PSI - Issue 11

Luigi Ascione et al. / Procedia Structural Integrity 11 (2018) 202–209 L. Ascione et al./ Structural Integrity Procedia 00 (2018) 000 – 000

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7

8. Comparison with the American ACI 549.4R-13

The American ACI 549.4R-13 provides design recommendations based on the acceptance criteria described by AC434 (2013). According to these criteria, the FRCM composite stress-strain behavior is assumed to be bilinear, where the slope of the first un-cracked stage is mainly governed by the matrix mechanical properties whereas the slope of the second cracked stage, E f,ACI , depends on the embedded mesh textile mechanical properties and on the matrix-textile interaction. The bond properties of the FRCM composite with respect to a specific substrate are not investigate during the acceptance procedure. The parameters needed to design a strengthening intervention with FRCM composites are obtained by tensile tests on FRCM coupons using the clevis-grip method (Arboleda et al. 2015). These parameters are the slope of the cracked stage E f,ACI and the effective tensile strain  fe = f fu /E f,ACI ≤0.012, where f fu is the ultimate tensile stress of the FRCM coupon. The effective tensile strain in further limited in the case of shear strengthening. When designing a FRCM strengthening intervention, perfect adhesion is assumed at all material interfaces and plane sections are assumed to remain plane. Since only the cracked stage of the stress-strain behavior of the FRCM coupons is considered in the acceptance criteria and the effective tensile strain is limited to 0.012, the maximum stress in the FRCM composites is generally significantly lower than f fu , which in turn leads to a general underestimation of the strengthened element maximum capacity. The level of safety obtained with the American and Italian guidelines cannot be compared directly. Indeed, the American approach provide different partial safety factors depending on the failure mode (brittle or ductile) and reduces the maximum capacity of the strengthened element employed a pre-determined factor. The Italian guidelines comply with the Eurocode approach and employs characteristic and design values of applied loads and material resistances according to the semi-probabilistic limit-state method. An FRCM strengthening intervention to a masonry wall subjected to out-of-plane load is designed in this section following the ACI 549.4R-13 and Italian Guideline approaches. The design is carried out considering average values of the material properties and neglecting all partial safety coefficients. The wall is made by clay bricks, has a width of b =1.8 m, a height of h =3 m, and a thickness of t =0.3 m and it is subjected to a vertical compressive load N =22 kN. The measured compressive and flexural tensile strength of the masonry are equal to f m =9 MPa and f mf =0.52 MPa, respectively. The cracking moment of the unstrengthened wall is: 9. Example of FRCM strengthening of a masonry wall against out-of-plane load

2

6   N b t   

  

M f

6.94

 



kNm

(2)

cr

mf

b t

The wall is strengthened on one both sides applying one layer of a carbon FRCM system with an overall thickness t F =10 mm. The carbon mesh textile has a nominal thickness t f =0.047 mm, a tensile strength  u,f =2000 MPa, an elastic modulus E f =203000 MPa. Bond tests on the specific support considered provided the conventional strain  lim,conv =0.58%. Tensile tests on FRCM coupons provided the slope of the cracked stage E f,ACI =80000 MPa, and the effective tensile strain  fe =1.00%. Since according to the Italian guideline approach, in the case of out-of-plane strengthening, the conventional strain is   lim,conv =  ·  lim,conv =0.87%, the bending strength of the strengthened panel is:

F 2 2 t t

t

  

  

  

  

( α)

R m 1 M f b k x k x t b    2 f

E

31.22 kNm



 

(3)

lim,conv f

2

where k 1 and k 2 are stress block coefficients of the masonry parabola rectangle stress-strain behavior considered (D’Antino et al. 2018), and x is the neutral axis depth. Applying the ACI 549.4R-13 approach, the bending strength of the strengthened panel is: