Structural rehabilitation is an activity with many dimensions. Unusual situations have to be evaluated in “out of box” thought process to arrive at their long term solution and also to ensure that they don’t reoccur. Routine rehabilitation actions are well defined and if they are executed scientifically by following all its fundamental principles, they end up giving an easy success. But when the structural damages are unique and unusual, they have to be independently and specifically evaluated to ensure that their rehabilitation scheme is successful even in absence of their previous references. Use of appropriate construction chemicals, few “out of box” considerations for design considerations as well as construction techniques, preservation of heritage / monumental values and many more; help in working out the most appropriate rehabilitation scheme.
The author has come across many such typical situations which were unique and unusual without any past reference so working out their tailor made solution was a challenging task in which “Out of box approach” helped. Such incidences are summarized in this paper without giving names of Organizations where these situations were faced.
Intensity of Structural damages has increased over last few years all across the world. Assessment of these damages, leads us to their reason of occurrence and then their rehabilitation methodology is worked out based on many factors. Routine damages like structural cracking, settlement of cantilever extension in a structure, water / chemical leakage, etc have past histories which can be taken as references to develop their tailor made rehabilitation scheme. They attract comparatively less effort as compared to rehabilitation scheme of unusual or unique damages. Few such cases are discussed below.
A) Chemical industry:
There was an unfortunate explosion in a chemical industry. Its intensity was very high causing severe damages to structures inside the industry premise and outside of the industry. The most seriously damaged structure was one RCC silo to store a by-product. Product conveyors which were passing above the silo had collapsed during the explosion and had fallen on the silo.
Critical path of the re-start-up schedule demanded readiness of this silo on top priority without which the actual process plant could not be restarted. So the most severely damaged structure was to be rehabilitated on topmost priority.
The entire rehabilitation plan went like this.
Damages of the silo were analysed with appropriate NDT and were quantified.
Beam column joints were weak due to few construction deficiencies.
Roof support beams of this portal frame structure were heavily damaged.
It would have been expensive and time consuming to repair those members at 24 meter height; under their designed rehabilitation method hence suitable options were explored by brain storming amongst executing Engineers and structural designer.
The most appropriate option was to dismantle top members of roof of the silo in their position but for this portal structure, removing its top members was not permitted.
Again options to solve the situation with speedy and cost effective solution were explored.
The best solution was unique, It was to convert the silo structure from “portal” to “simply supported” structure with structural steel truss, perlins and top sheets to cover the silo at top. Thus originally designed portal structure became a structure with long column effect in absence of top members of the portal so a grid of columns of required height was constructed at 3.00 Meter distance to compensate long column effect.
Though it was slightly costlier than other options, it was preferred because it was far too less time consuming.
Accordingly structural steel roof members were fabricated, erected and covered with roof sheets.
Simultaneously beam column junctions were grouted and then fibre wrapped to ensure their integrity with ductility.
This heavily damaged RCC silo was rehabilitated in minimum possible time with ensuring the highest integrity and stability for the structure. Records have shown that the rehabilitated structure has been successfully functional since last about two decades. As it can be uniqueness of rehabilitating this damaged structure was changing its structural arrangement from portal type to simply supported type.
B) Rehabilitation of structural steel members:
Rehabilitation of concrete members has been largely discussed and defined at many platforms and case studies but rehabilitation of structural steel members has been rarely addressed.
It was once reported in a chemical industry that structural steel members of its main production unit were distorted and had lost their shape and stability. This had affected load transfer capacity of the damaged members calling for stoppage of the production unit.
The entire damage assessment exercise and rehabilitation plan went like this.
- The damages were analysed. Subsequently inference were drawn that distortion had taken place due to their displacement.
- Plant history revealed that there was neither explosion nor earthquake since it was operational.
- As this was more challenging the entire structure was closely examined.
- It was observed that column base-plates were welded to insert plate provided at the top of column foundation (normal practice is to connect column base-plate with foundation is through foundation bolts).
- It was also observed that structural steel beam and column junctions were welded (normal practice is to provide bolted connections at beam column junction for structural steel members unless it is specially called for which could not be found out).
- One more significant observation was of continuous leakage and accumulation of water and chemicals around plant premise. It was there since commissioning of the plant. Path of leaking liquid was closely traced and it was found that leaking liquid was going to subsoil through many small holes on and around the floor.
- Subsoil survey was carried out in this area to analyse soil condition and its load bearing capacity. It could be observed that its load bearing capacity was grossly lost. Few cavities were also found at subsoil level generating hollowness in the subsoil medium.
- This had resulted in settlement of column foundations which were rigidly connected with steel columns in superstructure. So with settlement of foundations the superstructure steel columns were also pulled down. As the settlement for all foundations was not of same magnitude, pull down load which acted on all superstructure columns was different. This had caused their distortion and steel columns lost their shape.
- Hence reasons of damage as concluded after these set of observations included settlement of foundations due to continuous leakage of liquid and welded connections. Welded connections don’t allow movement of members which take place due to temperature effects of expansion and contraction. Majority of this structure being of structural steel members, their coefficient of expansion / contraction due to thermal shocks is high which invariably calls for allowance for micro movement of the steel members without losing their integrity. This was missing in this plant.
- Following such these significant observations, following action plan was worked out.
Fill the subsoil space with fresh earth having strength and stability for supporting the floor and superstructure.
Support the superstructure on scaffolding and release foundations from its loading.
Dismantle the settled foundations and reconstruct them as per their structural design on fresh and well consolidated soil.
New foundations were provided with foundation bolts to support superstructure columns.
Fresh structural steel columns and all other members were fabricated and erected as per structural drawing. Fresh structure was erected with bolted connection instead of welded connection.
Original configuration of the entire production unit was restored as per plant design and the plant was restarted. It is operating well since it was restarted before about a decade.
C) Effluent leakage through concrete storage tank:
It was reported in one of the chemical industries that effluent had started leaking from walls of concrete tank within twenty four hours of filling it with effluent. The effluent had pH between 2 and 4 so it was of acidic nature. Concrete surface of tank was coated with protective coating from inside before taking effluent in it so failure of the coating to resist the corrosive attack of chemicals present in the effluent was one of the probable reasons of this leakage.
However on closely examining the tank, some important observations were found out. Leakage was more and significant from construction joints and bottom of the tank. Visual observations revealed that concrete quality was good and no surface damages were found. Rebound hammer test results had also confirmed this.
However visual observations also revealed that bottom level of slab was slightly tilted at one corner. In the process of finding out the construction history of the tank, it was revealed that the construction was carried out during monsoon and tank was resting on piles. It was also observed that in a tank of 40 meter length, there was no expansion joint which permits any concrete structure to breath, expand and contract with temperature changes.
Detail survey of tank structure and its foundation was carried out and it was found that piles on one corner had settled due to which the tank had bodily settled in that corner. This had caused differential settlement, loss of its integrity and internal cracks through which effluent had started leaking out the concrete walls. According to the said observations, the reason of water leakage can be derived as missing expansion joint and differential settlement of RCC water retaining tank resulting in gap formation within the concrete wall.
Action plan for rehabilitate the tank structure was drawn like this.
Support the tank with scaffolding to temporarily transfer its load.
Design new end bearing type piles at new location near settled area to equally distribute the load of tank.
Construct piles as per design with pile cap.
Construct beam to support tank body as per new design.
Connect the entire new construction with existing construction of tank and transfer load. Release scaffold after this.
Provide compatible protective coating on internal surface of tank and allow it to cure for time as specified by manufacturer.
Load the tank with effluent.
- D) Conclusion:
Uniqueness in all the three cases discussed above is briefly summarized below.
Change in type of structure from “portal” type to “simply supported” type.
Continuously ignoring leakages can bring in settlement of foundation and floor. Connections between superstructure and foundation or between beams and columns should allow adequate ductility and space foe te structure to breath.
Construction of foundation should be avoided as far as possible during monsoon. However should it become necessary to construct foundation for building or pump or any unit, its design allowances should accommodate their resting on loose soil. Expansion joint in any structure (steel or concrete) is necessary when its length is more than 20 meters.
It can be concluded form all these cases that it is always necessary to assess the damage from grass root. The actual cause of damage has to be clearly identified before going ahead with defining the rehabilitation scheme. Unless root cause of damage is not identified its rehabilitation scheme can’t be completely successful because in some unique situations, apparent or visual observations or even few NDT results might not be adequate to reach grass root reason of the structural damage. Method adopted in all above mentioned cases to reach upto grass root reason of damage is analysis based on “Forensic Engineering approach”. It is indeed a logical and scientific all-round analysis of the damage occurrence which is implemented by elimination method of all possibilities of causes of damage with respect to desired behaviour of the structural element.
Fundamental knowledge, scientific analysis of damage and prevailing environmental / climatic condition are should be meticulously addressed in handling unique and unusual cases of structural rehabilitation.