Introduction to Lateral Force Resisting Systems

CE 636 - Design of Multi-Story Structures T. B. Quimby UAA School of Engineering

Basic Concepts

The LFRS is used to resist forces resulting from wind or seismic activity.

Buildings are basically big cantilever beams. They are supported on one end only and the loads are perpendicular to the beam.

As in a beam, buildings are designed for strength (shear and flexure) and serviceability (deflection).

Constraints

Strength Shear Flexure

Serviceability Deflection Spatial Requirements

Structural Systems for Cantilever Beams

Braced Frame (Vertical Truss) Moment Frame Infilled-Frame Shear Wall (~ solid beam) Tube systems Combinations of the above

Braced Frames

Braced Frames are basically vertical truss systems.

Almost exclusively steel or timber. Highly efficient use of material since forces are

primarily axial. Creates a laterally stiff building with relatively little additional material.

Has little or no effect on the design of the horizontal floor system.

Good for buildings of any height. Bracing may intrude on the spatial constraints. May be internal or external.

Types of Bracing

Different types of bracing Single Diagonal Double Diagonal Chevron Bracing Story height knee bracing (eccentricity

braced frames May be single story and/or bay or

may span over multiple stories and/or bays

Bracing

Large Scale Bracing

Multiple Floors Multiple Bays

Moment Frames(AKA Rigid Frames)

Columns and Girders joined by moment resisting connections

Lateral stiffness of the frame depends on the the flexural stiffness of the beams, columns, and connections.

Economical for buildings up to about 25 stories. Well suited for reinforced concrete construction

due to the inherent continuity in the joints. Design of floor system cannot be repetitive since

the beams forces are a function of the shear at the level in addition to the normal gravity loads.

Gravity loads also resisted by frame action.

Moment Frame Behavior

Note the bending in the typical beam, column and joint.

Infilled Frames

Common in many countries. Used for buildings up to 30 stories. Steel or concrete frame infilled with

concrete or masonry. Infill behaves as a strut in compression. Tension contribution is ignored. Due to random nature of masonry infill, it is

difficult to predict the stiffness and strength of this system.

No method of analyzing infilled frames has gained general acceptance.

Infilled Frames

Shear Walls

Generally constructed with concrete, masonry, or plywood. Sometimes steel.

Shear walls have high in-plane stiffness and strength.

Well suited for tall buildings up to about 35 stories.

Shear walls may intrude on the spatial constraints. Best suited to residential and hotel construction.

Can be used around elevator and/or stair cores.

Shear Wall Building

Coupled Shear Walls

Special case of shear walls. Two or more shear walls in-plane,

coupled with a stiff beam or slab at each level.

Tends to behave like a moment frame system with very stiff columns.

The coupling reduces lateral deflections.

Forces in the coupling elements can be quite large.

Coupled Shear Wall Building

Free body of left shear wall has additional reactions from the coupling members.

Wall-Frame Structures

Combination of shear walls and rigid frames or combination of braced and rigid frames.

Shear walls and braced frames tend to deflect in a flexural mode while the rigid frames tend to deflect in a shear mode.

In a wall-frame structure, both the shear walls and rigid frames are constrained to act together, resulting in a stiffer and stronger structure.

Good for structures in the 40-60 story range.

Wall-Frame Building

Tube Systems

The basic idea is to make a rectangular tube out the the perimeter of the building.

The tube is made up of closely spaced columns connected by stiff spandrel beams creating very stiff moment frames.

Frames parallel to direction of force act like webs to carry the shear.

Frames perpendicular to the direction of force act as flanges. Flange forces are not uniform.

Best applied to rectangular or circular plans. Suitable for both steel and concrete. Use for buildings of 40 stories or more. Frames are repetitive and easily constructed.

Tube System

Gravity Loads taken by frames and interior columns.

Aesthetically, the system gets mixed reviews because of the small windows and the repetition.

Tube Variations

Tube-in-Tube or Hull-Core Inner tube is usually around an elevator or service core

and can be made very stiff with shear walls or braced frames.

Bundled Tubes Introduces additional “web frames” which reduces shear

lag which makes flanges more efficient. Allows for more architectural variation. Sears Tower, Chicago

Braced-Tube Utilizes a large scaled braced frame in place of rigid

frames Allows for wider columns spacing and smaller spandrels.

Outrigger-Braced Structures

Structural “depth” is increased (i.e. the moment of inertia of the structure is increased)

Shear strength is unchanged. Utilizes a braced core with stiff

outriggers to mobilized outer columns in tension and compression.

4 to 5 outriggers appear to be the economical limit.

Outrigger-Braced Structures

Under Lateral Loads: Columns on one side

are in tension Columns on other

side are in compression.

Suspended Structures

Used primarily to achieve some architectural purpose.

Floor are hung from a truss on an upper level Tension members can be smaller than columns

would be in same place. Accumulated lengthening of tension members may

cause extreme deflection problems at lowest hung floor. This can be controlled by hanging 10 or less floors from a single truss.

Limited to “shorter” structures since structural depth is small at base, making lateral deflections large.

There are several variations on the theme.

Suspended Structures

Suspension does little to help the LFRS.

Core Structures

Core carries all gravity and lateral forces.

Core may be braced frame or shear wall.

Floors are cantilevered off of the core.

Creates a column free interior. Building width is limited by

capabilities of the cantilever. Building height limited by stiffness of

core. Structurally inefficient.

Space Structures

Three dimensional triangulated frame.

Highly efficient and relatively light weight.

Bank of China building in Hong Kong is a classic example.

Ingenuity required to get the gravity and lateral loads from the floors into the space frame.

Hybrid Structures

Combinations of the various types of systems.

There are almost limitless combinations. May be necessary to achieve architectural

goals. (“Postmodern” architecture intentionally tries to get away from simple prismatic building shapes.)

The development of large scale computer based analysis has made design of odd shapes possible.