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** INTERNATIONAL JOURNAL OF PURE AND **

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**RETROFITTING OF BUILDING AGAINST BLAST EFFECT **

**P. J. GUDHE1, R. K. WATILE2**
1. College of Engineering & Technology, Babhulgaon, Akola, (M.S.), India.

2. Department of civil engineering, College of Engineering & Technology, Babhulgaon, Akola, (M.S.), India.
**Accepted Date: 13/03/2015; Published Date: 01/04/2015**

**Abstract: Retrofit design procedures to improve the survivability of reinforced concrete (RC) columns to blast loads are described. **
These procedures allow conventional RC columns to survive large explosive loads at very close standoffs, and have been demonstrated
using carbon fiber reinforced plastic (CFRP) materials. The design procedures for blast loads were adapted from those already used to
develop seismic retrofits for RC columns but differ because they consider both shear and bending failures and include effects peculiar to
blast loads. The response of simple RC columns subjected to constant axial loads and lateral blast loads was examined. The finite element
package ANSYS was used to model RC column. For the response calculations, a constant axial force was first applied to the column and the
equilibrium state was determined. Next, a short duration, lateral blast load was applied and the response time history was calculated. The
analysis and design of structures subjected to blast loads require a detailed understanding of blast phenomena and the dynamic response
of various structural elements. This gives a comprehensive overview of the effects of explosion on structures. Analyses were conducted to
demonstrate the effectiveness of composite wrapped columns for improving the survivability of existing reinforced concrete multistory
buildings to attacks by explosives. The results indicate that under some circumstances composite wrap can be an effective means to
retrofit an existing facility to lessen its vulnerability to blast loads.

**Keywords-**Blast Effects Reinforced Concrete, FRP.

**Corresponding Author: MR. P. J. GUDHE **

**Co Author: MR. R. K. WATILE **

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**INTRODUCTION **

Due to different accidental or intentional events, the behavior of structural components subjected to blast loading has been the subject of considerable research effort in recent years. Conventional structures, particularly that above grade, normally are not designed to resist blast loads; and because the magnitudes of design loads are significantly lower than those produced by most explosions, conventional structures are susceptible to damage from explosions. With this in mind, developers, architects and engineers increasingly are seeking solutions for potential blast situations, to protect building occupants and the structures.

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Numerical analyses have demonstrated that structural collapse of a multistory building as a whole may start by the failure of perimeter columns on the first floor. Retrofit techniques consisting of strengthening the columns with round steel jackets were assessed and found to markedly increase the survivability of the columns.

The effect on the survivability of columns wrapped with composites will be examined using the
same numerical analyses methods. This study also includes the use of different charge sizes and
standoffs so that architectural considerations that limit the threat (*e.g.*, increasing standoff) can
be evaluated.

**RETROFIT DESIGN PROCEDURE **

The design procedure determines the required thickness of the composite wrap needed for
strengthening RC columns for a given blast design load in both flexure and shear. The initial
step in the procedure is to determine the response of the non-retrofitted structural component
to the design threat. Once the response of the non-retrofitted column is determined, checks
are made to determine if any enhancement is required. For the first check, the peak
displacement response is compared to a maximum allowable displacement. If this allowable is
exceeded, flexural enhancement of the column is required. The column is then checked for
adequate shear resistance and ductility capacity, and if deficient, hoop wrap is added. Flexure
enhancement is typically achieved with strips composed of unidirectional fibers, and shear
enhancement is provided by a unidirectional fabric, as shown in Figure 1. **Carbon (CFRP), **
**E-Glass (GFRP), and Kevlar (KFRP) may be used for the fiber of the composite. **

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**Figure 1 Illustration of a retrofit of a square RC column showing both composite strips for **

**flexure strength enhancement and hoop wrap for shear strength enhancement. **

**Flexural Capacity**

For the design procedure, a single degree-of-freedom (SDOF) model is used to estimate the
column's flexural response, to determine the need for retrofits and. where needed, to design
them. For the non-retrofitted columns, the resistance function detailed in the DAHS manual is
used, which accounts for both compressive and tensile membrane actions. However, if the
column is strengthened in flexure (e.g., by using the vertical composite strips shown in Figure
1), the DAHS resistance function must be modified to take into account the added strength. In
the case of columns with discontinuous flexural retrofit at the top and bottom, the column
jacket significantly enhances the moment capacity only at the column mid height. If the retrofit
is extended beyond the top and bottom of the column, it is possible to enhance the moment
capacity at those locations as well. The ultimate resistance, *r,h* of the column is given by *rl{ — *

*8(Mn,,+M^,)/L *, where *Mm, *is the moment capacity at the top and bottom of the column, *Mf/,, *

is the enhanced moment capacity at the column mid height, and *L *is the column height.

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produce an inordinately conservative flexure design and to underestimate the needed shear capacity. The corresponding diagonal and direct shear capacities are also shown in the figure 2.

For the K&C design procedure, the maximum lateral column displacement at the center is
limited to a fraction *(Fj< *1) of the displacement *duc* (i.e., A7/ = F</£/„<•). Beyond *dlic, *which is

associated with the peak compression membrane force *rlic* (Figure 2), the resistance function

decreases, producing unacceptably high lateral displacements for a column? The peak
resistance displacement *duc*is given as a fraction of column height and width?, as duc=0.033L *< *

0.5h. If compression membrane behavior is not included, *ru* given by flexural capacity alone is

used for the peak resistance and the displacement is limited to Af to prevent column instability

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**Shear Capacity**

Columns with poorly confined concrete that are expected to sustain large plastic rotations require enhancement to their ductility capacity. The ductility capacity of a column depends on the amount and distribution of transverse reinforcement within the plastic hinge region. This hoop reinforcement increases confinement of the core concrete, thereby increasing its ultimate compressive strain capability, and also provides lateral restraint against buckling of the longitudinal reinforcing bars. These effects have been quantified for seismic retrofits of reinforced concrete bridges (Priestley et al., 1996). The transverse reinforcement provided by the hoop wraps also enhances the diagonal shear capacity.

The resistance function related to flexural behavior can only be developed if the shear capacity exceeds the bending resistance capacity. Initially, diagonal shear capacity may be the weak link. If diagonal shear is prevented (e.g., by wrapping the column), and the bending strength is sufficient, the next failure mode that needs to be considered is direct shear at the column-floor interface. The design procedures checks that both shear capacities equal or exceed the shear demand.

**Diagonal Shear Capacity**

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The shear demand can be derived from the peak (i.e., ultimate) flexural resistance function, *V0 - *

*ruLI2. *This formula assumes that the dynamic shear demand is equal to the static shear

demand. However, a dynamic shear increase factor may be used; these factors are typically provided in terms of load duration and peak, and the natural frequency, of the column (e.g., Crawford, 1982). In actual design, the transient shear demand is usually assumed to be equal to the static shear demand, as indicated in the DAHS manual.

**Concrete. **In the DAHS Manual, the concrete contribution to the shear capacity is given by:

Where *Ka*= age increase factor (1.1 at 6 months, 1.15 thereafter)

* Ke = *static increase factor (about 1.1)

* Kd= *dynamic increase factor in tension

For quasi-static loading, this value of the shear capacity is limited to about 1/6 of the square root of the actual concrete strength in MPa, or 2 times the square root in psi units. This is the same value as in the ACI 318 recommendations, which typically allow 2 times the square root, and similar to the P-397 recommendations.

**Steel. **In the DAHS Manual, the steel contribution is given by:

Where, *Av * *= *area of shear reinforcement within length, *s *and width, *b*

* B * = width of rectangular section

* fdy * *= *reinforcement dynamic yield stress

* s = *length measured in span direction

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where *fsj- *= allowable increment of jacket stress = 0.004 *Ej < *0.5//7

*0= *angle of compression strut to column axis, typically 35°

*D = *column diameter

*tj *= wrap or jacket thickness

*ej = *jacket modulus of elasticity = *Es*if steel, *Ej *if FRP

*fvj = *ultimate stress for jacket For rectangular columns, the shear strength enhancement

provided by a rectangular jacket is:

.

where *h = *column dimension in shear

**Direct Shear Capacity**

Direct shear capacity is typically larger than the diagonal shear capacity and the shear force associated with the flexural capacity of the column. Hence, the diagonal shear and flexural capacities must be unusually high before consideration is given to a direct shear failure. Direct shear failure formulae in the DAHS manual are derived from original K&C work on construction joints (K&C, 1973). For monolithic joints the formula for the direct shear capacity is:

Where *Nc = *applied axial compression force per unit width (N/mm)

*AYf= *area of shear friction reinforcement per unit width (mm /mm) *d=*distance from the

extreme compression fiber to the centroid of the tension reinforcement (mm) *f'dc= *concrete

dynamic ultimate compressive stress (MPa)

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The direct shear capacity cannot be enhanced by typical wrapping systems, which stop at the column's ends. But since direct shear failure does not often govern, designs to prevent it, although doable, are not often important or needed.

**CONCLUSIONS**

Based on the studies available in the literature, the ultimate objective is to make available the procedure for calculating the blast loads on the structures with or without the openings and frame structures. Also to study the dynamic properties of reinforcing steel and concrete under high strain rates typically produce by the blast loads. From this part of the study, an understanding of how reinforced concrete columns respond to blast loads was obtained.

The following observations and conclusions are drawn from this study

1. The finite element analysis revealed that, for axially loaded columns, there exists a critical lateral blast impulse. Any applied blast impulse above this value will result in the collapsing of the column before the allowable beam deflection criterion is reached.

2. The column response to non-uniform blast loads was shown to be significantly influenced by higher vibration modes. This was especially true for the unsymmetrical blast loads.

3. The comparison between the normal strength column and the higher strength column showed that the critical impulse for the higher strength column case is significantly higher. This increase can be attributed to the added stiffness.

4. The surfaces of the structure subjected to the direct blast pressures can not be protected; it can, however, be designed to resist the blast pressures by increasing the stand-off distance from the point of burst.

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**REFERENCES **

1.P. Desayi and S. Krishnan (1964), “Equation for the stress-strain curve of concrete”. Journal of the American Concrete Institute, 61, pp 345-350.

2. S. Unnikrishna Pillai and Devdas Menon (2003), “Reinforced Concrete Design”, Tata McGraw-Hill.

3. Schmidt, Jon A. (2003), “Structural Design for External Terrorist Bomb Attacks”, STRUCTURER magazine, March issue.