Under the PPVC: tower cranes
By Alex Dahm and Heinz-Gert Kessel13 July 2020
PPVC (prefabricated pre-finished volumetric construction) is becoming increasingly popular. It requires high capacity tower cranes
Prefabrication is a broad category that refers to a construction process where building elements are constructed off-site and then shipped to the construction site for installation. Generally, prefabrication can be categorised as either two-dimensional building panels (2D), like walls, floor systems, or three-dimensional modules (3D). This is where an entire unit of a building is constructed off-site.
Fully fitted out modular units require the least amount of on-site construction time as all plumbing, electrical, internal finishes and fixtures have typically already been installed at the factory. This PPVC (prefabricated pre-finished volumetric construction) method increases productivity and sustainability of modularisation to its highest level in comparison to traditional on-site construction methods.
1.) Savings in construction time: The most obvious benefit is saving construction time. Site establishment, earth work and foundations can all be in progress on site at the same time as modules are fabricated in the factory off site. A six-storey hotel, for example, can be built on the factory floor in three months and assembled in only 20 days on site. This time saved has financial benefit, as there is a faster return on equity as rental revenue.
2.) Quality assurance: No weather-related delays affect factory-based module construction. Assembly line production and robots can raise building construction into today’s industry standard of productivity.
3.) Safety: More offsite construction means fewer hours working at height.
4.) Traffic reduction: Modular construction can reduce the number of site deliveries by 90 percent and decreases the average travel distance of workers to the site by 75 %.
5.) Overcome recruiting problems: 80 % of the traditional labour activity can be moved offsite to the module manufacturing facility. For this workforce the daily commute remains unchanged from project to project while the few experts to fix the modules on site can be offered an attractive skilled job.
6.) Reducing noise, dust and carbon dioxide pollution on site: The construction phase on site is reduced as is the amount of heavy equipment. Electric tower cranes for module placement minimise the noise and pollution impact on the neighbourhood and environment.
7.) Waste and space reduction: Modular construction cuts net waste in half compared to conventional construction. Modules delivered just in time under the crane hook reduce the requested storage area for construction material.
8.) Sustainability: Nearly 90 % off site industrialised module production has great potential for green and lean construction, reducing the greenhouse gas emission during fabrication as well as during the lifetime of a building.
Cost drivers for PPVC are:
1.) Module transport restrictions: Restrictive road regulations, abnormal load transport permits and transport restrictions on inner city sites sets economic site limits. Overall transport size is restricted to 3.4 metres wide, 3.2 metres height and 12 metres length. Maximum weight is 30 to 40 tonnes. Transport is 20 % of the PPVC cost drivers tending to increase.
2.) Crane size: Above six floors tower cranes are preferred due to the space needed for erection. Two types of PPC module system are in general use. The weight of steel modular units is between 8 and 20 tonnes while concrete modular units weigh 25 to 35 tonnes. Concrete modules are often preferred in residential buildings due to durability and ease of inspection. Less carbon intensive module construction will see more hybrid and light timber modules because the production of cement accounts for an estimated 5 % of global carbon dioxide emission.
Adding to the module weight will be purpose-built self-balancing lifting frames. These prevent the introduction of inward forces into the modules and accommodate the individual centre of gravity of every module for precise positioning. For these reasons PPVC projects require high capacity tower cranes. Experience in Singapore shows the initial cost and rental rates of tower cranes above 30 tonnes capacity is responsible for 15 % of the PPVC project total. Recent tower crane developments, however, for the PPVC market will help contractors to keep crane costs of upcoming projects more easily under control.
Projects delivering the greatest cost savings with the PPVC construction method are those with the highest level of repeatability. Examples include student accommodation, hotels, residential towers and affordable homes. These are exactly the type of building for which demand is rising as a result of the global trend for urbanisation.
PPVC construction can be the solution where there is a labour shortage, where traditional construction is expensive and where there Is a need to build rapidly, due to extreme housing shortages. Housing is needed in Australia, France, the UK and the USA, which recently announced one million housing units to be built by 2020. Governments in Australia, Singapore and the UK are making offsite construction a strategic priority. Singapore’s Housing and development Board (HDB) adopted the concrete PPVC method for 35 % of its projects in 2019. In Australia the government is seeking to create “twenty-minute cities”, so-called because of the travel distance in which people can fulfil their daily needs. It will increase the number of high-rise residential towers to reduce the carbon footprint. Other municipalities around the world will follow, again boosting the market for PPVC projects.
Singapore is the biggest testing ground for modular construction today. Early experiments elsewhere revealed the challenge for the lifting equipment. The 12-storey Habitat 67 apartment complex built for the Expo in Montreal, Canada, in 1967 is a manifestation of the potential of prefabrication. The 354 concrete modules, each weighing 80 tonnes, were placed using a Dominion Bridge rail-travelling, pedestal-mounted purpose-built stiffleg derrick crane. It offered 82 tonnes of capacity at a radius of 30.5 metres and 100 tonnes at 18.2 metres. To level the concrete modules at each corner of the square Francon lifting frame, an individually operated hydraulic jack was installed at the end of the lifting rope.
Also in 1967, another PPVC project started, in San Antonio, Texas, where its economical design and short construction time are notable. The 22-storey Hilton Palacio de Rio hotel was completed in just 10 months. Modular construction cut this two-year job to nine months. A total of 496 guest room modules were placed in 46 days using a 300 tonne Manitowoc 4000W ring crane. It had a customised 50 tonne capacity, 82 3 metre boom with canted top to increase usable load height. The fully fitted out 35 tonne modules, including furniture, were lifted into place within an accuracy of ¾ inch (19 mm) using a purpose built load spreader with Helicoguide counterbalancing. This consisted of a Sikorsky helicopter motor and 60 h.p. engine.
Instead of the planed 10 complete rooms a day, builders ended up lifting in 35 modules a day, illustrating the value of having the right lifting device to achieve full benefit from modular construction.
On today’s PPVC projects a twisting module is usually put into the right orientation by people pulling on taglines. It is an inefficient approach needing too much manpower and can be dangerous. Instead of taglines like the Helicoguide of old, an all new suspension device using gyroscopes can be used. Several products using gyros to control load rotation and eliminate taglines have been launched in the last couple of years. Japanese tower crane manufacturer Kitagawa has one called Gyro-master. The modular Everest series self-contained spreader developed by the Verton Group in Australia uses a remotely operated variable effort rotator to control suspended loads up to 1,500 tonnes.
Remote control helps improve safety by removing riggers from the lifting zone. Cycle time is reduced by as much as 25 % and fewer riggers are needed on site.
Kitagawa offers units up to 75 tonnes capacity and there are already units in used in Japan up to 125 tonnes. This equipment should be used with a self-balancing lifting frame to minimise time attaching the load and exact positioning. Such a lifting collar frame can also offer multiple slinging points to cope with the module’s structural integrity. It also means no need for a Christmas tree-like arrangement of lifting beams increasing the required head height of the crane.
An idea behind Tokyo’s Nakagin Capsule Tower of 1972 was that each of the one-room pods measuring 2.5 x 2.5 x 4 metres and bolted using four high-tension bolts to the lift core, could be replaced any time without disrupting the rest of the building. Modules were intended for replacement every 25 years which gave the flexible structure a life expectancy of 200 years. Unfortunately, no lifting device was developed to insert the capsules so only the top capsules can be removed. Replacing a lower one requires removing all above it.
Moving up to date, the hot spot for PPVC projects has been Singapore since 2016. It was driven by an ambitious government roadmap to improve construction industry productivity. Initially the light weight modules could be handled by 300 tonne-metre class tower cranes. Module weight has increased dramatically in the last three years to reduce cost and raise productivity.
It has severely impacted tower crane size and design. At first high capacity models were unavailable so cranes designed for industrial projects had to be adapted. In the last year new models have been appearing to address the needs of PPVC.
Singapore is a typical tower crane rental market. Some contractors like to operate high capacity tower cranes in pilot projects but tower crane rental companies can only realise the amortised cost after a few more PPVC projects, before the savings can be passed on to developers.
To improve the availability of large cranes there must be confidence in the future of concrete modular construction. At present authorities in Singapore prefer the heavy concrete modules for residential buildings. In other regions concrete structures are viewed less favourably in terms of carbon dioxide emission. Lighter hybrid modules incorporating timber and steel are preferred and the cranes can be smaller.
Each construction site needs its own crane concept but there are some general rules for a common high-rise residential tower construction:
1.) If possible trucks delivering the modules should be unloaded close to the building and the tower crane be positioned in the centre of the site to minimise requested working radius and capability of crane. Unlike conventional precast, PPVC modules are usually too big to be stored on site. Just in time transport and installation could be most efficient.
For reinforced concrete modules, including load spreader, capacity between 28 and 45 tonnes is needed, to a working radius of 35 metres, depending on the crane location and building shape. That translates as an 850 to 1,600 tonne-metre capacity class machine. For steel modular and hybrid units about 15 to 25 tonnes lifting capacity is required to a working radius of 30 or 35 metres. That means 450 to 900 tonne-metre class cranes.
2.) To reach all the site the capacity needs to be available right out to the end of the crane’s jib.
3.) Rental periods are shorter with PPVC than conventional construction. In Singapore conventional construction of a typical residential tower is about 16 months. PPVC, typically with 2,000 modules per residential project, will reduce jobsite duration for the crane to 8 to 12 months. Building a podium or lift shaft, by slip forming concrete, can be done with smaller cranes, thereby reducing the time needed for the large cranes. Considering the costs for mobilisation and demobilisation, however, it is still usually more economical to choose one size crane for the entire project.
4.) In Singapore tower cranes are not allowed to oversail neighbouring sites. Short counter jibs and flexible jib length are therefore required. In some cases, the out of service slew lock has to be used, which is easier in very short jib conditions.
5.) Minimising environmental impact during construction is becoming more important, especially on inner city sites. In Singapore working hours in the construction industry are stringently regulated. No work is permitted from 22.00 hours on Saturday until 07.00 on Monday. Noise limitations include a 1 hour limit for continuous noise of 65dBA or higher within 15 metres of residential zones. Carbon dioxide emission and transport traffic to the site have to be reduced as far as technically possible.
6.) Luffers show true value in very tight areas. In most cases, however, a compact hammerhead crane with reduced jib length and adapted to the site conditions is more cost effective. It is up to 40 % less to manufacture and is faster to rig. In addition, these cranes generally produce lower reaction forces, have lower power consumption and can simplify anti-collision management.
7.) Where hammerhead cranes are used, a flat top design is preferred due to, in most cases, faster set up times and lower overall height. A flat top crane is easier to change jib and counterjib lengths because there are no pendants.
8.) Handling modules on a typical PPVC construction means using the maximum load moment of the crane for most of its working time. Using the full length of the jib means a single trolley is preferred. In addition, it is preferable to use all the available jib length to place modules al the way to the maximum available outreach of the crane. Hence when using a saddle jib crane a single trolley is preferred instead of a double trolley arrangement common on other heavy lift tower crane designs.
9.) Precise lifting is more critical than speed. Soft start frequency inverter systems with micro move control are in demand. Also popular are a camera system to follow the hook block and one to show hoist rope spooling. Using smart GPS sensors on the lifting frame allows transmission of real time information to the construction software. Data logging allows interrogation of the entire crane operation via the internet. Nanyang Technological University researchers had such a smart crane at the Signature at Yishun condominium site. Each precast module had its own radio frequency identification. It could be tracked from leaving the factory to being hoisted into place. A computer model calculated the ideal lifting path for the crane for each load. Added together productivity was up by between 10 and 20 % and time savings of up to 30 % were recorded.
10.) Singapore allows a maximum free standing height of 40 metres. A maximum of 40 metres is allowed above the last tie back to the building in other regions so high free standing capacity is a popular request to save money on ties. For road transport a maximum component dimension of 3 x 3 metres is requested.
On the job
The first 40 storey towers of the Clement Canopy project in Singapore were 18 to 32 tonne concrete PPVC modules. They were placed by two standard Liebherr 1000EC-H40 Litronic cranes with 40 tonnes capacity at 27.1 metres radius. Jibs were 34.5 metres with a double trolley. Standard features such as micro-move, EMS electronic monitoring and the 17 metre short counter jib qualified this crane for heavy PPVC construction work back in 2017.
At the same time two more 1,000 tonne-metre class units were available in Singapore. These were Krøll K1000s, with 40 tonnes capacity at 26.2 metres radius but using a single trolley. They had a custom built 18 to 24 metre short counter jib instead of the normal 27 metres.
During the next development stage of PPVC construction, in 2018 and 2019, crane cost control became as important as raising the load moment to cover a wider working radius as well as making the cranes more compact to serve overlapping construction areas with a number of cranes. Flat top models here proved their worth.
Rental specialist Tiong Woon Corp. (TWC) ordered 20 units of the Yongmao STT1330 and STT1830. The STT1330 is available in 64 tonne and 50 tonne versions. It can lift 64 tonnes at a 21 metre radius. Capacity is 34.25 tonnes at the end of the 35 metre jib. The STT1830 lifts 64 tonnes to 30.5 m and has a tip load of 46.6 tonnes on a 40 metre radius.
According to site conditions the counter jib can be reduced to 15.2 metres for jib lengths up to 40 metres. Latest of the big Yongmao models is the all-new STT983-50t with a 15.2 metre counter radius, 50 tonne capacity (4 falls) and 22.97 tonne tip load at 45 metre radius. More than 20 units are scheduled for delivery to Singapore in 2020.
More for TWC
In 2019 ten more heavy lift flat tops were ordered by TWC, this time the T1200-64W from Zoomlion. The 17.1 m counter jib version allows the 64 tonne capacity crane a tip load of 37.18 tonnes at 35 metres radius.
Comansa has a long record of success in modular construction with its 2100 series. Since 2018 the 50 tonne capacity 21LC1050 comes with six modular counter jib lengths up to 20 metres for 35 metre jib radius. At 30 metres outreach the capacity is 43.67 tonnes. In 2019 the 21LC1400 was added, with PPVC construction in mind. It has a modular counter jib so at 18.6 metres long, with 34.4 metre jib, the tip load is 48.4 tonnes.
Jaso has sent four of its 64 tonne J1400 giants to Singapore. Designed in 2019 this crane offers four modular counter jibs, with an 18.1 metre minimum radius. Capacity is 36 tonnes at 40 metres radius. Despite its size the complete crane can be shipped in standard high cube 40 foot containers.
At the time of writing a smaller version of the Jaso, between the J1400 and J700, was under development. It will also offer four counter jib lengths to accommodate PPVC site restrictions.
Another new entrant to the Singapore market will be the flat top Krøll K860F, a reinforced version of the K830F. It retains the larger model’s design optimised for transport and rigging. Capacity is 48 tonnes at 19.6 metres radius and 24.8 tonnes can be lifted at 34.9 metres radius with the newly-designed compact single trolley. On their first job site four cranes will be rigged with just 20 metre and 34.9 metre jibs and counter jibs of 16.1 metres.
Wilbert TowerCranes sees a market for PPVC projects coming up in Europe. Capacity of the all new WT1000 e.tronic is 48 tonnes at a 22 metre radius and 26.5 tonnes can be handled at 40 metres.
At even more cramped sites in Singapore, luffers like the new Yongmao STL1400A, with 50 tonnes capacity at 27 metres, or 34 tonnes at 36 metres, are in use. Electrically driven Favelle Favco and Krøll luffers are also sound alternatives for heavy modular construction. On the drawing board in Japan is a Kitagawa luffing jib tower climbing crane especially suitable for heavy PPVC projects. It will boast a compact 8 metre tail radius, 64 tonne capacity out to 36 metres radius and 30 tonnes to 51 metres in 4 fall operation. It will sit on a 64 metre free-standing and compact 2.5 x 2.5 metre tower system.
In Europe, the UK’s construction industry has shown the most advanced way to raise residential towers following PPVC principles. It is demonstrated in London on 101 George Street’s 135 metre-high, 44-storey building of 546 apartments. It comprises 1,524 hybrid steel modules with concrete bases lifted into place using two Terex CTL650F45 luffers. These cranes offer a 42 tonne capacity on three falls mounted on top of the twin cores completed in advance using slipform construction.
On this job a one-bedroom apartment is built from two modules, one for the living room and kitchen and the other for the bed and bathroom. The 23 differently configured modules are 95 % finished when they arrive on site. Delivery and installation is at a rate of 10 to 12 modules per day in a 32-week programme. Mast climbers are used to fit the cladding which frees up the cranes for installing the modules. The project is scheduled for completion in May 2020.
More environmentally considered modular construction materials are being tested. A 14-storey timber-framed luxury apartment block in Bergen, Norway, was erected using a Potain MD365. It placed the 14 tonne timber modules and the 12 tonne pieces of laminated timber framework bracing the building structure. Completion was in 2015 and now another spectacular 18 storey wooden apartment building is on the drawing board. Toronto Tree Tower is inspired by the Montreal habitat 67 building. This time, however, its prefabricated modules of cross-laminated timber are designed to be put together in a similar way to the bricks of a Lego toy construction set.
Most PPVC projects show a 50 % reduction in construction time at the building site when compared with traditional construction but they end up being just 10 % cheaper. It is not only due to the large lifting equipment required. More than this it indicates a need to raise productivity simply by doing more PPVC projects and making modular construction mainstream.
Modular tower crane design has allowed even the requirements of Singapore’s demanding market be quickly addressed by most major crane manufacturers. For 2021 a slower supply of new large tower cranes for PPVC construction can be expected in comparison to the boom years of 2018 and 2019. In other core markets, however, labour shortages and large-scale unmet demand for housing will intersect and make the idea of modular construction particularly relevant. Experience collected in Singapore and by other PPVC pilot projects around the world can help keep costs for efficient and, at the same time environmentally conscious lifting equipment, under control.