Directions: Fazlur Khan was extended to incorporate interior and exterior structures. The primary lateral loadresisting system defines whether a structural system is interior or exterior. Give examples (with description) and
pictures of interior and exterior structures.
Buildings are more than just walls, floors, and ceilings—they are masterpieces of engineering, designed
to resist forces, support weight, and inspire future generations. Whether it’s a skyscraper in a bustling city or a
historical cathedral, the structural systems inside and outside a building are key to its stability and function.
Let’s explore the interior and exterior structural systems that make buildings stand strong against gravity,
wind, and earthquakes. As future engineers, understanding these systems is crucial. It connects the concepts we
study to real-world problems we’ll solve as engineers.
A. The following interior structures are possible:
1. Hinged Frame
A hinged frame consists of structural components connected by hinges, allowing the joints to
rotate freely under load. This system is primarily used in structures where flexibility is needed, as the
hinges help distribute forces without restricting movement. The structure responds to external forces
by allowing the frame to bend and move. This movement helps reduce the risk of failure due to high
forces like wind or earthquakes.

Single-Hinged Frames: These frames have a one-hinged connection, often seen in
small bridges and folding structures.

Double-Hinged Frames: Both ends of the frame are hinged, common in arched spans
like pedestrian bridges.

Triple-Hinged Frames: Provide the most flexibility and are used in movable or
retractable structures like sports arenas with sliding roofs.
Bailey Bridges, often used
in military and emergencies, is an
excellent example of hinged
frames. These portable bridges can
be quickly assembled and provide
a flexible yet stable means of
crossing rivers or other obstacles.
The hinged joints allow the bridge
to flex under traffic loads,
preventing stress concentrations
that could cause failure.
Figure 1. Bailey Bridge
2. Rigid Frame
A rigid frame is a type of frame where the joints are fixed, meaning they cannot rotate. This
type of frame is commonly used in buildings or structures that must resist both vertical loads (such as
the weight of the building) and lateral forces (such as wind or earthquakes). The rigidity of the joints
allows the frame to transfer bending moments and shear forces across its elements, making it a strong
and stable choice for taller buildings or structures in seismic zones.
 Single-Span
Frames:
Basic
structures used for small-scale
applications like garages or singlestory warehouses.
 Multi-Span
Frames:
Include
multiple rigidly connected spans, used
in industrial buildings or long bridges.
 Portal Frames: Designed for large
open spaces, such as factories or
aircraft hangars.
Taipei 101, a skyscraper in Taiwan, uses
a rigid frame to resist strong winds and
earthquakes. The rigid frame provides stability
by transferring forces from the structure to the
foundation, ensuring the building remains
upright during seismic events. This system is
essential for skyscrapers in areas prone to both
high winds and earthquakes.
Figure 3. Taipei 101
3. Braced Frame and Shear-Walled Frame
Braced frames use diagonal braces between vertical columns to resist lateral forces, such as
wind or seismic loads. These braces can be made of steel or concrete and are typically arranged in a
triangular shape, which is ideal for resisting horizontal forces. On the other hand, shear-walled frames
use thick concrete walls placed within a building to provide lateral stability. These walls work by
transferring the forces to the foundation, providing strong resistance against movement.
 Cross-Braced Frames: Feature diagonal
members forming an "X," often seen in steel
structures.
 V-Braced Frames: Diagonal braces meet at
the midpoint of a horizontal member, reducing
bending forces.
Figure 4. Braced Frame

Concrete Shear Walls: Provide high
stiffness, used in earthquake-prone regions.

Core Shear Walls: Central walls often
encase elevators and stairwells.
Figure 5. Shear-Walled Frame
Salesforce Tower in San Francisco
employs a combination of braced frames
and shear walls to protect it against
earthquakes. The braced frames help
reduce lateral movements, while the shear
walls ensure that the structure is stable,
even during strong seismic events. These
two systems together allow for the
construction of tall buildings in
earthquake-prone
regions
without
sacrificing safety.
Figure 6. Salesforce Tower
4. Outrigger Structures
An outrigger system is a structural design where horizontal beams, or outriggers, connect the
central core of the building to its outer columns. This design helps to resist lateral forces such as wind
and earthquakes by distributing the forces evenly across the building's structure. The outriggers act as
levers, extending the base of the structure to reduce the sway caused by wind or seismic activity,
making it particularly effective for tall, slender buildings.

Single-Tier Outriggers: Include
a single level of horizontal
beams, effective for moderately
tall buildings.

Multi-Tier Outriggers: Multiple
levels of outriggers are used in
super-tall skyscrapers.

Virtual Outriggers: Hidden or
indirect outrigger connections
that serve the same purpose.
Figure 7. Outrigger Structures
Burj Khalifa, the tallest
building in the world, uses an
outrigger system to provide
stability against strong winds
and seismic forces. The
outriggers extend the core to
the
building’s
exterior
columns, enhancing the
lateral stiffness and making
the building more resilient
during extreme weather
conditions. This system
contributes to the building's
overall
stability
and
performance.
Figure 8. Burj Khalifa
B. The following exterior structures are possible:
1. Buttresses
Buttresses are external supports that counteract the lateral forces exerted on a building's walls.
In traditional architecture, these were often used in large, heavy buildings like churches and cathedrals,
where the weight of the structure could cause the walls to buckle or collapse. Flying buttresses, which
are extensions of the support from the walls, allow for large windows, making them ideal for gothicstyle cathedrals.

Solid Buttresses: Thick and sturdy, used in medieval fortresses.

Flying Buttresses: Elegant arched supports are seen in Gothic architecture.

Counterforts: Reinforce retaining walls to prevent overturning.
Notre-Dame Cathedral in
Paris is a well-known example of a
structure that uses flying buttresses.
These supports help counter the
pressure from the walls of the
cathedral, allowing for the creation
of large stained-glass windows.
This not only improved the
building’s stability but also added
an aesthetic element to the
cathedral’s design.
Figure 9. Notre-Dame Cathedral in Paris
2. Diagrid
A diagrid structure is made up of a diagonal grid of beams and columns. The design is
especially effective in distributing loads across the structure and minimizing material use. The system
is typically used on the exterior of the building, which results in a visually distinctive look. Diagrid
structures are highly efficient because the diagonal components carry both vertical and lateral forces,
reducing the need for additional internal support.

Triangular Diagrids: Simple triangular patterns,
widely used for their efficiency.

Hexagonal Diagrids: Create a honeycomb-like
pattern, adding a visual flair.

Curved Diagrids: Used for domes or irregularly
shaped structures.
Figure 10. Notre-Dame Cathedral in Paris
Hearst Tower in New York City employs a diagrid design. The system provides enhanced
strength and stability while also allowing for a more flexible interior layout. The diagonal
members reduce the amount of steel used compared to traditional frame structures, making them
more sustainable and cost-effective.
3. Exoskeleton
The exoskeleton system places structural elements on the exterior of a building, supporting the
entire load of the building from the outside. This system is often used to maximize usable interior
space and give a building a distinct, futuristic appearance. Exoskeletons can consist of steel or concrete
elements, depending on the design, and allow the building to withstand external forces such as wind
and seismic activity.

Structural Exoskeletons: Load-bearing systems that handle gravity and lateral forces.

Aesthetic Exoskeletons: Focused on visual impact but provide minimal structural
support.

Hybrid Exoskeletons: Combine aesthetic appeal with structural functionality.
Centre Pompidou in
Paris is a famous example of
a
building
with
an
exoskeleton structure. Its
visible exterior supports
(such as pipes, ducts, and
steel beams) not only
contribute to the building's
structural integrity but also
give it a unique and iconic
appearance.
Figure 11. Exoskeleton Structure
4. Space Truss
A space truss is a three-dimensional triangular framework used for supporting roofs or large
spans. These structures are lightweight but incredibly strong, making them ideal for buildings with
large, open spaces. Space trusses are often used for domes or large exhibition halls where the load
needs to be evenly distributed across the entire structure.
 Flat Space Trusses: Used for roofing
large halls or stadiums.
 Curved Space Trusses: Provide a
dome-like structure, often seen in sports
arenas.
 Double-Layered Space Trusses:
Offer enhanced strength by using two
parallel truss layers.
Figure 12. Eden Project in the UK
Eden Project in the UK uses space trusses to support its large geodesic domes. The trusses are
arranged in a way that distributes weight evenly, allowing for the creation of expansive, open interior
spaces with minimal material usage.
5. Super Frame
A super frame consists of large, stiff frames used in the construction of very tall buildings.
These frames are designed to resist high wind and seismic loads, providing a strong and stable
structure for skyscrapers. Super frames typically consist of rigid beams and columns that are
strategically placed to enhance the overall strength of the building.

Single-Tier Super Frames: Feature one dominant frame system.

Multi-Tier Super Frames: Have multiple levels of frames, distributing loads more
efficiently.

Diagonal Super Frames: Include diagonally arranged mega-columns for added lateral
stability.
Petronas Towers in Malaysia are supported by a super frame system. The towers' frame
provides structural strength against the high winds common in the region, ensuring the building
remains stable and secure.
Figure 13. Petronas Towers in Malaysia
6. Tube Structure
Tube structures are buildings that use a hollow, tubular design for their outer walls. The
structure behaves like a cylindrical tube, with the external walls acting as the primary load-bearing
elements. Tube systems are often used in tall buildings to resist lateral forces such as wind. This system
allows the building to maintain its structural integrity while keeping the interior open and flexible.

Framed Tube: Uses a dense grid of columns and beams on the façade.

Tube-in-Tube: Combines an outer framed tube with an inner core for added strength.

Bundled Tube: Multiple tubes interconnected to form a stronger framework.
Willis
Tower
(formerly Sears Tower)
in Chicago uses a
bundled tube design,
where multiple smaller
tubes are connected to
form a larger, stronger
structure. This design is
highly
effective
at
resisting wind forces and
has allowed the building
to remain one of the
tallest in the world.
Figure 14. Willis Tower in Chicago
In the modern world, engineers face challenges like urban density, climate change, and natural disasters.
Structural systems play a key role in solving these problems. Tall buildings like the Burj Khalifa wouldn’t exist
without outrigger systems. Tube structures and rigid frames allow cities to grow vertically while resisting
earthquakes and wind. For us as students, understanding these systems equips us to innovate.
References:
Allen, E., & Iano, J. (2019). Fundamentals of Building Construction: Materials and Methods (7th ed.).
Wiley.
Ching, F. D. K. (2014). Building Structures Illustrated: Patterns, Systems, and Design. Wiley.
Leet, K. M., Uang, C.-M., & Gilbert, A. M. (2018). Fundamentals of Structural Analysis (5th ed.).
McGraw-Hill Education.
Arup. (n.d.). Innovative engineering projects. Retrieved from https://www.arup.com
Notre-Dame
Cathedral.
https://www.notredamedeparis.fr
(n.d.).
Official
restoration
details.
Retrieved
from
Skidmore, Owings & Merrill LLP (SOM). (n.d.). High-rise project archives. Retrieved from
https://www.som.com