Top down construction involves constructing a building or portion of a building in the reverse order compared with conventional construction techniques. Typically this is construction of upper level elements followed by lower level elements.
Top Down Construction Philosophy
In construction, like most things in life, time is money. In project management terms, you may hear of items which “lie on the critical path”. The critical path of a project is the chain of events and tasks which must be completed sequentially in order to finish that project.
When constructing a building, the majority of the structure is a critical path item. You can’t lay the carpet or install the doors until you complete the foundations, columns then slabs. However in high-rise buildings the majority of the façade can be constructed off the critical path, the lower level façade can be erected while the structure of the upper levels are being completed above.
Because the time to complete the base structure heavily influences the projects overall duration, finding ways to reduce the length of time it takes to complete these critical path elements can save significant time and cost.
This is where top down construction comes into play. Top down construction allows the construction of multiple structural elements concurrently.
Top Down Construction Methodology for Basements
You may see top down construction most commonly used for basement construction. The basement construction can take up a significant portion of the overall construction time for a project. For a thorough look at the types of basement construction and piling in general, take a look at THIS article.
During the first phases of top down construction, the basement wall piles are constructed and the associated capping beams. Construction of these elements occurs at natural ground level.
After the piled walls are complete, the columns are constructed. There are two options for constructing these columns for top down construction:
- Pile columns
- Plunge Columns
A pile column is simply a conventional pile which has been designed as both a pile foundation as well as a column. The piles are constructed from ground level and are constructed deep enough to extend beyond the lowest basement level with adequate socket length to provide foundation support. Sufficient reinforcement is provided in the full length of the pile for it to later act as a column within the basement floors once the soil is excavated.
The downside of pile columns is they are often large in diameter. They also have a rough concrete finish due to the concrete being cast against the soil during their construction. There is also larger tolerances for pile construction meaning that the final column location within the building may not be as accurate as conventional column construction.
A plunge column uses a slightly different technique:
- The pile hole is dug to the required toe level
- Wet concrete is placed in the dug hole to the required level as well as the reinforcement cage.
3. A plunge column frame is installed around the pile hole. The plunge column frame assists in ensuring that the later installed column is level and plumb.
4. The plunge column is inserted through the plunge column frame and into the dug hole
5. The column is attached to the plunge column frame and is lowered into the still wet concrete.
6. Final adjustments are made to the plunge column frame to ensure that the column is in the required position and is square.
7. Once the concrete is set, gravel is backfilled around the column (between the column and the dug pile hole) to ensure that the column remains plumb.
8. The plunge column frame is removed and the plunge column construction is nwo completed.
Returning to the top down construction sequence for the basement, after the columns are constructed the ground floor slab is poured. One or more temporary access openings is provided at ground floor to allow the removal of soil and installation of building materials both in and out of the basement.
After the ground floor slab is completed, excavation of the basement can commence beneath the slab. Because the full depth of the basement needs to be fully excavated, temporary ground anchors are usually provided to temporarily support the basement walls until the slabs are completed.
As excavation of the basement continues deeper, the upper floors of the tower can continue to be constructed.
Eventually, the basement excavation bottoms out at at the required level. The lowest basement floor is constructed, followed by the next floor above and so forth. As the basement floors are constructed from bottom to top, the tower levels above ground floor continue to rise also.
Eventually the basement floors are completed. As the basement floors are completed, the ground anchors are “de-stressed” progressively. The building tower above continues to climb after the basement is fully completed.
Top Down Construction for High-Rise Buildings
Top down construction can also be adopted for elements of the building above ground level.
A mixed-use high-rise tower may contain multiple usage across its height. A building might range from apartment buildings in the upper tower, split by an amenities level (swimming pool and gym), commercial offices in the podium all above a large entrance lobby at ground floor.
All of these different uses require different column arrangements. having a totally column free apartment floor would result in excessively thick tower floor slabs, meaning significant cost and mass to the building. However the entry lobby experience at ground level would be negatively impacted if it were filled with a forest of columns.
This often results in a complex transfer structure within the podium floors due to the miss-alignment in grid arrangement between the ground floor and tower levels. The transfer structure may be full depth trusses, very deep transfer beams or multi level transfer walls. Transfer structures can be costly and time consuming to build.
This arrangement makes top down construction an ideal solution to reduce the overall construction time for a multi-use high-rise building such as this example. The aim is to accelerate construction to the typical repetitive tower floors by skipping over portions of the podium floor transfer zone. Then the transfer levels and tower floors are constructed at the same time in parallel.
Lets consider an example of this, its a development over rail. It is not possible to continue columns through into the rail corridor.
This results in a very large span which would be inefficient to repeat on every floor above the rail corridor. Therefore a transfer is introduced, a form of spanning truss with a depth that takes up multiple floors (three in total).
This arrangement requires hanging columns and non-typical framing within the transfer zone. The floors immediately above the transfer zone are a series of office floors for a high-rise tower. The transfer zone can be constructed with a top down approach in more or less a similar way to the basement example explored earlier.
The process is to construct the structural “skeleton” of the transfer zone. The structural skeleton is the structural members which are required to immediately support the tower floors above. Once the skeleton is in place, the tower levels can be commenced while the slabs and secondary structure which make up the transfer zone are constructed in a top-down fashion.
Top Down Construction Advantages
The top down construction method doesn’t suit every project. However the projects which benefit from using this methodology can enjoy the following advantages:
- For a basement application, it allows the foundations to be constructed at ground level. If piles are proposed as the foundation system it provides a number of benefits for, firstly the piling rig does not require demobilisation and re-mobilisation between the basement wall construction and the foundation construction. This can make the piling process more efficient, especially if access to piling rigs in your local market is restricted. It also means that the piling rig doesn’t need to be dropped into the excavated basement hole with a crane or driven down lengthy temporary soil ramps.
- In terms of safety, a top down constructed basement provides better protection to the general public. Before the excavation begins on the basement dig, the constructed ground floor slab provides a protection lid preventing un-intended access to the basement.
- In wetter climates and seasons, having a lid on the basement excavation means that excavation works are not as effected by inclement weather.
- The basement lid can also reduce dust and noise through excavation activities.
- The biggest benefit to top down construction for basements is that the construction time can be greatly reduced provided the right soil conditions, building heights and geometries lend themselves to using this type of construction sequencing.
Top Down Construction Disadvantages
Ever wonder why top down construction isn’t used more widely than conventional bottom up construction techniques? here is a list of some of the disadvantages of top down construction with can lead it to not being a preferred option.
- If there are access restrictions to the site, there are only so many concrete trucks, concrete pumps and reinforcement deliveries which can be accommodated at any one time. If the top down portion of the project cannot be built truly parallel with the bottom up portions of the project it begins to lose its value.
- The sequencing, logistics and management of this type of construction can be more complex and require more resources to get it to work.
- This construction type works better with projects of larger scale. If the time savings are not substantial using top down methods the added complexity costs are not offset enough by the time savings. It is important to note that constructing a basement with the top down approach takes longer compared to conventional construction sequencing, the real saving is in the overlap portion of building the tower along with the basement portion.
- For top down basement construction, the columns are required to be oversized due to their slenderness in the temporary condition while the basement floors are being constructed, this results in more cost being added to the structure.
There are other examples and reasons to use top down construction on different projects. If you have had experience in using top down construction on your projects feel free to share your experiences and insights via a comment below.
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