1. Structural Elements & Design Zach Gutzmer, EIT Civil and Environmental Engineering South Dakota State University

2. Overview Structural Design Requirements Equilibrium Structural Elements Materials and Cost Modeling/Simulation

3. Structural Design Requirements The goal of every structural engineer is to design a safe and efficient structure. –i.e. “Strength” and “Serviceability” must be met Strength = Is it strong enough? (forces = loads) Serviceability = Can it be used? (deflection characteristics and stability)

4. Hold up! ……what is a structure?

5. Structural Design Requirements A structure is an assembly of structural elements: –Axial –Beam –Column To design a structure, we need to design each element for the load it carries.

6. Structural Design Requirements Gravity Loads –Dead Load: e.g. self weight –Live Load: e.g. occupants, furniture, vehicles Lateral Loads –Wind Load –Earthquake Load –Hydraulic Load –Earth pressure

7. Equilibrium 3 states of equilibrium: When loads are applied to a structure, the structure will want to move. (Newton’s 1 st Law) We want structures to stay in place and only deform/deflect! StableNeutralUnstable

8. Equilibrium Newton’s 3 rd Law: For every action, there is an opposite and equal reaction. The Reactions are provided by supports –“A” is pin that prevents translation in 2 directions –“B” is a roller that prevents translation in 1 direction The supports need to provide both force and moment equilibrium.

9. Equilibrium Force equilibrium = All of the forces acting on the structural element (in each Cartesian direction) sum to zero. Moment equilibrium = The moments of the forces acting on a structural element (in each Cartesian direction) sum to zero.

10. Equilibrium For this example in 2D: The supports must satisfy: ΣFx = 0 ΣFy = 0 ΣMz = 0

11. Equilibrium How the forces move from Action to Reaction is called the load path. Determining the load path will allow you to classify each element (axial, beam, column) in the structure, and it will help you understand how the structure functions. Each element is classified according to the type of internal forces it carries.

12. Structural Elements: Axial Axial elements are subjected to loads along their axis: Axial members are subjected to either Tension or Compression

13. Structural Elements: Axial Axial members develop one internal force to carry loads: Time for a tootsie roll!!!

14. Structural Elements: Beams Beam elements are subjected to loads along the member’s axis AND loads transverse to the member’s axis.

15. Structural Elements: Beams Beams develop three internal forces to carry loads: –V c = internal shear force* –N c = internal normal force –M c = internal bending moment* Beams are Flexural Members (Flexure = Bending) *Governs the design

16. Its all in how you use “I”t! I = moment of inertia = MOI Its tough to demonstrate MOI, but its easy to demonstrate deflection! MOI is inversely proportional to the deflection (Were strength and serviceability requirements met?) Beam Demonstration

17. Structural Elements: Columns Columns are beam elements with high axial loads Nc governs the design Look familiar?

18. Column Demonstration Yardstick Column –The strength capacity is: 1300 lbs –When you try to break it, what happens? It Buckles! –Buckling is a stability issue with compression members. Buckling depends on geometry –Which way does the column always buckle?

19. Material and Cost What materials are you going to build with? How strong is it? (Strength) How flexible is it? (Serviceability) Is it readily available? (Cost) Is it easy to work with? (Cost) Some materials do not work for every type of element: Pushing rope

20. Modeling/Simulation Once we know the loads and have selected the materials, its time to design and start modeling our new structure: –Computer analysis –Scale models The initial design is rarely the final design. The model will go through many revisions and tweaks to optimize the structure for strength, serviceability, and cost.