Retrofitting of Concrete Buildings Using Steel

Debashis Datta, Senior Manager (Civil & Structural), Institute for Steel Development & Growth, Kolkata.

The retrofitting becomes necessary to enhance the performance of structures including those encountered loss of strength due to deterioration or which have crossed their expected life. The success of retrofitting depends on the actual cause and the measures adopted to prevent its further deterioration. The process involves repair, retrofit, renovation and even part reconstruction.

Steel elements in the form of plates, channels, angles, I-sections along with anchor bolts / through bolts could be used with proficiency for retrofitting of a damaged RCC building. This paper has elaborated in detail different aspects of cracks developed in concrete structures with remedial retrofitting measures along with sketches and calculation methods. Some generic examples of retrofitting methodologies applicable to Global and / or Local failures in concrete have been cited in this paper. Along with the conventional measures of retrofitting some special measures / details using steel have also been discussed on case to case basis.

Use of steel profiled sheeting has been demonstrated to replace or retrofit a damaged concrete roof. Steel sections and sheets being lighter their use does not add extra dead load on the existing structure but offers strength to the structural element as well as system.

Detection and Prevention

Any structure needs periodic repairs and regular maintenance to continue its performance of withstanding the existing loads. But due to changing static as well as dynamic behavior of the applied loads, the structures undergo deterioration and ultimately loose material strength to develop cracks. In case of RCC structures, the cause of the failure could be known by studying the crack pattern and its depth, which helps in identification of cause and prescribing measures for repairs/retrofitting to enhance life expectancy of the structure.

The conventional design methodology ensures a life expectancy of an RCC building structure as about 70 years. But studies indicate that RCC structures develop deteriorating signals at half of structure life, demanding repairs / retrofitting / renovation. INSDAG had carried out one such study on the bridges of Indian railways, which establishes this fact by lots of examples all over India. Hence, it is necessary to understand the effective preventive measures against the development of cracks in RCC elements.

Global Symptoms and Local Symptoms

The structural behaviour and its failure mechanisms could be classified broadly as Global and Local based on the symptoms manifested in its failing members. If the redundancy of the structural system as a Global is high, the non-performance of a few members may not lead to the absolute failure of the structure unless redundancy reduces to zero. But the failure of main members may impair the Global system. It is easier to carry out retrofit on the structural elements, if it is confined to the Local one and does not affect the Global system of the structure.

During repair or retrofitting, care needs to be taken towards the release of residual stresses in the existing system by appropriately propping and supporting and the redistribution of the stresses due to the new system. The best method is to apply props and supports to structural members being repaired / retrofitted to get the residual stresses released. Even after the structure gets retrofitted, it is desirable to keep the props and supports in place for some time and remove them sequentially so that the new stress pattern in the structural element can be redistributed evenly without effecting any undue stress concentration at any point of time.

When the symptoms of failure lead the structure to a mechanism i.e. the state of failure, it requires additional props / supports to hold the system during the retrofitting activities. Even sometimes, it is required to renovate a part of the structure as a measure of retrofitting because that improves the health of the structure and increases its life expectancy.

Steel Elements Used in Retrofitting

Steel elements in the form of plates, channels, angles, I-sections along with anchor bolts / through bolts could be used for retrofitting a damaged RCC building. The detailing of the joints of Steel members with the cracked RCC members requires study of the nature of cracks, their extent and depth. Since, it is very easy to give the steel members different shapes, it could be detailed in the most favourable way to fit into the system according to its necessity and available spaces. Some specific failures and its retrofitting measures are explained below.

An RCC beam is prone to fail by either flexure and / or by shear force. The stressed zone for shear in the beam is located near the supports and cracks develop at an angle of 450 starting from the point of support. But the stressed area for the flexure is near the mid-span and its failure crack develops vertically.

In some structures, the RCC beam may undergo torsional failure which shows cracks over the peripheral surface of the member. In some specific cases, it may also undergo failure due to biaxial bending which impairs the concrete section through diagonal cracks started at corners of the section and propagate inward at an inclined angle. Some details are furnished below as suggested measure of retrofitting to prevent such failure.

The RCC column may fail as a compression member through buckling developing horizontal cracks at about mid height or it may fail as beam-column with similar nature of cracks. So, it requires close observations to detect the cause of failure in concrete and prescribe its remedial measures. In case of failure of column under compression, the concrete will tend to spall off near the point of crack. But in case of beam-column failure such spalling is absent. This is due the dominance of compression force and the bending moment. Stocky RCC columns may fail due to yielding impairing the concrete of the member through crushing and spalling. But such cases are not very common in practice.

But the most complicated situation arises when the failure in RCC is affected by the combination of these causes. In such a case, the detailing of retrofitting requires due consideration of all such aspects leading to failures including physical situations. The detailing may be made simpler to ensue the retrofitted joint takes up the different types of loads separately. But depending on the specific requirements, the measures of retrofitting could be applied as per the Engineer’s discretion.

In case of excessive differential settlement in the structural system cracks may appear locally at the beam-column junctions which may lead the structure to form a mechanism slowly as the crack increases. At the joints, the reactive forces act in combinations and failure of a joint means reduction of redundancies depending on the number of connecting members, which is not at all desirable. Hence, all the aspects of load combinations need to be carefully considered while prescribing a fail-safe retrofitted beam-column connection.

Design Methods of Retrofitting Using Steel Members

The design methodologies of retrofitting of RCC structures using steel elements are described with examples on case by case basis. While designing a retrofitting of structural element, the full strength of the parent members needs to be ascertained for the retrofitting of structural element to be able to withstand the full strength. This is because accurate assessment of the loss of capacity of the deteriorated concrete members is difficult and cumbersome.

Beam-Column Junction ^(1,2)

Three types of details of the retrofitting system have been shown in Fig.1. The horizontal and vertical components of the full strength of the bracing member shall be withstood by the Steel bolts fixed with the column and beam respectively either as inserted or through type. Type-1 shows anchor bolts inserted within the concrete and Type -2 shows anchor bolts as drilled through the concrete whereas Type-3 shows anchor bolts as through type but outside the concrete members fitted with back end plates.

Steel Jacketing of RCC Columns ^(1,3,4)

Existing concrete columns may fail under buckling due to loss of compression capacity of longitudinal rebars due to corrosion, expansion of its diameter and opening of its lateral ties with subsequent spalling of concrete covers. They may be subjected to excessive shear force during their performing life for which the columns had not been designed and the columns expand laterally. To increase the strength of the columns in such cases, the existing concrete and longitudinal reinforcement is confined within a steel jacket filling the gap with non-shrink grout and thereby its shear capacity is increased and the buckling mode of failure within the height of the columns is also arrested. The different systems of steel jacketing have been shown in Fig.2.


As per Aboutaha et al., 1999, the shear strength of the jacket (Vj) can be calculated by considering the jacket to act as a series of independent square ties of thickness and spacing t_sj, where t_sj is the thickness of the plates. For rectangular columns,

V_j = A_sj x (f_sj x d_sj) / s_sj

Where,
A_sj = Total area of assumed square tie = 2 x t_sj²
f_sj = allowable stress of jacket = 0.5 x yield steel of steel of jacket
d_sj = depth of the jacket
s_sj = spacing between the square ties = t_sj

If the column is not subjected to excessive shear force and the loss of its compressive strength is manifested with continuous propagation of predominantly horizontal cracks, the steel jacket may be extended from one connection to the other and the increased capacity of the jacketed column may be calculated by using the following methods. The ultimate compressive strength Ncu and the buckling strength Nccr of the jacketed column are given by:

N^c_u = A_c x r_u x F_c

N^c_cr = A_c x σ^c_cr

Where,
A_c = Cross-sectional area of a concrete column
F_c = design standard strength of existing concrete
σ^c_cr = Critical stress of concrete column
r_u = 0.85 = reduction factor for concrete

The critical stress, σ^c_cr = [2 / {1 + sqrt (λ_c⁴ +1)}] x r_u x F_c for λ_c ≤ 1.0
σ^c_cr = 0.83 x exp {C_c x (1 - λ_c ) } x r_u x F_c

In which
λ_c = (λ_c / p) x sqrt (€^c_u)
€^c = 0.93 x (r_u x F_c)^0.25 x 10^-3
C_c = 0.568 +0.00612 x F_c
λ_c = Slenderness ratio of the concrete column

Strengthening of RCC Beams ^(1,2)

Additional steel beams connected through the concrete beam by steel bolts share the load and relieve the concrete of its stresses. The load is shared proportional to the relative rigidities (EI) of the existing concrete beam and the added steel beam. There should not be any void in between the steel beam flange and the concrete slab. It should be dry packed with wooden wedges. Otherwise horizontal bracing system needs to be introduced in between two steel beams to reduce the lateral torsional buckling of the additional members. Sometimes flexible channel sections could be wedged from underside the slab while giving an upward deflection of the slab and release its excessive stresses (refer Fig.3). While retrofitting an RCC beam failing in flexural tension, the additional requirement of tensile reinforcement is compensated as shown in Fig.4.

Case Studies

The roof of a church in West Bengal required to be renovated without disturbing the masses going underneath. Steel profiled sheets had been placed below the damaged roof with the help of Steel beam grids as supporting structure. The existing roof was removed with jack hammers and the new one was cast with RCC over the Steel sheets which acted as sacrificing shuttering material. In this case, the thickness of the new roof could be optimized by use of Embossed profiled Steel sheets instead of Plain profiled Steel sheets because it reduces the requirement of rebars and thickness of the concrete section (refer Fig.5).

Dome of a Heritage Building, West Bengal

The Dome structure of a heritage building in West Bengal needed to be reconstructed because it was weired out over ages. Steel frames bent in the shape of the dome along with thin Steel sheets have been fabricated and placed below the existing concrete structure. The deteriorated old concrete was removed with jack hammer without disturbing the official activities going underneath and new concrete was cast with some shear studs made of Steel flats monolithically inserted within it. The steel support had been kept in place as permanent shuttering which also adds aesthetics to the dome structure (refer Fig.6).

Bracings in Building in Kolkata

In Kolkata, the columns in one heritage building have been relieved of bending stresses by addition of cross bracing systems using steel angles, plates, bolts etc (Fig.7). By use of bracings, the moment resisting type of the frame and its members has been converted to braced frame and the members were subjected to axial loads only. In this way, the columns were relieved from the flexural stresses induced in them due to the existing system.

Some original special Recommendations

Sometimes in specific cases, unconventional structures are required to be designed and constructed. Hence, the nature of cracks developed in the concrete is so diverse that it becomes very difficult to detect the cause of the failure and hence, prescription for its remedial measures becomes very difficult. As a case study, the cracks observed in a Stock Pile structure can be discussed. This structure was overloaded to double its capacity for over about seven (7) months. After a few weeks, cracks of different patterns started to develop in the concrete hopper walls, its supporting beams and the expansion joints got separated slowly. The nature of cracks were varied and it was a very difficult scenario to detect the causes of such failures so that the structure could be retrofitted and be back to its operation. The analysis of the structure in computer was done and depending on the location of cracks as the weaker points the excessive stressed zones were relieved through additional supporting members. While doing so additional parts were connected to the existing structure with such rigidity that maximum forces were drawn by these additional members.

The cracked members were retrofitted with steel elements as discussed earlier and the connection of old and new structural systems were done with steel plates, through bolts etc. to take care of the reactive forces like shear, bending moment etc.

Heritage structures need special sensitivity in retrofitting so as not to disfigure their appearance and many such buildings have been successfully reconstructed in different countries. We often come across disasters of old buildings, some of which are declared heritage buildings, due to ageing and loss of strength of the materials of construction. The life expectancy of such buildings could be enhanced considerably by using steel in special details by which the damage of the structural elements like columns, beams and slabs could be repaired or retarded.

Observations and Conclusion

The deteriorated and damaged RCC structures necessitate retrofitting through proper measures to make it functional for the rest of its life. Steel has a very high strength to weight ratio and is available in different shapes or it could be rolled or folded to the desired shapes. Hence, the retrofitting of the RCC building could be executed using different steel elements ensuring the safety and longevity of the deteriorating structure.

References

• IIT, Madras – Handbook on Seismic Retrofit of Buildings, 2010.
• Newman Alexander – Structural Renovation of Buildings – Methods, Details and Design Examples.
• Teaching Resource for “Structural Steel Design” Volume I, II & III: INSDAG Publication prepared by IIT Madras, Anna University and Structural Engineering Research Centre (SERC), Chennai under Dr. R Narayanan’s leadership.
• The Korean Society of Steel Construction (KSSC)– Intern- ational Journal of Steel Structures December 2005, Vol. 5, Number 4.
• Debashis Dutta, retrofitting of concrete buildings using steel.

Acknowledgement

This article has been reproduced from the proceeding of 'National Conference on Repair & Rehabilitation of Concrete Structures' organized by ICI western U.P Gaziabad, IA Sructural Engg, and Association of Structural Rehabilitation, with the kind permission of the organisers.