Professor, Institute of Engineering Mechanics, Harbin. China 2. Ph.D Student, Institute of Engineering Mechanics, Harbin. China 3

Transcription

1 The 14 th World Conerence on Earthquake Engineering COMPARISON OF FRP-RETROFITTING STRATEGIES IN CHINESE AND ITALIAN CODES J. W. DAI 1, Y.R. WANG 2, B. JIN 1, 3, D.F.ZU 4, Silvia Alessandri 5, Giorgio Monti 5, Nicola Nisticò 5 1 Proessor, Institute o Engineering Mechanics, Harin. China 2 Ph.D Student, Institute o Engineering Mechanics, Harin. China 3 Proessor, Harin Institute o Technology, Harin China 4 Master Student, Institute o Engineering Mechanics, Harin. China 5 Proessor,Dipartimento di Ingegneria Strutturale e Geotecnica, SAPIENZA Università di Roma, Via A. Gramsci 53, 00197, Italy jwdai@iem.net.cn ABSTRACT: In recent years, FRP (ier-reinorced polymer/plastic) has een widely used in retroitting o R/C structures due to its high tensile property. Many research works and actual applications aout FRP retroitting or civil structures were carried out y oth domestic and international organizations. However, dierent technical speciications are used in FRP retroitting practice or R/C structures in dierent countries. As an example, related speciications aout FRP-retroitting strategies in Chinese and Italian codes are compared in this paper. The strategies o the oth codes or FRP-retroitting concrete structures are consistence in essence rom principle point o view, nevertheless, the calculation methods o the onded length, lexural strengthening, and shear strengthening are dierent rom each other. The reasons leading to distinctness and sameness are also interpreted concretely. The dierences would illuminate the designer to pay attention to the related issues during calculation. KEYWORDS: FRP-retroitting, R/C structure, Chinese and Italian codes 1. INTRODUCTION In recent years, FRP, a kind o new material applied in retroitting ield o civil engineering, has een paid close extensive attention or its advantages such as high strength, high elastic modulus, corrosion resistance, atigue resistance, and easy processing. At home and aroad, many researchers have already done massive research work on the perormance o FRP; what s more, they conclude reinorcing unction o FRP rom three aspects: theoretical analysis, experiments, practical engineering; and then they compile their own national speciication. Dierent countries have dierent opinions on the applications o FRP. Based on the dierence, this paper is going to compare the FRP-strengthening strategies in Chinese and Italian codes in detail. The strategies o the oth codes or strengthening concrete structures are uniorm in essence rom principle point o view, nevertheless, the calculation methods o the onded length, lexural capacity, and shear capacity are dierent rom each other. The dierences and sameness would deepen designers understanding o FRP and illuminate designers to pay attention to the issues during calculation, so that the FRP is eectively and widely applied in the civil structures throughout the world. 2. COMPARISON OF FRP-STRENGTHENING STRATEGIES IN CHINESE AND ITALIAN CODES 2.1 Comparisons on the Bonded Length The onded length in Italian codes The ultimate value o the orce transerred to the FRP system prior to deonding depends on the length, l, o the onded area. The optimal onded length, l e, is deined as the length that, i exceeded, there would e no increase in the orce transerred etween concrete and FRP. The length, l, should e longer than the optimal onded length, l e, as represented in Figure 1. The optimal onded length, l e, may e estimated as ollows:

2 The 14 th World Conerence on Earthquake Engineering E t le = (2.1) 2 ctm Where E and t are young modulus o elasticity and thickness o FRP, respectively, and ctm is the average tensile strength o the concrete The onded length in Chinese codes The onded length (igure 2) should not e less than 200mm and more than the onding and extending length. Namely: E ε A the ond length max,200 (2.2) τ Where E is elastic modulus o FRP; ε is tensile strain in ull utilization section o FRP, according to the clause o Technical speciication or strengthening concrete structures with caron ier reinorced polymer laminate; A is the section area on tension ace o FRP; τ is the design value or onding strength etween FRP and concrete, the value is 0.5MPa and is the width o FRP. Beam FRP The moment undertaken y the steel due to the existing loads The improved moment ater strengthening 200m l Figure 2 the onded length in Chinese code Dierences and sameness on the onded length From Italian and Chinese codes, the calculation method o the ond length is quite dierent rom each other. The ormula o the ond length in Italian code is ased on the experimental results. It is related with the average tensile strength o the concrete, young modulus o elasticity and thickness o FRP. The ormula o the ond length in Chinese code connects with elastic modulus o FRP, tensile strain in ull utilization section o FRP, the design value or onding strength etween FRP and concrete, and thickness o FRP. Comparing two codes, the ond length in Chinese code depends on the orced state and the ormula is consistent rom a dimensional point o view; ut the ond length in Italian code is a ixed value once the material o FRP is determined and the ormula is inconsistent rom a dimensional point o view. 2.2 Comparison on Flexural Strengthening The lexural strengthening in Italian codes 1) Fundamental hypotheses ULS analysis o RC memers strengthened with FRP relies on the ollowing undamental hypotheses: Cross-eam sections, perpendicular to the eam axis prior to delection, remain still plane and perpendicular to the eam axis ater delection. Perect ond exists etween FRP and concrete, and steel and concrete. Concrete does not react in tension. Constitutive laws or concrete and steel are accounted or according to the current uilding code. FRP is considered a linear-elastic material up to ailure. 2) Calculation Flexural design at ULS o FRP strengthened memers requires that oth lexural capacity, ultimate moment, M, satisy the ollowing inequation: Sd M, and actored

3 The 14 th World Conerence on Earthquake Engineering M Sd M (2.3) For oth ailure modes, the position x o the neutral axis is computed y means o the translational equilirium equation along the eam axis as ollows: 0 = ψ x cd + As 2 σs2 As 2 yd A σ (2.4) Where cd is equal to the design concrete compressive strength, cd, suitaly reduced i it is necessary. The lexural capacity, M, o the strengthened memer can e calculated using the rotational equilirium equation as ollows: 1 M = ψ x cd ( d λ x) As 2 σs2 ( d d2) A σ d 1 γ + + (2.5) Where the partial actor, γ, should e assumed equal to In Equations (2.4) and (2.5), the non-dimensional coeicientsψ and λ represent the resultant o the compression stresses and its distance rom the extreme compression ier, respectively, divided y x cd and y x, respectively The lexural strengthening in Chinese codes 1) Fundamental hypotheses when memers reach the ending ultimate limit state, tensile strain o FRP is determined ased on the plane section assumption and is no more than ultimate strain o FRP in tension, ε. when considering the eect o secondary load, initial strain, ε i, on the extreme tension ier o concrete is calculated according to the plane section assumption eore strengthening. tensile stress, σ, o FRP is equal to products o elastic modulus, E, and tensile strain, ε, o FRP. Perect ond exists etween FRP and concrete, and steel and concrete. 2) Calculation In Chinese codes, lexural strengthening calculation o rectangular section is classiied into three types: when concrete compression height, x, is etween ξ h and ξ h 0 (Figure 3a), calculation ormula is as ollows. x ' ' ' M x c h0 + yaa( h0 a) + Eε A ( h h0) (2.6) 2 Concrete compression height, x, and tensile strain, ε, o FRP are determined y the ormula(2.7). ' ' cx= yas yas + Eε A 0.8ε (2.7) cu x= h εcu + ε + εi when concrete compression height, x, is less than ξ h (Figure 3), calculation ormula is as ollows. M yas( h0 0.5ξ h) + E ε Ah( 1 0.5ξ ) (2.8) ' when concrete compression height, x, is less than 2a, calculation ormula is as ollows. ' ' M y As ( h0 a ) + E ε A ( h a ) (2.9) Where M is the design moment including initial moment; As and A ' s are the area o steel reinorcement sujected to tension and the area o steel reinorcement sujected to compression respectively; A is ' area o FRP reinorcement; y and y are design yield strength o tensile reinorcement and design compressive strength o compressive reinorcement respectively; c is design concrete compressive strength; E is elastic modulus o FRP; x is concrete compression height; ξ is relative limit height o compression region when ultimate strain o FRP in tension and concrete compression ailure simultaneously

4 The 14 th World Conerence on Earthquake Engineering 0.8ε reach the value, cu ; ε i is ending moment acting eore FRP strengthening; ε cu + ε + εi ultimate strain o FRP in tension; ε is strain o FRP. ε is Dierences and sameness on the lexural strengthening The strategies on lexural strengthening o the oth codes or strengthening concrete structures are uniorm in essence rom principle point o view. The lexural capacity in oth codes comes rom three parts: concrete, steel ar, and FRP; however, details o calculation are dierent rom each other: (1) Partial coeicient o two codes. In Chinese code, partial coeicients o steel ar and concrete are 1.12 and 1.4 respectively; in Italian code, partial coeicients o steel ar and concrete are 1.15 and 1.6 respectively. From the partial coeicient, lexural strengthening calculation in Italian code is more conservative than in Chinese code. That will e demonstrated in the ollowing example. (2) Failure mechanism could e classiied into dierent types: ailure due to the rupture o the FRP system or ailure due to concrete crushing with yielding o steel in traction, while FRP strain has not reached its ultimate value. Failure o structure elongs to which type according dierent criteria in dierent codes: in Chinese code, criteria is concrete compression height; in Italian code, criteria is CFRP maximum tensile strain. 2.3 Comparison on Shear Strengthening 2.3.1The shear strengthening in Italian code Shear capacity o FRP strengthened memers can e evaluated as ollows: {,,,,max} V = min V ct + V s + V, V (2.10) Where V, ct and V, s represent concrete and steel contriution to the shear capacity according to the current uilding code, and V, is the FRP contriution to the shear capacity to e evaluated as CNR-DT 200/ indicated in the ollowing. Shear strength shall not e taken greater than V,max. This last value denotes the ultimate strength o the concrete strut, to e evaluated according to the current uilding code. In the case o a RC memer with a rectangular cross-section and FRP side onding coniguration, the FRP contriution to the shear capacity, V,, shall e calculated as ollows: 1 sin w V = β, min{ 0.9 d, hw} ed 2 t γ sinθ p (2.11) where the partial actor γ shall e assumed equal to 1.20; d is the memer eective depth, hw is the stem depth, ed is the eective FRP design strength to e evaluated as indicated in Section o Guide or the Design and Construction o Externally Bonded FRP Systems or Strengthening Existing Structures; t is the thickness o the adopted FRP system, β is the iers angle with respect to the memer longitudinal axis,θ represents the angle o shear cracks (to e assumed equal to 45 unless a more detailed calculation is made), and w and p are FRP width and spacing, respectively, measured orthogonally to the ier direction Figure 3 lexural strengthening calculations in Chinese code

5 The 14 th World Conerence on Earthquake Engineering (Figure 4). For FRP strips installed one next to each other the ratio w p shall e set equal to 1.0. In the same case o a RC memer with a rectangular cross-section and U-wrapped or completely wrapped conigurations, the FRP contriution to the shear capacity shall e calculated according to the Moersch truss mechanism as ollows: 1 w, ( cot cot V = d ed t θ + β) (2.12) γ p Where all symols have the meaning highlighted in item aove. For completely wrapped memers having circular cross-sections o diameter D when iers are placed orthogonal to the axis o the memer ( β = 90 ), the FRP contriution to the shear capacity, V,, shall e calculated as ollows: 1 π V, = D ed t γ 2 cotθ (2.13) In all Equations (2.11) to (2.13), it is allowed to replace the term p with the term p measured along the memer longitudinal axis, where p = p sin β The shear strengthening in Chinese code In Chinese code, shear strengthening o concrete eam should e calculated according to the ollowing ormula: V Vrc + V (2.14) 2n ω t V = ϕ εve h s + ω (2.15) ( ) 2 ε v = ( λ ) εu (2.16) 3 Where, V,represents shearing orce design value; Vrc is shear capacity o concrete eore FRP strengthening; V is shear undertaken y FRP; ε v is strain o FRP when memers reaching ultimate limit states o shear capacity; ε u is ultimate strain o FRP;ϕ is FRP strengthening ormal coeicient, completely wrapped: ϕ =1.0, U-wrapped:ϕ =0.85, side onding:ϕ =0.7; λ is shear span ratio, when load is uniorm load λ = 3.0 ; when load is concentrated load, λ = a/ h, i λ > 3.0 then λ = 3.0, i λ < 1.5, then λ = 1.5 ; n is onded layer numers o FRP; h is side onding height o FRP; s is spacing o FRP; t is thickness o FRP; ω is width o FRP Dierences and sameness on the shear strengthening The shear capacity in oth codes comes rom three parts: concrete, steel ar, and FRP; however, details o calculation are dierent rom each other: (1) Partial coeicient o two codes. In Chinese code, partial coeicients o steel ar and concrete are 1.12 and 1.4 respectively; in Italian code, partial coeicients o steel ar and concrete are 1.15 and 1.6 respectively. From the partial coeicient, lexural strengthening calculation in Italian code is more conservative than in Chinese code. That will e demonstrated in the ollowing example. (2)In oth codes, FRP strengthening coniguration is classiied into three types: side onding, U-wrapped, and completely wrapped eams. In Chinese code, three FRP strengthening conigurations are realized y three coeicients; in Italian code, three FRP strengthening conigurations are realized y three dierent ormulas. 3. AN EXAMPLE Figure 4 Notation or shear strengthening using FRP strips The example, which comes rom Italian code, shows dierences etween Chinese code and Italian code. It is part o examples o FRP strengthening design in Italian code to compare the onded length, lexural

6 The 14 th World Conerence on Earthquake Engineering strengthening and shear strengthening. 3.1 Geometrical, Mechanical, and Loading Data The uilding considered or design is shown in Figure 5 and Figure 6. Structural elements are deined as ollows: Main rectangular eams with cross-section o 30 cm x 50 cm (concrete cover d1=d2=3 cm). Secondary rectangular eams with cross-section o 30 cm x 40 cm (concrete cover d1=d2=3cm). Rectangular columns with cross-section o 20 cm x 30 cm (concrete cover d1=d2=3 cm). Figure 5 Building geometry (dimensions in m). Figure 6 Steel ars location or eams and columns. Material mechanical properties are as ollows: Concrete: R ck = 20 N/mm 2. Steel: FeB32k ( yk =31.5 N/mm 2 ). Loading conditions are deined as ollows: Live load at level 1: a 1 = 2.00 kn/m 2. Live load at level 2: a 2 = 0.50 kn/m 2. Snow (zone III, height a s < 200 m ): = 0.75 kn/m 2. Dead load due to looring (or each level): g = 6.00 kn/m 2. Factored loads acting at ULS can e evaluated as ollows: Level 1: q 1 = KN/m. Level 2: q 2 = KN/m. 3.2 Increase o Applied Load New loads are deined as ollows: Level 1: a 1 = 6.00 KN/m2. Level 2: a 2 = 4.00 KN/m2. New actored loads acting at ULS can e evaluated as ollows: Level 1: q 1 = KN/m. Level 2: q 2 = KN/m.

7 The 14 th World Conerence on Earthquake Engineering 3.3 Design o Flexural Reinorcement The lexural strengthening calculation o a eam with 5.5m span is used to explain the dierences etween Chinese code and Italian code in lexural strengthening Flexural strengthening calculation parameters with Italian code Design material properties are determined as ollows: Concrete ( ck =16.6N/mm 2,γ c =1.60, cd =10.38 N/mm 2, ctm =1.95 N/mm 2, ctd =0.7 ctm γ c =0.85 N/mm 2 ) Steel ( yk = N/mm 2, γ s = 1.15, yd = N/mm 2 ) FRP lexural strengthening is perormed y installing caron ier reinorcement using the wet-lay-up method with the ollowing geometrical and mechanical characteristics (0:α E =α =0.9): CFRP thickness: t,1 = mm. CFRP width: = mm. CFRP Young modulus o elasticity in iers direction (eam axis): α E E = N/mm 2 = N/mm 2. CFRP characteristic strength:α k = N/mm 2 = 2700 N/mm Flexural strengthening calculation parameters with Chinese code Design material properties are determined as ollows: Concrete ( ck =16.6N/mm 2, γ c =1.40, c =11.86 N/mm 2, ctm = 1.95 N/mm 2, t = 1.39 N/mm 2 ) Steel ( yk = N/mm 2, γ s = 1.12, y = N/mm 2 ) FRP lexural strengthening is perormed y installing caron ier reinorcement using the wet-lay-up method with the ollowing geometrical and mechanical characteristics: CFRP thickness: t,1 = mm. CFRP width: = mm. CFRP Young modulus o elasticity in iers direction (eam axis): E = N/mm 2 CFRP characteristic strength: k = 3000 N/mm Flexural strengthening calculation results According to Chinese code and Italian code, lexural capacity o FRP-strengthened memer and the onded length are as ollows: Tale 3.1 comparison on lexural strengthening results Code level Span[m] n lexural capacity [KN m] onded length[m] Chinese code Italian code Tale 3.1 shows comparison on lexural strengthening results. With the same materials and dierent codes, the results are quite dierent rom each other. Flexural capacity and onded length are KN/m and 0.85m respectively when Chinese code is utilized; however, Flexural capacity and onded length are 195KN/m and 0.11m respectively when Italian code is utilized. The calculation method o the onded length in Chinese code is ound to e conservative and the calculation method o lexural capacity in Italian code is ound to e conservative. The dierences etween the two codes are mainly caused y the calculation methods and partial coeicient. 3.4 Design o Shear Reinorcement The shear strengthening calculation o eam with 5.5m span is selected to explain dierences etween Chinese code and Italian code in shear strengthening Flexural strengthening calculation parameters with Italian code Design material properties are determined as ollows: Concrete ( ck =16.6N/mm 2,γ c =1.60, cd =10.38 N/mm 2, ctm = 1.95 N/mm 2, ctd =0.7 ctm γ c =0.85 N/mm 2 ) Steel ( yk = N/mm 2, γ s = 1.15, yd = N/mm 2 ) FRP shear strengthening is perormed y installing U-wrap caron ier reinorcement having the ollowing geometrical and mechanical characteristics (mode 1, Section :αE=α=0.9): CFRP thickness: t,1 = mm. CFRP width: = mm.

8 The 14 th World Conerence on Earthquake Engineering CFRP Young modulus o elasticity:α E E = N/mm 2 = N/mm 2. CFRP characteristic strength: α k = N/mm 2 = 2700 N/mm Flexural strengthening calculation parameters with Chinese code Design material properties are determined as ollows: Concrete ( ck =16.6N/mm 2,γ c =1.40, c =11.86 N/mm 2, ctm = 1.95 N/mm 2, t = 1.39 N/mm 2 ) Steel ( yk = N/mm 2,γ s = 1.12, y = N/mm 2 ) FRP lexural strengthening is perormed y installing caron ier reinorcement using the wet-lay-up method with the ollowing geometrical and mechanical characteristics: CFRP thickness: t,1 = mm. CFRP width: = mm. CFRP Young modulus o elasticity: E = N/mm 2 CFRP characteristic strength: k = 3000 N/mm Shear strengthening calculation results According to Chinese code and Italian code, shear capacity o FRP-strengthened memer is as ollows: Tale 3.2 comparison on shear strengthening results Code level Span[m] Section n shear capacity [KN] let support Chinese code right support let support Italian code right support Tale 3.2 shows comparison on shear strengthening results. With the same materials and dierent codes, the results are quite dierent rom each other. Shear capacity o let support and right support are 459KN and 380KN respectively when Chinese code is utilized; however, Shear capacity o let support and right support are 340KN and 261KN respectively when Italian code is utilized. The calculation method o shear capacity in Italian code is ound to e conservative. The dierences etween the two codes are mainly caused y calculation methods and partial coeicient. 4. CONCLUSION This paper compares the strategies o Chinese and Italian codes aout FRP-retroitting practice or R/C structures in detail. The FRP-retroitting related speciications o oth codes are essentially consistence in principle point o view, nevertheless, the calculation methods o the onded length, lexural strengthening, and shear strengthening are dierent rom each other. The dierence mainly comes rom two aspects: dierent partial coeicient and dierent calculation methods. Case study shows that Chinese code is more conservative in the ounded length; Italian code is more conservative in lexural strengthening and shear strengthening. The dierences would illuminate designers to pay attention to the issues during calculation and design. ACKNOWLEDGEMENT The authors grateully acknowledge the joint inancial support o the China National Science Foundation (Project No ), the National Major Basic Research 973 Program (No. 2007CB714205), the Science and Technology Support Program (No. 2006BAC13B ) o the Ministry o Science and Technology o P.R. China, through Institute o Engineering Mechanics, CEA. REFERENCES Technical speciication or strengthening concrete structures with caron ier reinorces polymer laminate. (2002). Guide or the design and construction o externally onded FRP systems or strengthening existing structures. (2004). Code or design o concrete structures. (2002). China uilding industry press.