CE 331, Spring 2006 Structural Design Process 1 / 3
Calculate
Loads on the
Structure
Structural design basically consists of selecting member sizes so that the response of the structure under loads meets the design criteria (illustrated in Figure 1 below).
Select Member
Sizes
Calculate the
Structural
Response
Check Structural
Response
Against Design
Criteria
Figure 1. Structural Design Process.
Design criteria typically fall into the following broad criteria:
• the structure should not collapse
• the structure should provide acceptable service
• the structure should be economical to construct
Each of the three basic criteria above can be subdivided more finely, as indicated in the second column of Table 1 below. For this class, we will analyze members to determine if they fail, we will calculate the maximum deflections of members and compare to acceptable limits, and (when we design) we will select the smallest possible member that meets the first two criteria.
Table 1. Structural Design Criteria
Basic Criteria CE 331 Criteria
Not collapse
Sub-Criteria
• prevent loss of life
• hospitals should remain in operation after an earthquake
• building should not be damaged during an earthquake
Members should not fail
Provide acceptable service
Economical to construct
• walls should not crack when building is loaded
• floor should not vibrate noticeably when loaded
• concrete cracks should remain very small
• members not needlessly large
• straightforward to construct
Deflections within limits
Use smallest possible members

CE 331, Spring 2006 Structural Design Process 2 / 3
Structural Failure
The normal procedure in designing a structural member so that it doesn’t fail is illustrated in
Figure 2 below. The figure illustrates that design involves an iterative analysis. The steps in a typical analysis are: first calculate the loads on the structure, then calculate the maximum internal forces on the particular member, and finally to check if the internal forces will cause the member to fail. A member can fail structurally in several different ways (called failure modes).
Calculate Loads on the Structure
Select Member
Sizes
Calculate
Internal Forces in a Member
(Axial, Bending
& Shear)
Check for
Failure
Figure 2. Design Process to Prevent Structural Failure.
Failure Modes
Failure modes can be divided according to the type of deformation in the member (axial, bending or shear) and whether failure is caused by a material failure (e.g. rupture or crushing of wood fibers) or a stability failure (buckling). The different failure modes studied in this course are illustrated below.
1.
Axial Failure
1.1.
Material Failure
"fibers" rupture
"fibers crush"
Tensile Failure Compressive Failure
1.2.
Axial Buckling Failure

CE 331, Spring 2006 Structural Design Process
2.
Bending Failure
2.1.
Material Failure fibers crush
2.2.
Lateral Torsional Buckling Failure fibers rupture
3 / 3
3.
Shear Failure (material failure)
Compression side of beam displaces laterally
Cross-section rotates
The structural engineer’s job is to predict whether a member will fail under the anticipated loading. Material failures occur when the load per unit area (applied stress) is greater than the material strength. Stability failures occur when the load applied over the member’s unbraced length exceeds the buckling strength.