Numerical Studies on Corroded Steel angle members

Journal of Structural EnginEEring 197 Vol. 43, no. 2, JunE - July 2016

Journal of Structural EngineeringVol. 43, no. 2, June - July 2016 pp. 197-205 No. 43-20

Numerical studies on corroded steel angle membersr. Vikraman*,, a. cinitha* and P.K. umesha*

Email: [email protected]

*cSir-Structural Engineering research centre, cSir-campus, taramani, chennai - 600 113, india.

received: ; accepted:

Keywords: Accelerated corrosion; carbon fiber reinforced polymer; compression strength; retrofitting.

Steel structures may have to be retrofitted due to various reasons. one of the major problems facing steel structures is corrosion, which effectively reduces the gross cross sectional area of steel members thus leading to higher stresses in the corroded area. reduction of member thickness results in the reduction of cross section properties thus affecting the buckling capacity of the members. Carbon fiber reinforced polymer (cfrP) is considered as suitable for structures in corrosive environment due to its high-strength, light-weight, and anti-corrosive qualities. in recent years, a continuous increase has been experienced in using carbon fiber reinforced polymer (CFRP) for structural strengthening as well as repair works. in this paper, residual capacity of the corroded members is evaluated by classifying the section according to the level of corrosion. a damage model was proposed by Kayser and nowak (1989) which evaluated the reliability of a corroded steel girder bridge over time. another theory called ‘interval probability theory’ was proposed by Sarveswaran (1999) for assessing the reliability of corrosion-damaged steel structures. using this theory,

the remaining thickness of a severely corroded element was represented by an interval number, which expressed the range over which there was uncertainty about the thickness. later, a methodology was developed by Hathout (2004) for assessing the reliability of existing transmission structures and lines in the presence of structural deterioration. Enhance the simplified method to calculate the residual capacity of corroded members according to the average residual thickness is evaluated. Due to corrosion, the capacity of the members reduces. the amount of reduction varies w.r.t the level and location of corrosion. Beaulieu et al. (2010) reported that, the corrosion results in significant changes in sectional area, causing major changes to the compressive capacity of the steel angle members. Studies on steel angle and tubular sections by cinitha et al. (2014) point out that apart from imperfections corrosion results in changes in geometric properties such as cross sectional area, moment of inertia and slenderness ratio. for same percentage of corrosion with different locations it is found that ultimate load carrying capacity reduced by 1.5%-37.91%. for corroded hollow tubular structural

steel structures corrode when exposed to the corrosive environment such as marine, industrial and humid and hence the strength of steel structural members reduces. steel angle sections are widely used in transmission line micro wave towers and their performance is effected as they corrode. This paper presents the numerical studies on uncorroded, corroded and retrofitted angle specimens considered as compression members and are compared with experimental results. The cross sections considered are IsA 70 × 70 × 5mm, 75 × 75 × 5mm and 100 × 100 × 6mm. Numerical studies were also conducted by using ABAQUS finite element (FE) based software and the results of the FE models are validated with that of the experimental studies and presented.

198 Journal of Structural EnginEEring Vol. 43, no. 2, JunE - July 2016

members with varying percentage of corrosion (20%-40%), it is found that ultimate load carrying capacity reduced by 10%.-25%. and aparna et al. (2014) studied experimental investigation of compressive strength of uniformly corroded steel angle members retrofitted with cfrP and concluded that external bonding of cfrP is a promising alternative strengthening technique for steel structures. this paper studies numerical investigation on corroded and retrofitted steel angle sections, corroded by galvanostatic method and retrofitted using CFRP composites with epoxy adhesives. the specimens were then subjected to compression testing in order to understand the strength performance and then numerical investigations were carried out and discussed.

Corrosion is defined as the physical interaction between a metal and its environment which results in changes of the metal’s properties and which may lead to significant functional impairment of the metal, the environment or the technical system of which they form a part. this interaction is often of an electrochemical nature. among the various forms of corrosion, present study focus on uniform or general corrosion. this is a surface phenomenon, which occur through uniform attack of metal resulting from the contact with certain strongly acidic or alkaline electrolytes as well as conditions of high humidity or moisture-laden atmosphere. this is the most common form of the corrosion, which will lead to the gradual thinning of members, accordingly for the greatest destruction of metal. as it occurs evenly over the entire surface, the rate of corrosion is often presented as a weight loss. uniform corrosion is very predictable, and is the basis of most corrosion prediction equations. also it has been pointed out that this type of corrosion is the most

serious form of corrosion observed on steel structures.

StrUctUrAl BEhAvioUr oF corrodEd, uNcorroded members

in order to understand the behavior of corroded and retrofitted angle compression members, experimental and numerical studies were conducted. numerical results were validated with experiments. totally nine angle specimens were considered, with three sets. Each set contains 1) angle uncorroded (auc), 2) angle corroded (AC) and 3) Angle corroded and retrofitted (arc). galvanostatic method with 3.5% nacl is adopted as electrolyte to achieve the desired amount of corrosion. The corroded specimens are retrofitted with one layer of cfrP strip by using araldite aW106 resin and HV 953u epoxy as hardener with a weight ratio of 10:8. the manufactures have provided the properties of cfrP as modulus of elasticity; 295600n/mm2, tensile strength; 378.2n/mm2; Density; 1.69E-9ton/mm3 and Poisson’s ratio as 0.33.

after achieving the desired amount of corrosion, the angle specimen to be retrofitted was properly grounded and cleaned with acetone in order to have a proper binding with cfrP. the resin and the hardener were then mixed in proportion and applied with a spatula to the pretreated surfaces. immediately after applying the resin-hardener mix to the surface with a painting brush, cfrP was properly pasted to the corroded portion of the specimen. after pasting cfrP to the corroded surface, one more coat of the resin-hardener mix was applied over the cfrP in order to have proper binding and to protect the fibers from getting damaged. The specimen details are shown in fig 1. the thickness and mass loss observed after corrosion is given in table 1.

taBlE 1tHicKnESS anD WEigHt mEaSurEmEntS of iSa 100 × 100 × 6 SPEcimEnS

iD Size of the specimen (mm)

length (mm) il1 (mm) il2 (mm) fl1 (mm) fl 2 (mm) initial weight (kg)

final weight (kg)

auc- 100 100 × 100 × 6 1000 6.272 6.767 - - 16.55 16.55ac - 100 100 × 100 × 6 1000 6.456 6.334 5.59 5.27 16.49 15.87

arc - 100100 × 100 × 6 (c) 1000 6.6 6.54 4.997 5.494 16.94 -100 × 100 × 6 (r) - - - 8.22 8.165 - 16.61

*AUC-Angle Un corroded, AC-Angle Corroded, ARC-Angle Retrofitted after Corrosionil1-initial thickness of leg1, il2-initial thickness of leg2FL1-Final thickness for corroded/retrofitted leg1, FL2- Final thickness for corroded/retrofitted leg2.


The details of materials required for retrofitting are as follows

i) carbon fiber reinforced polymer (cFrP)

Carbon fiber is defined as a fiber containing at least 92% carbon by weight. Carbon fibers generally have excellent tensile properties, low densities, high thermal and chemical stabilities in the absence of oxidizing agents, good thermal and electrical conductivities, excellent creep resistance.

ii) Adhesive

in the present study, the adhesive used for binding cfrP to steel angle section was araldite aW 106 resin/Hardener HV 953u epoxy adhesive. it is a multi-purpose, viscous material that is suitable for bonding a variety of materials including metal, ceramic, and wood. adhesive and hardener are having different specific gravity and viscous properties.

ExPErimEntAl SEt-UP

in the present study compression tests were conducted under force control method using 500kn hydraulic jack in order to develop a stable post-buckling behaviour,

ID

AUC - 70 × 51000

1000

1200

345

Corroded region

Corroded and Retrofitted region



345

Corroded region

445

Corroded region

445

345

Corroded region

Corroded region

345

Corroded region

7075

100

55

6AC - 70 × 5

ARC - 70 × 5

AUC - 75 × 5

AC - 75 × 5

ARC - 75 × 5

AUC - 100 × 8

AC - 100 × 6

ARC - 100 × 6

LOCATION OF CORROSION LOCATION OF RETROFIT

Fig. 1 Details of the corroded, uncorroded and retrofitted angle specimens


as shown in fig. 2, the displacement in axial in–plane buckling and out-of–plane buckling directions were measured during the tests to observe the nonlinear behaviour of the angle members. the axial and lateral direction displacements were measured using linear variable differential transducer (lVDt). the strain was measured by using strain gauge the strain gauge and lVDt were placed at one third of the overall length from the bottom flange i.e (corroded region).

fig. 2 Experimental set up

Numerical studies

the steel angle members considered for the present study are intermediate columns with yield strength of Fy (yield stress) as 360 n/mm2, Fu (ultimate stress) as 466 n/mm2, E (young’s modulus) as 210000n/mm2, density as 7850 kg/ m3. the behavior of corroded angle members under compression are numerically modelled with ABAQUS finite element software. This study is aimed to simulate compression behavior of uniformly corroded and retrofitted angle specimens. uniform corrosion is modelled by thickness reduction and CFRP retrofitted behavior is simulated with

Simpson integration method. the solid element used is c3D8r. it is an eight nodded linear hexahedral brick element and is used for modelling the relatively small leg thickness of the angle specimen compared to the other dimensions which results in local buckling when subjected to axial compressive load. a kinematic coupling restraint is defined to constrain the motion of the top flange plate to the regions below where all the translational and rotational degrees of freedom are specified. Reference point is then established in the top flange plate which passes through the centroid of the section under consideration. Boundary conditions are

taBlE 2DEtailS of iSa 70 × 70 × 5 anD iSa 75 × 75 × 5 SPEcimEnS BEforE anD aftEr corroSion

iD Size of the specimen (mm) length(mm) il1 (mm) il2 (mm) fl1 (mm) fl 2 (mm) initial weight

(kg)final weight

(kg)auc- 70 70 × 70 × 5 1000 5.05 5.06 - - 13.91 13.91ac- 70 70 × 70 × 5 1000 5.071 5.099 4.294 4.406 13.55 12.05

arc- 7070 × 70 × 5 (c) 1000 5.007 5.034 4.648 4.572 13.51 -70 × 70 × 5 (r) - - - 8.405 8.51 - 13.31

auc- 75 75 × 75 × 5 1200 5.201 5.171 - - 15.65 15.65ac- 75 75 × 75 × 5 1200 5.277 5.254 4.091 4.152 16.51 15.75

arc- 7575 × 75 × 5 (c) 1200 5.276 5.331 4.44 4.537 16.56 -75 × 75 × 5 (r) - - - 6.895 6.836 - 16.26

(a) Boundary condition

(b) Meshed part

(c) Simulation of uniform corrosion effect

fig. 3 numerical model


then given with all translation degrees of freedom at top surface nodes except the vertical displacement as fixed and all degrees of freedom restrained at bottom. figure 3 shows various stages of numerical model. then axial loading condition is simulated by applying load to the reference point. the Static, riKS approach is used to solve nonlinear problems. in this

method, load magnitude is considered as an additional unknown and thus loads and displacements are solved simultaneously.

the results of interest are the current displacements and the loads which may be referred to a load proportionality factor.

the ultimate load is obtained by multiplying the load given with the load proportionality factor.

The retrofitted elements are modelled in three stages, initially the uncorroded region is modelled, followed by corroded region. in the second stage the corroded and uncorroded regions are assembled with tie constraint. In the third stage, the CFRP retrofit is separately modelled as a shell element (S4R) and fixed to the region of corrosion with a tie constraint. the wrapping of cfrP to the corroded surface is achieved by treating corroded surface as master and cfrP shell surface as slave and the regions are tied together with

ARC - 100X6 AC - 100X6 AUC - 100X6

Fig. 4 Comparison of numerical and experimental modes failure for the retrofitted, corroded and uncorroded specimens

400.00

AUC 100 EXP

AC 100 NUM

AUC 100 NUM

ARC 100 EXP

AC 100 EXP

ARC 100 NUM

Axi

al lo

ad (k

N) 300.00

200.00

100.00

0.000 1 2

Axial displacement (mm)3 4 5

fig. 5 Validation of numerical results with experiments


tie constraint. then after same procedure as corroded specimen is followed to simulate the compression behavior. apart from numerical simulation, the iS:800-2007, iS:802 and aScE10-97 and rankine’s gordan method were used to calculate the theoretical capacity of uncorroded members and results were compared.

taBlE 3loaD BEaring caPacity of corroDED anD

uncorroDED SPEcimEnS

S.noiS: 800-

2007rankien’s method

iS;802 & aScE

10-97

nu-merical method

Experi-mental method

Pd (kn)auc - 70 165.65 169.241 160.732 168.397 172.153ac - 70 - - - 126.142 123.270

arc - 70 - - - 158.038 159.750auc - 75 170.18 172.794 162.494 186.913 187.759ac - 75 - - - 117.709 111.839

arc - 75 - - - 180.981 184.564auc -

100 320.788 322.202 276.297 338.040 327.470

ac - 100 - - - 204.150 203.500arc -

100 - - - 244.980 242.140

Validation of Numerical results

the numerical and experimental studies were carried out in order to assess the strength and behavior of corroded steel angle members. in this work, steel angle sections were uniformly corroded and retrofitted with CFRP. In order to study the behavior of uniform corrosion on the angle members, six corroded members were taken, out of which three was retrofitted with CFRP. The ultimate strength as well as deflection for various weight loss percentages were studied. Where corrosion was modelled for a height of one-third from the bottom. they were modelled using aBaQuS and the results were validated with experiments. a comparison between the analytical, numerical and experimental values of the load carrying capacities of the uncorroded, corroded and retrofitted specimens were tabulated as shown in Table 3. from the analysis, it was observed that the mode of failure in corroded members was due to local buckling. it was because of higher compressive stresses caused due to the reduction in the cross sectional area of the effected corroded region. comparison of numerical and experimental failure modes were observed typically for the retrofitted, corroded and uncorroded specimens as shown in fig. 4. Both numerical and experimental results match well for uncorroded, corroded, corroded

ARC - 70X5 AC - 70X5 AUC - 70X5

ARC - 75X5 AC - 75X5 AUC - 75X5

fig. 6 modes of failure observed for iSa 70 × 70 × 5mm, 75 × 75 × 5 mm sections


and retrofitted studies and are shown in Fig. 5 the study has been further extended on iSa 70 × 70

× 5mm, 75 × 75 × 5mm sections. the specimen details, thickness and mass loss observed after corrosion are given in table 2. the observed modes of failure were shown in fig. 6.

70×5 AUC

75×5 ARC

70×5 AC

100×6 AUC

70×5 ARC

100×6 AC

75×5 AUC

100×6 ARC

75×5 AC

Axi

al lo

ad (k

N)

0 1

350300250200150100500

2Axial displacement (mm)

3 4 5 6 7

fig. 7 load vs axial Displacement behavior curves

the observed modes of failure were shown in figs. 6-7. it was observed from load vs axial displacement graph that, as the percentage of corrosion increases, the ultimate capacity of the members decreases.

coNclusIoNs

this paper presents numerical studies carried out on uncorroded, corroded, corroded and retrofitted steel angle sections as compression members. totally nine compression members were considered , out of which six were corroded to a specified level corresponding to weight loss. the capacity of corroded members were observed between 20% to 40% of their uncorroded capacity. the reduction in capacity of many uncorroded members were predicted using iS: 800-2007 and rankine gordan formula, which gave a closer value compared to the experimental and numerical results. the numerical, analytical and experimental study confirms that there is a drastic reduction in the load carrying capacity of the corroded member compared to the uncorroded specimens. from the compression test carried out on the specimens, it was concluded that corrosion has a major impact on the failure mode of the member. for the uncorroded members, buckling was observed at mid height whereas in the case of corroded members, the critical region of failure shifted towards the location of minimum thickness region. out of the six corroded specimens, three was retrofitted with cfrP and tested under compression. from the study it was found that 15% to 35% improvement in strength of

the retrofitted specimens with CFRP compared to the corroded specimen. thus external bonding of cfrP has been clearly established as a promising alternative strengthening technique for steel structures.

AcKNowledgemeNT

the authors thank the staff of the Structural testing laboratory and advanced material laboratory for the co-operation and suggestions provided during the investigations. this paper is published with the permission of Director, cSir-Structural Engineering research centre, chennai, india

rEFErEncES

1. aparna Ben, Vikraman.r, cinitha.a, umesha.P.K, Eapen Sakaria, compressive Strength of uniformly corroded Steel angle members Retrofitted with CFRP, International Journal of Emerging technology and advanced Engineering - iSSn 2250-2459, Vol. 4, issue 8, august 2014.

2. cinitha. a, umesha.P. K, and nagesh r. iyer, an overview of corrosion and Experimental Studies on corroded mild Steel compression members, KSCE Jl. of Civ. Engg., Vol. 18, (6), 2014, pp 1735–1744.

3. Beaulieu. l.V, legeron f and langlois S, ―Compression strength of corroded steel angle members, Jl. of Construct. Steel Res., Vol.66, 2010, pp 1366–1373.

4. Sarveswaran V, Smith J.W, ―Structural assessment of corrosion damaged steel beams using minimum capacity curves, Jl. of the Struct. Engr., Vol. 77, no. 14, 1999, pp 17–23.

5. Jack.r.Kayser. andrzej. S. nowak, capacity loss due to corrosion in steel- girder Bridge, Jl. of Struct. Engg., Vol. 115, no. 6, 1989, pp 1525–1537.

6. aScE 10-97, Design of latticed steel transmission structures, 1997.

7. iS: 802-1992 (Part1/Sec2), use of structural steel in overhead transmission line towers-code of practice, BiS, new Delhi.

8. iS: 800-2007, general construction in steel-code of practice, BiS, new Delhi.

(Discussion on this article must reach the editor before September 30, 2016)

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Numerical Studies on Corroded Steel angle members