behavior of composite open web steel joist under …

Journal of Engineering Science and Technology Vol. 15, No. 5 (2020) 3314 - 3333 © School of Engineering, Taylor’s University

3314

BEHAVIOR OF COMPOSITE OPEN WEB STEEL JOIST UNDER IMPACT LOADING

MARWAH S. ABDULJABBAR*, WAEL S. ABDULSAHIB, MOHAMMED J. HAMOOD

Civil Engineering Department, University of Technology, Baghdad, Iraq *Corresponding Author: [email protected]

Abstract

The present study is devoted to investigate the behaviour of the composite open web steel joists with different parameters under the effect of impact loading. The experimental work involves testing three composite joists to investigate the influence of drop weight, and span-to-depth ratio under impact loading. Four impactor heights were considered for each specimen. The steel joists were 3000 mm span, and the composite concrete slab was 400 mm wide, and 90 mm overall depth overlaid on a profiled steel sheet. The results are presented in terms of recorded time histories of impact and reaction loads, mid-span and quarter-span deflections, and crack pattern of the composite open web joist. Based on the experimental results, the deflection and cracks were found more pronounced when the span-to-depth ratio of the steel joist was decreased. On the other hand, decreasing the drop weight to the half resulting in reduced the maximum deflection, and the cracks were limited at the impact zone.

Keywords: Composite open web steel joist, Composite structures, Drop weight impact test, Headed shear studs, Low velocity impact loading.

Behavior of Composite Open Web Steel Joist under Impact Loading . . . . 3315

Journal of Engineering Science and Technology October 2020, Vol. 15(5)

1. Introduction The composite open web steel joist system consists of a topped concrete slab poured on a profiled steel sheet, supported over open web steel joist. The concrete slab is linked to the steel joist by means of shear connectors to increase the ability of the floor system to span long distance, and reduce deflection [1]. Many structures, in the last decades, experienced an increased risk of potential attacks worldwide. This fact makes a public concern to the researchers and designers to understand the behaviour of structures against any threating activities or sudden loading, such as blast or impact load, to improve their safely [2, 3]. Impact loading can results in cracks propagating through the structural elements with a serious consequence [4]. In addition to impact loading resulted from an explosion, impact of free fallen objects, or simply due to human vibration, attracted attention of researchers to study the resistance of different types of structure members to ensure their safety under any accidental load [5, 6]. Therefore, most experimental studies concentrated their efforts to understand the behaviour of RC beams and slabs under impact load [7-9], however, experimental studies carried out in this area for composite open web steel joists are few studied and little information was collected. For example, a study carried out by Avci et al. [10] to excite a single- and three- span footbridges by four types of excitations, chirp, sinusoidal, heel drop and walking. This paper focused on examine the effect of bottom chord continuity on the deflection and vibration characteristics of the floor system. The results showed that removing and re-installing these extensions decreased the frequency response function of the footbridges.

However, an intensive study is necessary to attain a better understanding of the impact behaviour of the composite open web steel joist. Consequently, the objective of this paper was to present in details a well-instrumented experimental program to properly investigate the impact response of composite open web steel joists in terms of impact force, reaction force, mid-span and quarter span deflections, and crack pattern. It is worth to mention that this paper is part of a research project of testing composite open web steel joists under drop weight impact machine to provide a basis for future research in this field. The parameters of this paper include the span-to-depth ratio of the composite open web joist, and also include effects of impact mass and impact height on the behaviour of such structures.

2. Experimental Work

2.1. Fabrication of the test specimens The guidelines of the Standard Specification for Composite Steel Joists [11] was used to design the composite open web steel joist specimens. The specification recommended that the span/depth of the steel joist is between 12-30, and the steel top chord should have an area approximately 50 - 70% of bottom chord area.

The steel used in the manufacture of the chords and web sections in this experiment had slenderness ratios conform to the limitation provided by the specification as follows:

For top chord interior panels: 𝑙𝑙𝑟𝑟 = 27.05 < 90

For top chord end panels: 𝑙𝑙𝑟𝑟 = 27.05 < 120

3316 M. S. Abduljabbar et al.


For bottom chords all panels: 𝑙𝑙𝑟𝑟 = 17.83 < 240

For web members (in tension): 𝑙𝑙𝑟𝑟 = 65.28 < 240

For web members (in compression): 𝑙𝑙𝑟𝑟 = 65.28 < 200

Also, the specification recommended that for the shear studs shall have a minimum of 13 mm concrete cover with slab thickness not less than 51 mm.

2.2. Description of test specimens A normal concrete mix with weight proportion of 1 Portland cement: 1.2 fine aggregate: 1.8 coarse aggregate, with 0.3 tap water and 0.8% superplasticizer has been used in the preparation of the concrete slab of the composite open web steel joists. The cement was complied with the of the Iraqi Specification No.5/1984 [12], and the aggregates were agreed with the Iraqi Specification No.45/ 1984 [13]. The compressive strength of the hardened concrete has been tested using standard concrete cylinders of size of 150 mm diameter and 300 mm length according to ASTM C39/C39M - 14 [14]. The typical dimensions for the concrete slab are 400 mm width with slab thickness of 90 mm.

Three composite open web joists have been constructed with 3 m span and two span-to-depth ratios, 13.5 and 15.5, as illustrated in Table 1. The specimen denoted by (N13.5ROI) corresponds to normal concrete slab followed by span-to-depth ratio, web members type, over-connected shear connection degree, and impact mass of 50 Kg. While the composite open web joist with (II) notation indicates that the impact mass was 25 kg.

The top chords of the steel joists were fabricated using back-to-back double angles of 2L 50×50×5 mm with yield strength of 371.66 MPa, and 2L 76×76×5 mm with yield strength of 326.66 MPa for the bottom chords.

The typical schematic drawing of the composite joist specimens is presented in Fig. 1. The composite slab was reinforced at mid height by welded wire fabric of 6 mm bars with square spacing of 150mm c/c in both directions. A total of 28 double rows shear connectors with 16 mm diameter and 75 mm length, after welding, were placed in the strong position for all test specimens. The shear connectors were welded through circular openings made in advance in the profiled steel sheet on the steel top chord. The number of shear connectors designed for all specimens was exceed the tensile force of the bottom chord to ensure ductile failure. Plywood formwork with the welded wire mesh were placed before pouring the aforementioned concrete mix. Then, the specimens were cured with wet burlap for 28 days, as shown in Fig. 2.

Table 1. Summary of test specimens’ details. Specimen Span

Length (m)

Steel Joist Depth (mm)

Concrete Strength (MPa)

Impact Mass (kg)

N13.5ROI 3 222 41.80 50 N15.5ROI 3 193 43.40 50 N13.5ROII 3 222 43.59 25



(a) Composite joist layout.

(b) Composite joist cross-section.

Fig. 1. Typical specimen's details.

Fig. 2. Casting and curing of the composite slab.



2.3. Impact loading test setup and procedure The impact test was conducted by using a drop weight machine. The main parts of the impact test-rig are:

1. A steel frame of 3250 mm total height was used to freely drop the masses onto midspan of the specimens.

2. A vertical tube guide was used for the falling mass to ensure midspan impact with diameter of 145 mm.

3. Two drop masses of 50 kg and 25 kg, including the impact load cell, were used in this research study. The selection of these masses was expected to fall in a storage, for example. The striking tip of the masses had a hemispherical shape with a radius of 50 mm.

4. The weight of 50 kg was dropped on two specimens (N13.5ROI and N15.5ROI), while the weight of 25 kg was dropped on the specimen (N13.5ROII) only.

5. For all samples, the drop masses were repeatedly dropped to the same specimens onto midspan from four heights: 500 mm, 1000 mm, 1500 mm, and 1700 mm.

A load cell with capacity of 200 kN was installed to the drop weight to measure the impact force, while another load cell with capacity of 100 kN was placed beneath one end of the composite joist seat to measure the reaction force, as shown in Fig. 3(A and B). The midspan and quarter span deflections were recorded using a laser displacement sensor and linear variable differential transformer (LVDT), respectively. The strains of the concrete slab, top chord, and bottom chord were monitored through three stain gauges mounted at the midspan of the composite joist. In addition, a velocity sensor and an acceleration sensor were fixed to the drop weight and the steel open web joist, respectively. National Instruments PXIe-1062Q data acquisition system with data rate of 204 kilo sample/second controlled by a computer was used to record the data, see Fig. 3(C). Lab VIEW 2018 was used to save results from the present tests.

The vertical movement was restrained by specially designed simply supports, whereas rotation was permitted through using round steel bar underneath the steel plate. Locations of the instrumentation used in this test and arrangements of the supports are presented in Figs. 4 and 5, respectively.

Fig. 3. (A) Reaction load cell with capacity of (100 kN), (B) Impact load cell with capacity of (200 kN), (C) Data acquisition system.



Fig. 4. Impact loading test setup.

Fig. 5. Support arrangement used for the impact test.

3. Results and Discussion This section presents the impact test results in terms of time histories of impact force, reaction load, midspan deflection, quarter-span deflection, and crack pattern of the three composite open web steel joists.

3.1. Crack pattern and overall failure mode The cracks developed after each strike was marked and the crater depth and diameter were measured manually. Figure 6 shows the overall crack patterns for all specimens, and the crack pattern at each drop height. For specimen (N13.5ROI), no damage was observed at a drop height of 0.5 m, while a longitudinal and transverse crack were extended from the impact zone at 1.5 m drop height. At 1.7 drop height, the cracks were widened and extended significantly. No transverse cracks were marked at the shear span, but the delamination between the concrete slab and the profiled sheet was observed along the specimen after the fourth impact.



For specimen (N15.5ROI), four hairline cracks were observed at 0.5 m drop height, then with the further of increment of the drop height to 1.7 m, new cracks were formed under the impact point. The delamination was noted extensively in one side of this specimen, and the transverse cracks were spreading along the longitudinal axis of the concrete slab more as compared to previous specimen. For specimen (N13.5ROII), one hairline crack was observed at 0.5 m drop height, then with the increase in the drop height, the cracks were further formed at impact loading point. No visible delamination or transverse cracks were observed along the concrete slab, except within the midspan of the specimen.

(a) (N13.5ROI) Composite joist.

(b) (N15.5ROI) Composite joist.



(c) (N13.5ROII) Composite joist.

Fig. 6. The overall crack pattern and crack propagation at each drop height for the tested specimens.

3.2. Deflection-time histories As previously mentioned, vertical deflection at the midspan and quarter-span were measured with a laser displacement sensor, and LVDT sensor for each drop height, respectively. The maximum deflections at these locations are summarized in Table 2. It can be noticed that as the ratio of span-to-depth of the steel joist decreased from 13.5 to 15.5, the increase in deflection reach about 109 % at the fourth strike, whereas when the weight changed from 50kg to 25kg, the deflection decrease about 55% at the same strike. The response of deflections of the composite joists are shown in Fig. 7. It can be seen that decrease in the composite joist’s stiffness at first and second strikes causes increase in deflection at the third and fourth strikes for all specimens.

Table 2. Maximum mid-span and quadrant deflections for each drop height.

Specimen Drop height (m)

Maximum deflection at the midspan

(mm)

Maximum deflection at the quadrant span

(mm)

N13.5ROI

0.5 8.26 1.55 1 8.82 1.98

1.5 11.71 2.74 1.7 12.86 2.84

N15.5ROI

0.5 13.43 0.88 1 17.62 1.63

1.5 24.05 2.21 1.7 26.97 3.26

N13.5ROII

0.5 2.66 0.50 1 3.62 0.92

1.5 4.69 1.12 1.7 5.73 1.23



Fig. 7. Time histories of mid-span and quadrant deflections.

3.3. Load-time histories Two specimens, (N13.5ROI) and (N15.5ROI), were tested by dropping a 50 kg weight, while 25 kg was dropped on specimen (N13.5ROII) only. All the composite joists were impacted from four different heights at the midspan. Figure 8 shows the time histories of the impact and reaction forces of the tested composite joists during the impact process. From this figure, it can be noted that the impact and reaction responses of all composite joists have a similar pattern. At the beginning of impact process, the impact and reaction force of specimens started with a primary wave, followed by many cycles before they dampen. Table 3 presents the impact and reaction forces for each drop height. It can be observed that the maximum impact force decreased with the decrease in span-to-depth ratio by a percentage more than 3 %. This may be attribute to the composite joist stiffness which decreased as the span/depth ratio decreased. For specimen (N13.5ROII), the percentage of decreasing of impact and reaction forces is more than twice for all drop heights when it compared with the control specimen (N13.5ROI). This difference in forces between the composite joists is compatible with the smaller mass of the falling weight, i.e. decreasing the drop weight decreases the impact and reaction forces.



Fig. 8. Time histories of impact and reaction forces.

Table 3. Maximum impact and reaction forces for each drop height.

3.4. Strains It was mentioned earlier that three strain gauges mounted on the top and bottom chords, and the composite concrete slab. Figure 9 shows the strain- time histories of all specimens for each drop height. In general, the strain measurements of the bottom chords increased with increasing drop heights, but lower than the yield strain of the steel material for all specimens. Also, for top chords strain measurements, the compression peak strain value of specimen (N15.5ROI) at the 1.7 m drop height was about 20% higher than that of specimen (N13.5ROI). While specimen (N13.5ROII) showed compression strain values at first and second drop heights, then the strains

Specimen Drop height (m)

Impact force, Pb (kN)

Reaction force, R (kN)

N13.5ROI

0.5 10.89 4.77 1 17.44 7.16

1.5 20.47 9.03 1.7 21.04 9.54

N15.5ROI

0.5 9.50 4.55 1 15.73 7.81

1.5 18.84 9.33 1.7 20.55 9.97

N13.5ROII

0.5 5.07 2.38 1 6.36 3.12

1.5 8.62 4.29 1.7 9.29 4.58



transformed to tensile after the third and fourth drop heights. Also, it can be observed from the same figure that the compression strain values in the composite concrete slabs increased with drop heights for all specimens, except for the reference specimen (N13.5ROI), in which the excessive cracks at the impact region had significantly reduced the strains after the third drop height.



Fig. 9. Time histories of strains of the tested specimens.



4. Analysis of the Impact Results

4.1. Impact and inertial forces It is well established that in the early stage of impact event, the impact force is resisted by the specimen in the form of inertia forces to maintain balance, in addition to support reactions. The impact force acts as a point load in the midspan of the specimen, while the inertial force is equal to the magnitude of the mass times acceleration distributed throughout the body of the specimen.

In addition to the results obtained experimentally, the peak impact force may be estimated by calculation using a theory of mechanical energy conservation, as follows [15]:

1- The static midspan deflection can by calculated as

δst=(𝑚𝑚 𝑔𝑔) 𝐿𝐿3

48 𝐸𝐸𝐸𝐸 (1)

2- The impact factor (n) is

n=1+ �1 + 2 ℎ𝛿𝛿 𝑠𝑠𝑠𝑠

(2)

3- Then, the maximum impact force at the midspan of the specimen is obtained by

Pmax=n(mg) (3)

where δ st is the static vertical displacement of specimen; m is the impact mass; g is the acceleration due to gravity; L is the specimen span; E is the elasticity modulus; I is the cross section inertia of the specimen; h is the drop height.

Also, Bentur et al. [16] presented an equation to find the total impact force at any time t:

Pt (t) = Pb (t) + Pi (t) (4)

where Pt (t) is the total impact force at time t; Pb (t) is the true bending load at the midspan of the specimen at time t; and Pi (t) is the inertial force at the midspan of the specimen at time t, which equivalent to the distributed inertial force.

Therefore, in order to find the true bending load on the specimen, the inertial force should be subtracted from the total impact force [17-19]. In this experiment, the true bending load was obtained directly from the test through using the support reactions [19]. Also, since one load cell was mounted beneath the support, and steel yokes were used at the location of the support to make sure that the condition of simply supported specimen is valid, therefore, the true bending load can be obtained as follows:

Pb(t)=2R(t) (5)

Equation (4) can be rewritten as [19]:

Pi(t)=Pt(t)- Pb (t) (6)

Hence, the inertial force at any time t can simplified further as

Pi(t)=Pt(t) - 2 R(t) (7)

Table 4 presents a comparison of calculated results with experimental data for the maximum impact force, Eq. (3), and a computed inertial force for each drop heights by Eq. (7) at that peak impact force. The calculated peak impact force showed less than



half the experimental ones as can be noticed from the comparison. Also, as can be seen from the table that the contribution of the inertial force used to accelerate the specimens from their rest positions are very small at the time of the peak impact force, while the true bending load increased with increasing drop height. This means that the impact force at the midspan produced stress waves reached the supports within the impact duration, and the overall specimen was accelerated. This conclusion matches the experimental acceleration response well, which was mounted at 1000 mm away from the impact point. The accelerometer recorded a peak value approximately at the same duration when the impact force reached its peak value, as shown in Fig. 10.

(a) (N13.5ROI) composite joist.

(b) (N15.5ROI) composite joist.

(c) (N13.5ROII) Composite joist.

Fig. 10. Acceleration time histories of the tested specimens.



Table 4. Calculated impact load and inertia forces for each drop height.

Specimen Drop height

(m)

Calculated Impact Force (kN)

Inertial force, Pi

(kN)

𝑪𝑪𝑪𝑪𝑪𝑪. 𝑰𝑰𝑰𝑰𝑰𝑰𝑪𝑪𝑰𝑰𝑰𝑰 𝑪𝑪𝒍𝒍𝑪𝑪𝒍𝒍 𝑬𝑬𝑬𝑬𝑰𝑰. 𝑰𝑰𝑰𝑰𝑰𝑰𝑪𝑪𝑰𝑰𝑰𝑰 𝑪𝑪𝒍𝒍𝑪𝑪𝒍𝒍

% 𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑪𝑪𝑪𝑪 𝒇𝒇𝒍𝒍𝑰𝑰𝑰𝑰𝑰𝑰 𝑬𝑬𝑬𝑬𝑰𝑰. 𝑰𝑰𝑰𝑰𝑰𝑰𝑪𝑪𝑰𝑰𝑰𝑰 𝑪𝑪𝒍𝒍𝑪𝑪𝒍𝒍

%

N13.5ROI

0.5 3.40 1.35 31 12 1 4.57 3.12 26 18

1.5 5.48 2.41 27 12 1.7 5.80 1.96 28 9

N15.5ROI

0.5 3.15 0.4 33 4 1 4.23 0.11 27 1

1.5 5.05 0.18 27 1 1.7 5.34 0.61 26 3

N13.5ROII

0.5 2.29 0.31 45 6 1 3.12 0.12 49 2

1.5 3.76 0.04 44 0 1.7 3.99 0.13 43 1

4.2. Impact energy Since the impact energy depends on the mass and velocity of the drop weight, therefore, in this paper, each specimen impacted by the same drop mass with different heights, subsequently velocities, to study the effect of the impact velocities on the behaviour of the composite open web steel joist. The impact energy can be calculated from the common formulae of the kinetic energy (K. E =0.5 m ʋ2). The impact energy imparted to the specimens during multiple impacts can be summarized in Table 5. It can be noticed that the specimen (N13.5ROI) suffered from cracks extended adjacent to the impact zone after impacted from the fourth drop height (1.7 m), this leads to say that the impact energy of this specimen is 789.61 J. While the impact energy for specimen (N15.5ROM), which had approximately identical cracks profile after the same drop height, is 761.76 J. For specimen (N13.5ROII), the impact energy of this specimen is 60% lower of that of specimen (N13.5ROI).

Also, Table 5 shows the dynamic responses in terms of momentums of the specimens. It can be noticed that the amount of momentum increased with the increasing drop height with constant mass.

Table 5. Impact energies and momentums for each drop heights.

Specimen Drop height

(m)

Drop weight (Kg)

Impact Velocity (m/sec)

Impact Energy

(J)

Momentum (kg. m/s)

N13.5ROI

0.5 50 2.40 144 120 1 50 3.42 292.41 171

1.5 50 5.22 681.21 261 1.7 50 5.62 789.61 281

N15.5ROI

0.5 50 2.05 105.06 103 1 50 3.24 262.44 162

1.5 50 5.12 655.36 256 1.7 50 5.52 761.76 276

N13.5ROII

0.5 25 3.14 123.25 79 1 25 4.31 232.20 108

1.5 25 5.78 417.61 145 1.7 25 6.31 497.70 158



5. Conclusions The dynamic response of composite open web steel joist under impact loading using the drop weight test was investigated. Parameters of span-to-depth ratio drop height and weight variation were chosen. Impact and reaction load versus time, midspan and quadrant deflections versus time behaviour of the composite joists were studied. Based on the experimental investigations conducted the following conclusions are marked.

• The lower steel joist span-to-depth ratio exhibited superior damage under impact loading when compared to the higher ratio, and the deflection increased more than twice for the fourth height.

• Decreasing the drop mass resulting in reducing the maximum deflection to more than half for the final impact height, and the cracks are observed at the impacted area only when compared to the control composite open web joist.

• Increasing drop height resulting in increased the maximum impact, reaction forces and deflections for all specimens. A worth of mention, the last two strikes has more significant increase in deflection and crack propagation than the first two strikes. The reason may be return to decrease in the composite joist stiffness as the drop height increased.

• It is found that, with same impact mass but increasing the impact velocity, the inertia load may account less than 20 % of the total impact force for all specimens.

• Estimations of peak impact forces did not agree well with those experimentally measured. • The lower span/depth ratio results in lower impact energy of the specimen more

than 3% and using lower drop mass caused significant decreasing in the impact energy more than 60 % as compared to the reference specimen.

However, conclusions about the impact behaviour of the composite open web steel joist, especially stress wave propagation throughout the composite system still require to be verified by further experimental and theoretical studies.

Nomenclatures E The elasticity modulus, MPa g Acceleration due to gravity, which equal to 9.81 m/s2 h The drop height, m I The cross-section inertia of the specimen, ms K.E Kinetic energy, J L The specimen span, m m Impact mass, kg Pi(t) The inertial force at the midspan of the specimen at time t, kN Pb Impact force, kN. Pb(t) The true bending load at the midspan of the specimen at time t, kN Pt(t) The total impact force at time t, kN R Reaction force, kN Greek Symbols δ st Static vertical displacement of specimen, m



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