Flexural capacity

The primary failure mechanisms encountered in reinforced concrete buildings arc flexure, diagonal tension, and direct shear. Of these three mechanisms,. flexure is preferred under blast loading because an extended plastic response is provider prior to failure. To assure a ductile response, sections are designed so that the flexural capacity is less than the capacity of non-ductile mechanisms. 

Strengthening of the connections is often the most cost effective upgrade for existing buildings if it does not require removal of existing interior walls and equipment. For a member to absorb blast energy and be structurally efficient, it must develop its full plastic flexural capacity. This requires a substantial increase in shear capacity at the connections to avoid failure. 

Damages of concrete piers as shown in Fig. 1(a) were caused by lack of flexural capacity or shear capacity of existing structures against seismic forces. Particularly, extensive damages were observed at the base part of the pier and at the termination of longitudinal re-bars at mid-height. Therefore, in piers with inadequate lateral reinforcement, it is imperative to provide additional external confinement to insure ductility of the piers. 

The design of FRP-reinforced concrete members in flexure is analogous to the design of steel-reinforced concrete members. Flexural capacity can be calculated based on assumptions similar to those made for members reinforced with steel bars. The design of members reinforced with FRP bars should take into account the mechanical behaviour of the FRP materials (ACI-440.1R-03 2003). 

The advantages of externally bonded reinforcement over other methods of strengthening concrete structures include the ability to strengthen part of a structure whilst it is still in use, minimum effect on headroom, low cost and ease of maintenance (see Fig. 6.8, for example). The method has been in use for over 20 years, mainly to enhance flexural capacity, and has been found to produce effective and economical solutions to particular problems. 

Straps were installed along the outer face of the columns parallel to the columns axes (2 layers) to increase their flexural capacity. 

Type J-BI-18 specimen has the bent in joint bar anchorage details with 18 mm diameter flexural reinforcement for beam and column. Figure 13.14 shows the load and displacement relationships for J-BI-18 specimen for both push and pull directions. Specimen J-BI-18 was designed for the beam flexural capacity of 137 KN. However, as shown in Fig. 13.14, it collapsed at 99 KN in a joint failure mode in shear. Figure 13.15 represents the formation of crack in joint during the cyclic load test. 

Based on computations, confirmed by reinforcement strain variations, it is concluded that the flexural capacity of the joint specimen was not reached. Further, the failure is identified as a joint shear failure, but at a failure load (Vy = 240.2 kN) less than the ACI recommended capacity (Ycap = 383.9 kN) for properly detailed joints. 

For the KFUPM specimen designed with relatively high reinforcement ratio p = 0.01 (J-Bl-18), experimental results showed that the specimen collapsed due to failure of joint under shear, as the joint collapse load was lower than the flexural capacity of the beam (27.6 % lower). That was confirmed from the combined mechanistic/experimental computations and also further corroborated from DIANA results. 

An ultimate limit state analysis requires to establish the value of the FRP reinforcement so that the design factored moment, Msd, and the flexural capacity of the strengthened structure, Mr[Pg.69]

In order to evaluate the design flexural resistance of the FRP-strengthened member as well, in the presence of an axial force (combined compressive and bending stress), the principles introduced in the previous chapters are valid, as long as the dependence of the design flexural capacity Mr, of the strengthened member on the normal factored axial force, Nsi, is taken into account. 

Enhance flexural capacity or combined bending and axial capacity through the application of composites with fibers ranning in the same direction as the member axis and then in different directions as well... 

In these cases, it is deemed necessary to increase the column capacity under combined bending and axial load, aiming at turning the structure into a strong column-weak beam simation. The application of the hierarchy strength criterion implies an increase in the flexural capacity of columns and, thus, an increase of shear exerted at the nltimate states. As a consequence, snitable shear tests are required, eventually enhancing the strength towards this feature in order to avoid a brittle collapse. 

The CFRP strengthening comprised a transverse carbon fibre in-situ laminated fabric (to provide additional transverse flexural capacity), and after the installation of this fabric had been installed and cured, the surface cleaned and roughened, 112 pnltmded CFRP plates were installed. Figure 16.5 shows the installation of the CFRP strengthening (plates and fabric) system to the arch intrados of Minsterley Bridge. 

There is considerable uncertainty about the magnitude of the effective slab width in tension it is pointed out that slab bars which are in this effective flange width and are parallel to the beam increase its flexural capacity for hogging moment and hence the beam capacity design shears, as well as the likelihood of plastic hinging in the columns. 

Similarly, welded haunches and bolted brackets are used to move the plastic hinges away from the column, but also strengthen the existing coimection and seek to maintain the original flexural capacity of the beam. 

A similar approach was adopted by RILEM in a recent recommended method for calculation of flexural capacity, the as method [78]. The RILEM recommendations provide guidelines for the stress distribution as a function of the level of loading, characterized by the deflection or crack opening displacement. The... 

S. Altoubat, J.R. Roesler and K.-A. Rieder, Flexural capacity of synthetic fiber reinforced concrete slabs on ground based on beam toughness results , in M. di Prisco, R. Felicetti and G.A. Plizzari (eds) Fiber-Reinforced Concretes, BEFIB2004, RILEM Proceedings PRO 39, RILEM Publications, Bagneux, 2004, Vol. 2, pp. 1063-1072. 

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