Post date: Jan 25, 2011 4:47:48 PM
Structure modification and challenges for the unknown and structurally weak facilities with space constrain
INTRODUCTION
In planning for rehabilitation works, the initial activity in preparing methodologies for the said work shall be investigation for the current situation (as per EN 1504) as well as some desk investigations in regards to the history of the structure or target. These are vital pre-requisites to construe that the later proposed methods shall be the best solution in tackling the root(s) of cause and within the financial range which compliment with the value of (repair) work.
Structure repair which is a rehabilitation work will only give provision to make good and/or control the defects which may lead to degradation in strength, durability and robustness of the structure. However, when a major change is required, rehabilitation work shall be part or not even part the scope of works as desired by the client.
To make this article’s analysis and solution challenging, various factors shall be included into consideration during redesigning the proposed work. Therefore, it would best to simulate a water treatment plant renovation with the addition vertical force on the shell or the retaining/bearing wall. Why water treatment plant (WTP) as an example to simulate and to trigger the worst case scenario?
OBJECTIVES AND CRITERIA
The WTP facility is one of the most important facilities which provide clean and treated water to public. This also means that there is no allowable breakdown time during the intended modification execution period. The WTP requires the analysis of lateral forces besides the intended vertical load in taking the model and simulate into Ultimate Limit State (ULS) with the addition of earth pressure as well as partial factors for earth pressure. Aside from that, the geotechnical influences toward bearing capacity, groundwater effects and thus includes slope stability when the facility is using gravity and level as part of the processes. Aside from that, this facility usually is congested with other adjacent functional services and contamination is not given any provision.
The initial plan was very much about addition of CONCRETE BASED covering instead of other (material as) cover types. This is a CLIENT SET CRITERIA TO THE DESIGN and therefore the suggestion of other materials is not relevant in this discussion. If you are talking about light weight material, it would have been using the same solution deployed for water tank at Chiba, Japan to solve this same problem.
SCOUTING AND NOT INVESTIGATION
The whole processes in preliminary proposals are not based on the relevant investigations and there are no archived documentations as point of reference. This will make it rather difficult to consider the accurate capability and capacity of the said facility predominately the intended modification to the open tank. Aside from that, the tank is not an underground water tank which positioned on a bund-like flat platform at the slope and thus makes modification with extra vertical loading a risk for the stability of the slop. The facility has been operating for years and therefore it consist of other installation throughout the site and adjacent to the existing tank, this make it a constrained project side in order to execute work at site and tremendously have affected the possible solutions. There is a small stream 50meter away from the intended work and the tank has the massive capacity of treated water.
The plant has no plan to stop or halt the water supply and their processes and this also will complicate the selection of remaining construction methods. Any intended work should only progress if provision has been taken to ensure the treated water is not contaminated by any debris as the result of proposed modification work. In-situ techniques and protective membrane shall be the last option in mind.
DETERMINATION OF METHODS
Where most of the local contractors and consultants will addresses this issue and work by recommending the addition of trusses or universal beam with precast half slab which load directly on the tank shell by suggesting that the concrete will undertake compression easily. The shell of the tank was constructed for direct lateral load from the stored water and has very low capacity in taking vertical load. Imagine if we applied additional uniform load of 30kN/m² per meter run and the addition of up to 120kN/m² (over 500x500x10 metal plate) point load on the wall. This will be a bad design because this shear will cause flexure (based on design envelop) and leads to rupture to the shell and it has nothing to do with compression capability of the concrete itself.
THE ANALYSIS
From ESTEEM, the design will be correlated due to the limitation of the wall/lift shaft feature in taking vertical uniform load. The correlation applied shall be in the form of 500x500mm beam with 500x500mm column support. Say, the existing water tank shell would behave as the correlated designed beam as mentioned below.
The main reinforcement bar with 3T25 and the Nominal link shall be 2T16-200,
The parameters would be a simple supported beam of 7000mm,
Cover shall shall be 50mm with Concrete of grade 35/C30;
The load shall be derived from the addition of thin precast half slab
Uniform load when the end of the precast half slab rest directly on the tank shell while point load derived from the spliced universal beam or inverted tee channeling the weight of the precast half slab and the beams and tees.
The analysis concludes that the beam would not withstand a point load of 30kN at the center and without uniform load.
Then the analysis concludes that;
The analysis also concludes that the beam would not withstand a uniform load of 30kN distributed along the beam.
The analysis also showed that an even 2 point load of 20KN, at one third and two third of the beam indicates a failure.
By distributing the later said 2 point loads of 20KN into 400x500mm bearing plates also indicates a failure.
This analysis also showed that, regardless of the spacing of longitudinal and latitudinal reinforced bars and mesh at standard water tank, the possible stirrup designed at the top shell are not designed to carry extra vertical load.
The only reinforced concrete structure which will restrain vertical and lateral load would be the abutment and/or wing wall, in this case we encountered something design mainly for lateral force. Therefore it is impossible to have the proposed cover resting directly on the existing tank shell. However, the other two proposed solutions do look very promising unless considerations are taken into account.
GENIUS BUT IMPRACTICAL
The rather congested site which sits at bund and relative close to the non-protected slope (allowance of 1000-1300mm) does not allow excavations for the intended strip footing. From the site scouting, we relative understood that, the slope throughout the perimeter are not protected and stable, thus excavation/trimming will increase the cost which would likely includes a retaining wall. This also will contribute to the change of groundwater profile.
If the case that, a strip footing (very close to the existing slab, almost like jacketing or enlargement of the existing slab) is allowed, the design should consider on the effects of moment by the uplift effect of the groundwater if it is above the relative low level water tank. Couple with moment from the water reservoir, it will result in cracking at the slab or the footing itself. Shall the robustness of the design may prevent it but probably be aggravated by the newly disturbed bearing capacity (under the newly excavated strip footing), the newly erected column will start slide and slant due to swelling effect.
Now, the structure integrity is in doubt, as a whole. Cracking is never been anything but disqualification, non-conformance and problems for a water retaining structure as mentioned in BS 8002:1987 – Code of Practice for Design of Concrete Structures for retaining aqueous liquids.
The extraordinary method of distributing the load from the top to the wall and not through the pile initially is something we would propose for this kind of renovation and addition. However, after scrutinizing the design and bracing method, it is rather skeptical to transfer the whole load of approximate 40 ton from the precast half slab to the shell itself. Even though, this concept may work but it is reckoned that it would lead to cracking and degradation of the cover. The bolt or anchor which will permanently hold the system probably will experience tension, shear and combination of both. The possible single shear experienced by the bolt will not be effective on degrading concrete cover where it would be dislocated by the upward force caused by the moment/torque generated from the new frame withstanding the weight of precast half slab. This will lead to dilapidation of cover due to cracking or worst, exposed the passive concrete to possible aggravation, now the robustness of the existing structure is questionable.
Aside from that, the moment from the frame also clearly showed that, it is assisting the retained reservoir in increasing the flexure of the tank shell. Now, how much shall be spent in order to brace the tank shell directly from the lateral water force and also from the frame moment? This has yet to put into consideration the possibility of the bearing capacity to hold another 40tonnes on 157.7m² raft footing.
WHAT IS YOUR DESIGN SOLUTION?
So what are the possibilities that this cover could be constructed as needed by the client? Therefore, I challenge you in using innovative designs to ensure concrete cover would be added as client primary requirement.
My key solution (in TEKLA15 analysis currently) in solving this rare engineering problem would identified by these keywords; retrofit and non-rigid frame system.