Introduction to Structural Design

Architecture has recorded the great ideas of the human race. Not only every religious symbol, but every human thought has its page in that vast book.

- Victor Hugo

Structure of a Building

The primary function of a building structure is to support and transmit the building loads and forces to the ground.

Photos courtesy Tilt-up Concrete Association

Characteristics of a Structure

• Strength

• Stability

• Economic Value

Structural System

• Made up of many smaller structural sub-systems that work together to provide a strong, stable, and cost-effective structure.

• Examples of these smaller structural sub-systems include:

Structural engineers design these structural systems.

Structural Sub-System

• Made up of many structural elements that work together to provide a strong, stable, and cost-effective system.

• Basic structural elements include:

Structural engineers design these structural systems.

Forces and Loads

• A force is any action that causes a change in the shape or motion of an object.

• A load is a force that is supported by a structural element.

• The terms force and load are often used interchangeably.

SNOWWIND

Structural Member Forces

Tension – A force that stretches or pulls apart a member, resulting in the member elongating.

Compression – A force that squeezes or presses a member together, resulting in the member shortening.

Shear – A force that acts perpendicular to the axis of the member, causing the internal particles of the member to slide against each other.

Beam

• A structural element that carries a load that is applied transverse (perpendicular) to its length.

• Usually a horizontal member that carries a vertical applied load.

• The top fibers of a beam are in compression; the bottom fibers are in tension.

Cantilever Beam

• A special type of beam that is supported only at one end

• The top fibers of the cantilever are in tension; the bottom fibers are in compression

Column

A vertical structural element that carries an axial force in compression

COLUMN

Truss• A structural element that is composed of

smaller structural members typically configured in triangular arrangements

• Some truss members carry a tension force; others carry a compression force

Arch

• A curved structural element that spans over an open space

• In stone arches, the last stone to be placed at the top is called the keystone

KEYSTONE

Vault

A continuous arch that forms a ceiling or roof

Dome

A structural element that is made up of arches that radiate around a center point to create a half sphere

Hybrid Forms

• An infinite number of combinations of these forms exist

Design Process

The same design process used by engineers and designers in other disciplines can be used to design structural systems.

Architectural Program

• Establishes overall concept and design philosophy

• Includes– Location of building on site– Architectural style– Interior space planning– Design elements– Construction materials– Building systems

• Encompasses the first four steps of the design process

Structural Design Process

• An architectural program establishes preliminary structural systems.

• The structural engineer must analyze the structural systems and provide a strong, stable, and cost-effective design.

Structural Design Process

Identify Criteria and Constraints– Additional criteria needed:

• Types and magnitudes of applied loads

• Load path that each load will follow to the ground

Structural Design Process

Explore Possibilities– Consider alternate

structural configurations, materials, and spacing to improve the design or reduce costs

Structural Design Process

Select an Approach– Based on structural

performance, compatibility with the architectural program, and cost

– Based on the possibilities explored, select the optimal design

Structural Design Process

Develop a Design Proposal– Create a preliminary

structural design incorporating the selections for all structural elements

Structural Design Process

Model or Prototype– Create a model to

represent your structural design

• Sketches• Structural analysis software• 3D architectural software

Structural Design Process

Test and Evaluate– Reanalyze structure based

on element selections– Structural analysis

software– 3D architectural software

Structural Design Process

Refine– Revise the design as

necessary to correct problems with strength, stability, and compatibility with other systems

Structural Design Process

Create or Make– Based on the final design

after revisions– Create working drawings

of the structural design

Structural Design Process

Communicate Results– Distribute the drawings– Include in bid package– Include in construction

drawings

Image Sources

• Tilt-up Concrete Association

(www.tilt-up.org)