax < 50% increased slightly with number of cycles (N), as is shown in Fig. 8 for a typical slab (B4-10-08) and thereafter remained more or less constant. These slabs sustained 4.5 million cycles; this suggests that there was some work-hardening effect during fatigue loading and that the normal elastic fatigue loading could be sustained indefinitely. Ultimate static tests following 4.5 million cycles of repeated loading for slab (B4-10-08), as shown in Fig. 9, indicated that the ultimate loadcarrying capacity could still be developed. Similar failure modes were observed as for the static test, with no previous load history. The initial static stiffness before fatigue loading was 4500 kN/m and after 4.5 million cycles was 4800 kN/m. The ultimate static load of 11-3 kN was accompanied b,.: an end-slip of 0.86 mm. TABLE 7 Summary of Fatigue Tests Results for Group A

Specimen identi~y

Group identity

Load range (%)

Stain

Smax

Cycles to failure (Nf)

Stiffness at failure (k f)

kN/m B4-10-08 B6-10-08 B 11-24-08

I I I

4"0 4"0 4"0

30 45 55

* * 200

4850 4800 1610

B10-24-08 B 1-10--08

II II

40 40

55 70

700 500

1730 2300

B3-10.-08 B5-10.-08

III IV

25 55

70 70

440 0

1560 1700

* Deck did not fail after 4-5 million cycles.

G. ,I. Krige, ,I. Mahachi

260

6000

6ooo ~ ~ _ . ~ ~

~

E 4000 ~. Z v QO

aO 4) ~m

3000 . . . . . . . . . . . . . . . . . . . . . . Specimen B4-10-08 2000

.

1000

.

.

.

.

.

.

.

......................

0 0

I I I I 1 2 3 4 Number Of Cycles (N) (Millions)

Fig. 8. K - N curve for an uncracked Grup A slab.

f

10

8

i ~ j

Specimen B4-10-08

0

I

0

1

I

2 3 Deflection (ram)

4

Fig. 9. Ultimate static test after 4"5 M cycles.

For slab (B11-24-08), subjected to S m a x = 55%, a crack was developed at mid-span immediately after 200 cycles. The load range also dropped drastically. The load-deflection curve for this slab after 200 cycles is shown in Fig. 10. From the figure it can be seen that the stiffness was reduced tremendously from an initial value of 3800 kN/m to 1600 kN/m after

Dynamic behaviour of compositefloors

261

6

Specimen B l 1 - : ~ 4 - 0 8 5

4

.J 2

1

0

0

~

•

~r--

2 0 0 Cycles

Cycles

I

I

I

I

I

0.5

1

1.5

2

2.5

Deflection (ram)

Fig. 10. Static test for a cracked Group A slab after 200 cycles. 200 cycles. The same cracked slab was then tested for fatigue under a lower load range of (4.0-30%). The cracked composite slab (B11-24-08) showed better performance for a lower load range. The ability to continue to withstand further loading was probably due to the profile of the deck having a re-entrant angle that prevented the separation of the concrete and the steel deck. Load-deflection curves after various numbers of cycles (N) of repeated loading had two distinct slopes, the second, less steep, slope characterising the stiffness of the slab when the crack was fully open. A typical load-deflection curve after 840 500 cycles is shown in Fig. 11 and the curve of the stiffness (with the crack fully open) versus number of cycles is shown in Fig. 112.F r o m Fig. 12 the composite slab managed to survive 4.0 million cycles with a reduction in stiffness from 1880 to 1540kN/m. Ultimate static test after 4-0million cycles showed that the maximum ultimate strength was 5-45 kN. In this case, end-slip occurred early, at a load of 4-65 kN, and the maximum end-slip was 0.33 mm. For Group I specimens it was concluded that the behaviour of the composite slabs under repeated high load range was inferior to that observed for the low load range and was characterised by a crack after a few hundred cycles. Thus for S m i n = 4"00,/o, the endurance limit for 4-5 million cycles is about 50% of the ultimate endurance. Specimen group II: Sr, i . = 4 0 % . For the composite slab (Bl-10-08) subjected to a maximum load range of 40-70%, the slab failed after

262

G. J. Kri#e, J. Mahachi 8

4

A

z

8

"O O ..J

0

I

I

I

I

0.5

1

1.6

2

Deflection

2.5

(ram)

Fig. 11. Static test for a cracked Group A slab after 840 500 cycles. 1900

8peolmen B l l o 2 4 - 0 8 1800

E z o m O

1700

1600

160©

0

~

i

J

l

1

2

3

4

Number

O f Cycles (N) (Millions)

Fig. 12. K - N curve for a cracked slab.

500cycles, with a noticeable crack at mid-span, and the load range suddenly dropped. The initial static stiffness of 4185 kN/m dropped after 500 cycles to 2300 kN/m (a reduction of 45%). The slab was then subjected to a lower load range of 20-40%. For this load range the crack developed further and the stiffness was further reduced to 1736 kN/m. The slab could not carry the load range after 119 000 cycles.


263

Another test (B10-24-08) was performed with the same minimum load, Stain = 4 0 % , but with a maximum load reduced to 55% (6-5 kN). As with the above case, a crack developed at mid-span after 700 cycles. The slab was then tested for a lower load range (4-30%). The behaviour was similar to that of a G r o u p I cracked slab (Bll-10-08) with two distinct slopes for the load-deflection curves. F r o m these test results it was concluded that any maximum load exceeding 50% of the ultimate could not be sustained.

Specimen group I I I : S,~i, = 2 5 % and Sma x = 7 0 % . Failure of this slab, characterised by a central crack occurred after 440 cycles. The slab was then tested statically and the results show that the stiffness was reduced by a b o u t 60% from an initial value of 3800 kN/m. Specimen group IV." Stain=55% and Smax=70%. F o r this slab (B510-08) a crack immediately developed on applying the cyclic load. The initial static stiffness of 4300 k N / m was reduced to 1700 k N / m after the formationL of the crack. There was a corresponding increase in mid-span deflection and some creep. F r o m this test it was concluded that the composite deck will not be able to sustain any load range when Smin > 50% of the ultimate. 4.3.2 Group B The specimens for this group were further subdivided into two groups, depending on the minimum load, Smi,, and a summary of the test results is shown in Table 8 below. TABLE 8

Summary of Fatigue Tests Results for Group B

Specimen identi~:y

Group identity

Load range (%) Smi n

Sma x

Cycles to failure (Nf)

Stiffness at failure (k f) kN/m

B9-24-08 B 13-24-08 B**-24-08

I I I

4.0 4-0 4.0

45 55 65

* * 600

4880 3820 2350

Bt6-24-08 B15-24-08

II II

40 40

65 85

* 500

5500 1850

* Deck did not fail after 4.5 million cycles.


264

Specimen 9roup I: S min = 4 " 0 % . For the composite slab (B9-24-08) with Smax=45%, the behaviour under repeated load was similar to that observed for Group A specimens with Smax<45%. The composite slabs showed a work-hardening effect with a resulting increase in stiffness after a few thousand cycles and thereafter remained more or less constant, as shown in Fig. 13. Static tests after 4-5 million cycles showed that the ultimate load could still be attained. For the composite slab (B13-24-08) subjected to a maximum load, Smax = 55%, there was an observed decrease in stiffness with the number of cycles, Fig. 14. The decrease could be attributed to the deterioration in the bond between the steel deck and the concrete. The slab, however, managed to survive 4.5 million cycles with a reduction in stiffness of about 20%. For a maximum load, Smax = 65%, composite slab (B12-24-08) could not survive repeated loading. The maximum number of cycles (600) was accompanied by a shear bond failure, characterised by a crack at one of the loaded points. From these tests it was concluded that for Smin =4"0%, Smax is approximately 60% of the ultimate, if repeated loading is to be sustained. Specimen 9roup II: Smin =40%. For composite slab (B16-24-08) subjected to Smi, = 4 0 % and Smax =65%, the behaviour was similar to that of Group I specimens, with Smax <55%; whereas for a higher load range, 6000

5000

~

~

___

4000"

•

8000

Specimen

B9-24-08

2000

1000

J

0 0

1

2

L 3

i 4

Number of Cycles (N) (Millions) Fig. 13. K - N curve for Group I slab with

Sma x

=45%.

Dynamic behaviour of compositefloors

265

5000~

4800 1

Specimen B13-24-08

.... 4600H E

4400 0

4200 4000 3800 3600 3400

I

I

I

I

1 2 3 4 N u m b e r of C y c l e s (N) ( M i l l i o n s )

5

Fig. 14. K-N curve for Group II slab with Smax= 55%. Smax=85%, slab (B15-24-08) could not survive more than 500 cycles. Failure mode was a shear bond failure. Thus for Smin = 40%, the maximum load that can be applied repeatedly for a 2-point central line loading is about Smax= 7 0 % of the ultimate if fatigue failure is to be eliminated.

5 DESIGN PROPOSAL FOR BOND-LOK COMPOSITE SLABS

5.1 Design formulation 5.1.1 Group A specimens--central line loading The fatigue test results were plotted as Smax against Smi,, as shown in Fig. 15. ]From the data points, a lower bound line (design line) of Smax ----50% could be deduced, suggesting that the maximum load that can be applied in order to sustain repeated loading is 50%. 5.1.2 Group B specimens--2-point loading For this group, the plot of Smax against Smi, is shown in Fig. 16. Although having insufficient data, a suggested extrapolated design line is shown in the figure. The line is slightly above the no-failure points. The above two figures can be presented as modified G o o d m a n 4 diagrams, a,,; are shown in Figs 17-18.


266 8max(%)

100

100

80

80

60

60

40

40

20

20

0

I

o

20

i 40

i 60

L 80

0 100

Smin(%) I • Failure

+ No failure

.... *Deeign line l

J

Fig, 15. Group A, Smax VS Smi..

Smax('~) lOO

i

80

_ / ~

~' .... ~. .....

100

80

40 -

40

20

20

0

L 20

0

t 40

t B0

I 80

Smin(%) I

g

Fmllure

4- No failure

..... Deelgn line

Fig. 16. Group B, Smax VS Smi,-

100


s

s . (%) rain

267

max

(%)

., 1 0 0

100

80

8O

60

60

40

40

20

20

0

i

L

Fig. 17. Design chart for central line loading.

s (%)

s . (%)

mtlx

rain

100

lOO

80

80

60

60

40

40

20

20

0

~

~

h

~

0

Fig. 18. Design chart for 2-point line loading.

268

G. J. Krioe, J. Mahachi

5.2 Design method Analysis of load) Estimate the expected maximum number of repeated cycles (N) which the slab has to resist during its design life. Calculate the ultimate static strength (Pu) of the slab using rectangular stress block analysis. Determine the minimum and maximum design load levels for the specified loadings. For the given minimum load (Smi.) determine the maximum permissible load (Sma~)from the design chart (modified Goodman diagram) (Fig. 17 for central line loading and Fig. 18 for 2-point line loading). For a safe design, the maximum permissible load (Smax) must be greater than or equal to the maximum design load. For example, if Stain=20% then Smax=50% (Fig. 17) for central line load and Smax =65% (Fig. 18)for 2-point line load.

6 CONCLUSIONS Where Bond-lok composite slabs are subjected to repeated central line loading, as in industrial buildings, natural bond cannot be relied upon as a satisfactory form of shear connection, in particular when the load range is high. For low load range, provided that Smax< 50%, fatigue failure is not expected. For any Smi., Sinai > 50%, cracks will inevitably initiate, resulting in a loss of stiffness, increased mid-span deflections and ultimate failure of the slab. For 2-point line loading there is an improved endurance to repeated loading, as compared to the central line loading for a given minimum load.

ACKNOWLEDGEMENTS Financial support has been provided by the Foundation for Research Development (FRD) and The South African Institute of Steel Construction. Appreciation is extended to Pioneer Concrete for providing the concrete and Brownbuilt Metal Sections for the steel decks.

REFERENCES 1. Porter, M. L. & Ekberg, C. E,, Jr, Design recommendations for steel deck floor slabs, J. Struct. Div. ASCE, 102(ST11) (1976) 2121-2136.


269

2. Wyatt, 71'. A., Design Guide on the Vibration of Floors. Steel Construction Institute, CIRIA, Berkshire, 1989. 3. ACI Committee 215, Considerations for design of concrete structures subjected to fatigue loading, ACI J., 71(3) (1974) 97-121. 4. Conway, J. B. & Sjodahl, L. H., Analysis and Representation of Fatigue Data. Mar-Tesl: Inc., Cincinatti, Ohio, 1991, pp. 147-150. 5. SAISC, South African Steel Construction Handbook (Limit State Design), 2nd edn. SAISC, Johannesburg, 1992, pp. 14.50-14.53. 6. BS8110, ,Structural Use of Concrete Part I, British Standards Institution, 1985. 7. Paz, M., Structural Dynamics--Theory and Computation. Van NostrandReinhold, 2nd edn, New York, 1991, pp. 8-50.

Dynamic behaviour of composite floors

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