st-4 lecture for mid
DESCRIPTION
St-4 Lecture for MidTRANSCRIPT
CE 3223 Structure IV
Long Span Structures
• Concepts of long span structural systems.
• Structural elements
• Description of the behavior, types, advantages, disadvantages and applications of different Long Span Structures.
• Principle load types for large structures
• Structural system for tall buildings.
• Support structures of large span structures.
• Vierendeel truss.
• Folded plates.
• Design of wind pressure for a tall building.
• The distribution of earthquake load of a tall building.
• Dynamics of long span structures – wind loads and seismic loads.
• General Cable Theorem (GCT), Shear of Uniformly Loaded Cable (ULC), Tension in ULC, Length of ULC
• Suspension Bridge, Statically determinate Suspension bridge.
• Pre-stressed Concrete.
• Three Concepts, Types of Pre-stressed Concrete, Stages of Loading
• Loss of Pre-stress
• Analysis of Sections for Flexure.
• Analysis of the three hinged Arch.
• Water Tank Design.
Reference Books
ü Elementary Structural Analysis (4th Edition) - Charles Head Norris, John Benson Wilbur, Senol Utku
ü Design of Pre-stressed Concrete Structures (3rd Edition) - T.Y. Lin, Ned H. Burns
Concepts of long span structural systems
Structure with span larger than 20m can be regarded as long span structure. Throughout the
twentieth century the use of long span structures has broadened notably for communication,
commercial, industrial and leisure uses. The need for large covered spaces to indoor sports,
conference centers and huge arenas has provided challenges for architects and engineers in their
ambition to achieve efficient, economical and appropriate structural enclosures. Long span roofs
are today widely applied for sport, social, industrial, ecological and other activities. The
experience collected in last decades identified structural typologies as space structures, cable
structures, membrane structures and new – under tension – efficient materials which combination
deals with lightweight structural systems, as the state of art on long span structural design. In
order to increase the reliability assessment of wide span structural systems a knowledge based
synthetic conceptual design approach is recommended. Theoretical and experimental in scale
analysis, combined with a monitoring control of the subsequent performance of the structural
system, can calibrate mathematical modeling and evaluate long term sufficiency of design.
Long span structures need special investigations concerning the actual live load distribution and
intensity on large covering surfaces. Building codes normally are addressed only to small-
medium scale projects. The uncertainties related to the random distribution of live loads on long
span structures imply very careful loading analysis using special experimental methods.
STRUCTURAL ELEMENTS
Ø Arches
Ø Cables
Ø Truss
Ø Beam
Ø Column and Beam-Column
Ø Grid
Ø Frames
Ø Folded Plate
Ø Shell Structure
Ø Dome
Describe the behavior, types, advantages, disadvantages and applications of the following
structural elements –
Arch
Behavior
- 1 or 2 D (Dimensional Element) - Compressive member - Sufficient size proportion resist the buckling
Types
- Single- Arch
- Parallel – Barrel Vault
- Radial – Dome
Advantages
- Carry compression - Resist buckling - Suitable form for the materials which are weak in tension
Disadvantages
- Weak in tension - Provide large amount of support reaction - Limited load capacity (as reinforced is not used) - No geometric flexibility
Application
- In buildings and tombs - In the church - In the mosque
Cable
Behavior
- 1 or 2 D - Tensile elements - No compression or bending - Cable shape depends on the magnitude and position of the load or external load
Types
- Single
- Parallel
- Radial
- Orthogonal
Advantages
- Perfect tensile member - High strength - No buckling - Geometrically flexible - Aesthetically beautiful
Disadvantages
- Weak in compression - Excess values of horizontal and vertical components of support reactions - Required high strength and expensive supports
Application
- Bridge – suspension and cable stayed - Electricity transmission line - Complex cable truss
Truss
Behavior
- 2 or 3 D - Axial member - Carry both tension and compression
Types
- Space truss (3 Dimensional)
- Plane truss (2 Dimensional)
Advantages
- In midblock section, shear force is less or negligible - Can cover large amount of open span
Disadvantages
- Required diagonal and vertical bars - Geometrically rigid
Application
- Airport hanger
- Auditorium - Bridge
Shell structure
Behavior
- 2 or 3 D - Bending element - Can transmit the load effectively - No buckling - Load is carried by either tension or compression
Types
- The folded plate,
- The cylindrical barrel shell,
- The dome of revolution – parabolic, hyperbolic, cylindrical
- The folded plate domes
Advantages
- Highly efficient - Aesthetically acceptable - Economic
Disadvantages
- Analysis and design is complicated - Time consuming
Application
- Roofs - Gymnasium, cafeteria
Beam
Behavior
- 1 or 2 D - Bending element - Straight member - Can carry Axial Force(AF), Shear Force(SF), Bending Moment(BM) - Weak in tension, required reinforcement to carry tension
Types
- Simply supported
- Cantilever
- Skew cantilever
- Continuous
Advantages
- Easy to design and analyze - Need not to support the horizontal load - Suitable for almost all structures
Disadvantages
- Depth of beam increases with the increase of span - May cause shear failure
Application
- Use in all buildings
- Bridges
- Almost all structures
Folded plate
Behavior
- 2 or 3 D
- Bending element
- Act as a beam or slab
- When the ratio of span to width is small, behaves as a deep beam
Types
- Prismatic- if they consist of rectangular plates
- Pyramidal- when non-rectangular plates are used
- Prismoidal, triangular or trapezoidal
Advantages
- Advantage may be gained by increasing the thickness of the slab, so it will act as a
haunched beam and as a I section plate
- Easy in forming plane surface
- More adaptable to smaller areas than curved surface
Disadvantages
- Not adapted to as wide bay spacing, as barrel vaults
Frames
Behavior
- 2 or 3 D
- Bending and axial member
- Can resist AF, SF, BM, torsion, compression
- Can resist lateral loads
- Direction of loads and location of joints are not restricted
Types
- Braced frame- eccentric, concentric
- Moment resisting frame- OMRF, SMRF, IMRF
- Space frame
Advantages
- Provide support for gravity loads
- Can resist lateral loads
- Can carry vertical loads
- Geometrically rigid
Application
- Building structures
Principle load types for large structure
1. Dead load- Self weight, fittings, fixtures 2. Occupancy load – storage, machinery, furniture 3. Bridge load- vehicle 4. Snow and rainfall load 5. Lateral load – wind and earthquake load 6. Water pressure 7. Earth pressure 8. Ice pressure 9. Wave pressure 10. Temperature 11. Shrinkage 12. Misfitting
Structural system for tall buildings
Category A- Floor System
1. Beam Slab
2. Flat Slab
3. Plate Slab
Category B- Gravity Load System
1. Column
2. Wall
3. Frame
Category C- Lateral Load Resisting System
1. Tubular System
2. In-filled Frames
3. Shear Wall
4. Shear Wall Frame interaction
5. Braced Frame
6. Load Bearing Wall
7. Rigid Frame
Structural systems
Classification of long-span and complicated structures
In consideration of different active systems, there are four types of structural systems
• Form active structural systems
• Vector active structural systems
• Section active structural systems
• Surface active structural systems
Ø Form active structural systems
Form active structural systems are systems of flexible, non-rigid matter, in which the redirection
of forces is effected by particular form design and characteristic form stabilization
Example of structures:
1. Cable structures
2. Tent structures
3. Pneumatic structures
4. Arch structures
Ø Vector active structural systems
Vector active structural systems are systems of short, solid, straight linear members, in which the
redirection of forces is effected by vector partition, i.e. by multi-directional splitting of single
force simply to tensile or compressive elements
Example of structures:
1. Flat trusses
2. Curved trusses
3. Space trusses
Ø Section active structural systems
Section active structural systems are systems of rigid, solid, linear elements, in which redirection
of forces is effected by mobilization of sectional forces
Example of structures:
1. Beam structures
2. Frame structures
3. Slab structures
Ø Surface active structural systems
Surface active structural systems are systems of flexible or rigid planes able to resist tension,
compression or shear, in which the redirection of forces is effected by mobilization of surface
forces.
Example of structures:
1. Plate structures
2. Folded structures
3. Shell structures
Another classification of Structural System
In consideration of distributional load direction, there are two types of structural systems
1. Long Span One – Way Structural Systems
A one-way structural system is characterized by relatively large linear spanning elements in one
direction. Smaller spanning members are used to carry loads to the primary members. Typically,
one-way structural systems are used in rectangular framing bays.
Types of one-way structural systems
Steel beams & girders, Steel rigid frame, Flat steel truss, Pitched steel truss, Steel arch, Steel bar
joists, Steel joist girders, Pre-stressed concrete single T beam, Pre-stressed concrete double T
beam, Concrete arch, Wood glulam beams, Flat wood trusses, Pitched wood trusses, Wood arch.
2. Long Span Two-Way Structural Systems
As its name implies, a two-way system distributes load across two or more members. All
members in a two-way system are considered to be primary members. A two-way system is
most efficient when the shape is square so that there is equal distribution along the supporting
members. In a rectangular shape, the members spanning the short path carry more of the load.
Two-way structures have much more redundancy than one-way structures. They are also much
more difficult to analyze and design because of their static indeterminacy.
Types of two-way structural systems:
1) Space Frame – Basically, it is a 3-dimensional truss. Lots of redundancy built into this type of
truss system. It is very difficult to erect, since there are many members framing into a single
point.
2) Dome – Probably the most efficient structural system. A circular dome has vertical meridian
lines that act like vertical arches in compression and horizontal hoops that act in tension.
3) Thin-Shell Structures – Carries shear, compression and tension in the plane of the shell.
These structures are deformation resistant based on their shape. Examples of thin-shell
structures are vaults, hyperbolic paraboloids and folded plates.
4) Membrane structures – Similar to thin shell structures, membrane structures are also
considered to be form resistant. However, these fabric-like membranes can carry tension ONLY.
They are extremely lightweight. Their biggest disadvantage is that they change shape based on
loading and can also “flutter” in the wind. Membrane structures can come in various forms,
including tents, air-supported structures, domes, etc.
Vierendeel truss
The Vierendeel truss is a truss where the members are not triangulated but form rectangular openings, and is a frame with fixed joints that are capable of transferring and resisting bending moments. Regular trusses comprise members that are commonly assumed to have pinned joints with the implication that no moments exist at the jointed ends. This style of truss was named after the Belgian engineer Arthur Vierendeel, who developed the design in 1896. Its use for bridges is rare due to higher costs compared to a triangulated truss.
The utility of this type of truss in buildings is that a large amount of the exterior envelope remains unobstructed and can be used for fenestration and door openings. This is preferable to a braced frame system, which would leave some areas obstructed by the diagonal braces.
Another Definition of Vierendeel truss:
Vierendeel truss is an open-web truss with vertical members but without diagonals and with rigid
joints.
However, it is well established that Vierendeel trusses are less efficient than triangulated trusses.
Folded Plate Structure Folded Plate Structure is a thin-walled building structure of the shell type. Folded plate structures consist of flat components, or plates, that are interconnected at some dihedral angle. Structures composed of rectangular plates are said to be prismatic. In modern construction practice the most widely used folded plate structures are made of cast-in-situ or precast reinforced concrete (including pre-stressed and reinforced-cement structures). The structures are used as roofs for industrial and public buildings.
The main advantage of folded plate structures over other shells (such as cylindrical) is the simplicity of manufacture.
Folded plates are ideally suited for a variety of structure such as factory buildings, assembly halls, godowns, auditoriums and gymnasia, requiring large column free area.
Folded plate roof for gymnasium and cafeteria
Folded Plate Roofs
Folded-Plate Hut in Osaka
Basic Elements
The principle components in a folded plate structure are illustrated in the sketch above. They
consist of, 1) the inclined plates, 2) edge plates which must be used to stiffen the wide plates, 3)
stiffeners to carry the loads to the supports and to hold the plates in line, and 4) columns to
support the structure in the air. A strip across a folded plate is called a slab element because the
plate is designed as a slab in that direction. The span of the structure is the greater distance
between columns and the bay width is the distance between similar structural units. The structure
above is a two segment folded plate. If several units were placed side by side, the edge plates
should be omitted except for the first and last plate. If the edge plate is not omitted on inside
edges, the form should be called a two segment folded plate with a common edge plate.
Dynamics of long span structures – wind loads and seismic loads
Very long span suspension bridges are flexible structural systems. The introduction of these
cable suspended structures has been profoundly enhanced by the development of new structural
materials and computer methods of analysis. Cable suspended systems comprise both categories
of suspension bridges and cable stayed bridges. These flexible systems are susceptible to the
dynamic effects of wind and earthquake loads.
Wind loads and earthquake loads are the lateral forces on a structure. A fundamental problem in
dealing with these lateral forces is the computation of the magnitude of the wind load and the
earthquake load. The structural effects, the response of the structure to such random lateral
loads, and the subsequent design of an efficient lateral load resisting system, dictate very
sophisticated methods of analysis and design. Such methods include but are not limited to
classical methods of structural analysis, computer methods of structural analysis, experimental
methods, as well as other validation and verification methods.
The finite element methods present the engineer with a powerful structural analysis technology
reliant on modern digital computers. Preprocessors and postprocessors are available to facilitate
the input and output data of such advanced computers. The art in all this technology is to present
the engineer with results that can predict reliably the response of such complicated structural
systems. Linear as well as nonlinear response, aerodynamic performance, structural stability, the
choice of light materials for the superstructure, and other design considerations constitute the
essence of the problem.
Wind tunnels are available to help us understand the aerodynamic problem associated with the
structural vibrations of long span suspension bridges subject to wind loads. Shaking table
experiments also can help us understand the dynamic behavior of long span suspension systems.
Wind can produce the following effects on suspension bridges:
1. Wind lift and drag forces, ( Lift is the component of aerodynamic force perpendicular to the
relative wind and Drag is the component of aerodynamic force parallel to the relative wind.)
2. Aero-elastic effects (torsional divergence or lateral buckling),
3. Oscillations induced by vortex effects,
4. Flutter phenomena,
5. Galloping effects, and
6. Buffeting caused by self-excited forces.
All of the above effects require wind tunnel tests. It is very important to understand here that
studies are needed for the partially complete structure as well as the completed structure. The
performance of the structure under the effect of wind loads should be investigated during the
various construction stages of the suspension bridge. The construction period of large
suspension bridges should be wisely planned for seasons where no serious storm conditions are
anticipated. Proper prediction of the weather for extended time periods is important. If the
construction is contemplated for seasons with predicted storm activities, energy dissipating
devices and dampers should be used to reduce the magnitude of the vibrations on the partially
completed structure.
There are 3 types of wind tunnel tests on a suspension bridge:
1. Models of the entire bridge,
2. Taut strip models and
3. Sectional models.
The first category of wind tunnel models provides the engineer with the advantages of similitude
between model and prototype. These models are expensive to build and constitute a large initial
capital expenditure. Experience from previous designs indicates that a scale of 1 to 300 is
desirable. Other scales are also possible. The distribution of the mass in such complete scale
models is identical to the mass distribution of the real life structure or prototype.
The second category, or the taut strip model, consists of 2 wires that are stretched across the
wind tunnel. The response of such models to applied fluid flows in the wind tunnel is similar to
the response of the center section of the suspension structure.
The third category is made up of sections of the bridge deck in the span-wise direction. The ends
of these sections are supported on spring type foundations to allow motion in the vertical
direction as well as the rotational sense. The usual scales for such deck sections are within the
1/50 to 1/25 range. These sectional models are very important in determining the aero elastic
stability of the proposed deck system. These models allow us to further investigate the steady
state coefficients for drag, lift, and moment.