A Program Management Approach : Site Logistics

“The only reason for time is so that everything doesn’t happen at once.” - Albert Einstein

One of the primary challenges associated with super tall construction is the ability to efficiently move materials, manpower, and equipment.?A super tall project exceeds most other project types in the quantities of materials and manpower while facing the same ingress constraints of less tall buildings.?A well-developed site logistics study, taking into consideration owner requirements and schedule, will contribute greatly to the success of the project.??

Key Logistics Considerations

The following non-inclusive list contains items that must be considered on every project, whether super tall or otherwise:

Hoisting.?The ability to effectively move people and material vertically in and out of the building during the project is critical.?The extent to which placing the right manpower, close to the right areas, with the right material is a driving force for the overall schedule of super-tall buildings.?It is standard practice to put the building’s service elevators on the schedule’s critical path so that use during construction will be possible (enabling any externally mounted hoists to be dismantled, cladding completed, fit-out started, etc.).??

Canteens.?Canteens and dining/rest facilities must be located within the tower at reasonably spaced and accessible levels.?Owners should not be paying for labor downtime experienced with workers who would otherwise have to exit and re-enter the building during eating breaks.?Keeping all trades closer to their work areas is critical.

Batching Plant.?If space permits, it is recommended to have an onsite batching plant.?This will save time on deliveries, as many large cities have bans on truck traffic during commuting hours.?This will also allow for greater control of concrete supply and quality.

Onsite Testing Facilities.?These are typically outfitted in a twenty-foot shipping container and allow for onsite testing of concrete cubes, rebar tensile strength, etc.?This saves time for inspectors from not having to be offsite, and it allows for greater efficiency and frequency of inspection.?

Temporary Utilities.?Site drainage, temporary power, and temporary water need to be carefully planned and phased for the transition to permanent utilities during the construction process.?Without power and water, nearly all significant construction activity is unable to proceed.??

Vehicular Access & Parking.?This includes temporary access roads to the site from the main adjacent traffic arteries.?Access roads to the construction areas and to the site offices, lay-down areas, canteen facilities, and general assembly areas must also be considered.

Cranes.?Schedule requirements and floor plate size dictate the number of tower cranes. Structural work takes up the majority of crane time for super-tall buildings and, given that most projects allow no more than a four-day cycle per floor, it is common to see three (3) or four (4) tower cranes on supertalls.?Crane capacities are determined by the weight (and reach) of the heaviest lift. The cable drum may need to be custom designed, and if so, they may need to be procured at an earlier stage (and often imported).?Mega columns, outrigger elements, mechanical equipment, or building maintenance units are the “usual suspects” for the heaviest lift.?

Cranes must be placed within the constraints of the building design and the construction sequence.?Cranes and hoists are the arteries of high-rise building construction, and a comprehensive study is required before selection. Consideration of all trades, and in particular for the fa?ade, should be started early and decisions made whether the cranes will lift the fa?ade panels or whether oversized hoists will be used.???Finally, many supertalls will keep the final height undetermined and allow a provisional sum for additional height. In conflict with any guarded secret around height is the need for cranes to “know” well in advance what the final height will be.??

Case Study, Cranes at Burj Khalifa

Burj Khalifa installed three (3) tower cranes attaching the center core (Favco M440D, M380D, and M220D with capacities of 25 tons, 19 tons, and 11 tons, respectively).??

The heaviest pick for the tower cranes were the link beams that weighed up to 20 tons. Each crane was paired up with one of the three jump form systems and had to climb every four (4) floors as the center core progressed. The cranes sat on specially designed steel frames connected to embeds cast in the core walls every four (4) floors.??

Each tower crane was equipped with high speed lifting technologies, allowing cables to raise and fall at a rate of approximately 22 meters per minute.?Tower crane 1 (25 ton) was kept at the very top for the final spire erection procedure, a process that would take several months to assemble and jack into place. The base of tower crane 1 was located at Level 152, two levels below the transition of the Burj structure from reinforced concrete to structural steel.??

Though at this level the tower crane stopped “jumping” up the building, its height would continue to increase by inserting 38 masts with five levels of lateral support connected to the main structure to its final height of 700 meters, where it was able to reach both the ground level and spire.?Net hoisting minutes for a pick from ground level to 700 meters were approximately 12.7 minutes, placing even greater importance on ensuring the other facets of the project logistics plan were being enforced.?One simple mistake could cause hours of delay.

Tower Crane Dismantling.?Dismantling tower cranes requires as much planning as setting them up in the first place.?At the Burj Khalifa, the smallest of the three main tower cranes was actually dismantled and then reassembled to dismantle the largest tower crane.?Additionally, two recoveries (far smaller capacity) cranes were installed, dismantled, and reinstalled at various levels (using the building setbacks as working platforms) several times to assist in tower crane dismantling.?When the structural steelwork at the spire was completed at Level 160, tower crane 1 was used to dismantle tower crane 2. The recovery cranes and tower crane 3 were then used to dismantle tower crane 1, and the recovery cranes alone dismantled tower crane 3.?Recovery crane members were disassembled manually on the building setbacks and then taken down to ground level using the building service lifts.?It was a remarkable, time-consuming, and critical path activity to manage, and the overall process is illustrated in the two graphics below.?

Case Study, Hoists at Taipei 101

Since the cranes were predominately used for the 107,000 tons of structural steel works at Taipei 101, the contractor installed six (6) oversized high-capacity hoists, each capable of carry three tons or 30 people.?Most of the facade panels were transported by these hoists during the night.?The hoist climbed along the single mast from ground level to Level 91 (393 meters), achieving another world record of the highest construction hoist. It would take four (4) minutes, non-stop, from the ground to Level 91.?

Jump Hoists

The downside of exterior construction hoists is that they create temporary openings in the buildings that require come-back work. Until the hoists are dismantled, the building will not be completely enclosed, which impacts the completion of interior fit-out. Weather can also impact the operation of the hoists; rain and high wind can stop the hoists, interrupting the transportation of people and equipment. Most hoists travel at a speed of less than 1.5 meters per second. With frequent stops, the potential wait times will be long for super-tall buildings. Accordingly, a new solution for moving people up and down more efficiently has been developed.?The idea is to utilize a temporary machine room, which is self-climbing and jumps as the building progresses, inside the permanent hoistway. The permanent rails, shaft, and door equipment are used during construction. When the core reaches its final height, the temporary machine room will be replaced with the permanent one and the lift is converted into final service.?As the lifts, now commonly called jump lifts, are inside the building, with a proper roof installed at the top of the shaft, they are able to operate without interruption due to weather.?Even though these lifts are powered by temporary motors, they still travel three (3) to four (4) times faster than conventional hoists and improve people flow during construction. If the external hoists are no longer required, fa?ade work can proceed without the need for any come-back work.?The jump lifts do have limitations. Their size normally can only accommodate people and hand tools, leaving all materials and equipment hoisting to the tower cranes, which could impact the cycle time of the construction.??

?Lean Construction

There is a movement in construction to increase efficiency and safety by reducing waste in various forms.?Waste must first be identified, as it can come in the form of overproduction, wait time, inventory management movement, processing, effort, and under-utilization of labor.?While lean is a broader people development and measurement system, elements specific to the practice of lean in construction are highlighted herein.

Material Production & Delivery.?Excess inventory creates a housekeeping and logistics problem.?“Just in time” (JIT) delivery and concepts such as “nothing hits the ground” where construction tools and materials are placed on pallets with wheels for ease of handling are such approaches that can remedy and reduce inefficiency.?Close coordination among subcontractors on schedule, material lead and delivery times, and availability are all efficiency inputs and, if established early in a super tall project, can create predictable production rates while reducing onsite stored material (which takes up space).?Workshops should be established during preconstruction, as well as during construction, to constantly evaluate areas that can be continuously improved or modified.?

Prefabrication.?Prefabrication is another Lean tool, which though it is not a new concept, can be extremely effective in saving time, saving money, and improving quality.?Examples include assembling as much rebar as possible, either at the ground level or in the production facility in advance or using prefabricated bathroom units.?This will keep the working areas within super tall towers more clean and free of debris.?The same goes for larger members (nodes, outriggers, transfer beams, etc.), which can be more easily (and safely, and efficiently) assembled in locations other than where they will be finally installed.