High-rises needs special attention to lateral loads. Tall slender (Figure 1, left) buildings are in compression due gravity, but significant horizontal loads make it meanwhile a “cantilever” too. Most simple is a single cast monolithic core for lateral resistance, usually accompanied by columns dispersed around them (Figure 2, left). However, external constraints and usage might lead to combination of different frame solutions and limit or separate cores or allow only columns with bracing. High-rise slabs and joints on the other hand need stiffness, and also support significant construction (temporary) loads, so often they are cast on site, (Figure 1, right)
Figure 1. Left: Slenderness, roughly: b > H/6 not significantly slender, b < H/8 already need very careful attention to lateral resistances; Right: Loading from construction materials and activities may be dominant imposed load
In design, symmetric and centered core is ideal. Core will carry most lateral and big part of vertical load but can deflect very differently from columns that have high compression. Core fits stair, lift, technical and other areas needed on each floor, and are best casted with progressing formworks (Figure 2). Casting stiffening or diaphragm slab can be relatively fast using latest state-of-art formwork, lifting, casting, connection to support and punching shear resistance. Precast and composite methods can be used for all parts possible to further increase speed.
Figure 2. L: Various cores of wide but slender high-rise casted with climbing slip-formwork systems – R/bottom: climbing forms
Typical vertical construction challenges
- Enclosure (provide safety with access on perimeter and prevent objects from falling)
- Access and stairs (symmetrical centered core accommodating elevator, ready as early, is ideal)
- Crane (tower crane: frame support, water tightness issue; self-climbing requires slab opening)
- Boom (for casting: climbing, self-climbing/erecting or mobile on slab)
In addition special shape (See end: Precast in high-rises), and many openings in slabs for construction purposes (lifts, cranes, casting boom, electricity, water) or installations (HVAC), where prefabricated elements can allow partial casting. (fib 73 chapter in end for details)
Beneficial flat slabs could be pursued with high-rise building with stiff cores:
- post tensioning keeps slab straight under permanent loads, and bubble-deck cavities can further optimize slab weight, while shear rails enhance resistance at supports, this can be cost effective, but slow and tricky to tense from outside, and over supports, which can cause restraint
- precast slab can be done with bolted concrete or slim steel composite beams supporting hollow core or composite plank slab units: hollow core units minimized formwork and propping and concreting, camber is provided in steel section or concrete pre-tensioning
- thickness, diaphragm/lateral robustness design and cranage planning demanding
Figure 3. Slim floor in multi-storey. Composite columns (right) are expensive but can work for high-rise bottom floors or high compression zones, when reinforced concrete structures need stiffer or slimmer alternative (Note! Tube is a formwork too)
Post-tensioning precast slim floor handle gravity loads – additional stiffness need additional reinforcing against axial and uplift forces. Designing joints as reinforced chords and significant top-concrete of 75mm or more is used to ensure diaphragm action. (ACI/EN?, fib 73 - chapter 4)
Figure 4. Hollow core unit deck diaphragm action and chord reinforcing principles can be seen from precast frame handbooks
Costly precast hybrid structure with specialized connection design and supply can have hidden added value by compensating in formwork, propping and hook time savings. A LEED platinum (reference?) case finished 2019 in Frankfurt had secondary moment connected beams to balance girders, enabling relatively quick and slim hybrid slab system (Figure 5).
Figure 5. Hybrid or composite slab can utilize precast in beams or slab manufacturing and installation
Many high-rises in Australia are constructed on cores and simply supported relatively fast monolithic slabs. Threaded speed starter bars are tightened directly into anchors in grooves of concrete wall. This has also proved to add safety and avoid back injury, apart from slab pour speed and quality. High seismic or cyclonic area where slim structures are required might require all connections however rigid, If precast speed is needed, corbels and bolted moment connections increase.
Figure 6. Fast connection on walls of simply supported monolithic slab
Figure 7. Hand-made shaft, 3D model cut from precast shaft with stair/landing hanged on hidden corbels
Each high-rise building is every time a challenge for designers and builders. Aiming for the skies , they are the pyramids of our time. Most of time we build much more modest buildings for urgent purposes, but every now and then, sophisticated connection solutions and construction innovations have a chance to get into the broad daylight riding the skylines.
References
Fib 73 bulletin (from concrete structure publication series) – Tall buildings, “core” -chapters for this text:
- Intro: short history, table 1.1 for tall building definition
- Design, considerations: Figure 2.1 slenderness, 2.5 core, 2.5.1 Usage, 2.6-2.9 challenges from height to transport, function of building, buildability, drift (even dynamic, sway, collapse etc.)
- Frame combinations and principles in short
- Structural elements – 4.1 optimum floor; Columns: Figure 4.1 eccentricity, Table 4.2 shape comparison
- Foundations (geotechnical and structural FEM iteration in cycle)
- Buildability - construction: 6.1-6.3: form, core, members, 6.4 slab (in-situ, some hybrid/PC etc.)
- Loads: Figure 7.1 erection stage loads matter
Adjusta page/references/video
PSB page/references/video
DELTABEAM® page/references/video
Slipform & boom & precast construction references
Fib 74 bulletin – Planning and design handbook on precast buildings (link for purchase)
Precast frame analysis, Kim S. Elliott, 2016 (link for purchase)
Project links
William Street tower: Peikko references, 1williamstreet.com
The Jewel: Peikko references, jewelgc.com
Other Peikko high-rise references: Azure (2016, Canada), The Silo (2015, Denmark), Glasshouse (2015, Canada), INIT headquarters (Germany 2014), Renaissance II (2013, Canada), Aarhus City Tower (2013, Denmark), The Shard (2009, UK)
Precast in high-rises: Sede Unica (2011 Italy), OMNITURM (2019, Germany)
