ROSENWASSER / GROSSMAN CONSULTING ENGINEERS
RGCE HIGH-RISE CONCRETE ENGINEERING
EFFICIENT STRUCTURAL DESIGN
STRUCTURAL EFFICIENCY IS NOT ABOUT MAKING EVERY STRUCTURAL ELEMENT AS SMALL AS POSSIBLE.
The most efficient concrete building is the result of hundreds of interconnected decisions. Core configuration affects the lateral system. Column locations affect the architecture, façade, MEP systems and transfers. Slab thickness affects spans, punching shear, column locations, building stiffness and the overall height of the building. Optimizing any one of these decisions in isolation can create inefficiencies somewhere else.
RGCE approaches structural efficiency from the perspective of the entire building. Our goal is not simply to minimize pounds of reinforcing steel or cubic yards of concrete, but to develop a structural system that balances material quantities with architecture, constructability, floor-to-floor height, construction cycle and overall project cost.
Sometimes the most efficient solution means using less structure. Sometimes it means putting a little more structure in the right place to eliminate much more somewhere else.
IT STARTS WITH THE CORE
THE CORE IS THE BACKBONE OF THE BUILDING
The concrete core is the backbone of a high-rise concrete building. Establishing the right core configuration early in design is one of the most important decisions affecting the efficiency of the entire structural system.
The core must be appropriately sized and proportioned to resist wind and seismic forces efficiently while controlling building drift and movement. Its location is equally important. A properly located core can minimize torsional response and distribute lateral forces efficiently, while a poorly configured or located core can require additional structure throughout the building to compensate.
RGCE works closely with the architect early in design to develop a core that satisfies both the structural demands of the building and the architectural requirements for elevators, stairs, shafts and circulation. Getting the core right at the beginning establishes the foundation for an efficient structural system throughout the building.
DUAL SYSTEMS — MAKING THE ENTIRE STRUCTURE WORK
The concrete core does not necessarily have to work alone. RGCE frequently incorporates dual lateral-force-resisting systems, in which lateral loads are shared between the concrete shear walls and the building’s column-slab moment frames.
By engaging the column-slab frames as part of the lateral system, demand on the shear walls can be reduced while increasing the overall stiffness of the building. This can allow for a more efficient shear-wall configuration and, in appropriate buildings, reduce the height, number, or extent of shear walls required.
THE MOST EFFICIENT STRUCTURAL SOLUTION IS NOT ALWAYS THE ONE THAT USES THE LEAST MATERIAL.
Our objective is to develop the structural system that creates the greatest overall value for the building—balancing structural efficiency with architecture, flexibility, constructability and cost.
LET'S LAY OUT THE COLUMNS
SOLVING THE PUZZLE
Columns can be both fun and frustrating. In a high-rise building, laying out the columns is effectively solving a three-dimensional puzzle—and there is rarely only one solution.
A tower may contain several different residential floor plans stacked above entirely different uses at the base, including lobbies, retail spaces, amenity areas and parking. RGCE looks for column layouts that work across these different uses while maintaining efficient spans and minimizing the need for structural transfers.
Avoiding transfers is an important objective, but it is not the only one. The best column location structurally may not be the best location for the architecture, façade or building systems. Developing the right layout requires balancing all of these considerations.
A COLUMN DOESN'T EXIST IN ISOLATION
Apartment layouts can be particularly challenging. A column must be evaluated not only for where it lands on the floor plan, but for how it affects room layouts, the façade and how it interacts with the mechanical, electrical and plumbing systems surrounding it.
This is especially important in flat-plate concrete construction. Openings and penetrations near a column can reduce the punching shear capacity of the slab-column connection and interfere with the placement of flexural reinforcement. A column location that initially appears ideal may become problematic when it falls adjacent to a bathroom wet wall or another area requiring significant slab penetrations.
RGCE considers these conflicts while the column layout is being developed—not after the structural system has already been established.
REGULARITY HAS VALUE
Regular column spacing generally produces a more efficient concrete structure. Consistent spans simplify the slab system, reinforcing and formwork, while irregular spacing can introduce inefficiencies that repeat on every floor of the tower.
But not every span should necessarily be equal. Where possible, RGCE looks to position longer gravity spans adjacent to the building's shear walls. This directs additional gravity load into the walls, helping counteract overturning forces and potentially reducing the tension demands and reinforcement required in the lateral system.
The objective is therefore not simply to find locations where columns can go. It is to develop a column layout in which the structure, architecture and building systems work together.
THE RIGHT SLAB, NOT THE THINNEST SLAB
8 INCHES IS NOT ALWAYS THE ANSWER
Many design teams begin with the assumption that a residential flat-plate slab should be 8 inches thick. For many buildings, that works very well. But treating 8 inches as a fixed requirement can eliminate one of the structural engineer's most useful tools for developing an efficient column layout.
A slightly thicker slab can permit longer spans, greater flexibility in column locations and, in some cases, a more efficient building overall. The additional concrete in the typical floor slab needs to be weighed against what that flexibility may eliminate elsewhere in the structure.
The thinnest slab does not necessarily produce the most efficient building.
SOMETIMES A FEW INCHES CAN SAVE A FEW FEET
This becomes particularly important when the column layouts of different portions of a building do not align. Restricting slab spans to maintain a predetermined slab thickness can force additional columns or ultimately require a structural transfer.
A transfer structure may add two, four or even six feet of structural depth at a transfer level. In the right building, distributing a relatively small increase in slab thickness over the typical floors can eliminate that transfer entirely—potentially reducing the overall building height while producing a simpler and more direct structural system.
This is another example of why RGCE evaluates efficiency at the scale of the entire building, rather than optimizing an individual structural element in isolation.
THICKER SLABS CAN CREATE BETTER ARCHITECTURE
Longer slab spans can also provide significant architectural flexibility. In higher-end residential buildings, where room and partition spacing may exceed 12 feet, a thicker slab may allow columns to occur at every other partition along the façade rather than at every partition.
The same principle applies at building corners. Greater slab capacity can permit longer corner cantilevers, allowing the structure to accommodate column-free corner windows and other architectural features without introducing unnecessary transfers or complicated framing.
In these cases, the additional slab thickness isn't simply additional structure—it is being used to buy architectural flexibility.
AND THERE CAN BE OTHER BENEFITS
Increasing slab thickness can provide benefits beyond span capability. A thicker slab increases punching shear capacity and stiffness and can increase the contribution of the slab-column moment frames to the lateral system, improving the overall stiffness of the building.
Additional building mass can also be beneficial in the design of tall, slender towers, where dynamic response and occupant comfort may govern aspects of the structural design. In appropriate circumstances, increased mass can help reduce building accelerations and the perception of motion.
Finally, increased slab mass can improve floor-to-floor acoustic performance—a benefit anyone who has lived in a high-rise building can appreciate.