Structural Engineering

Forces acting on a structure

DEAD

LIVE

SNO

W

WIND

EARTHQUAKE

How are these forces resisted?

Structural Element Properties

• Shape– W Shape vs Tube Shape– Section Modulus, S– Moment of Inertia, I– Length, L

• Material– Steel– Allowable Bending Stress, Fb (ksi)– Young’s Modulus (Stress/Strain ratio), E (ksi)

Section Modulus, S and Moment of Inertia, I

Structural Element Properties

• Shape– W Shape vs Tube Shape– Section Modulus, S– Moment of Inertia, I– Length, L

• Material– Steel– Allowable Bending Stress, Fb = 33.0 ksi– Young’s Modulus (Stress/Strain ratio), E = 29,000 ksi

Common Failure Modes?

• Strength (Snapping) • Deflection (Movement)

Maximum Deflection Allowed = L/360

Maximum deflection exceeded

Common Failure Modes?

• Strength (Snapping) • Deflection (Movement)• Shear• Torsion

Typical Beam, Plan View

1. Top Down Plan View

3. Tributary Width, Trib2. Beam Length, L

4. Linear Force, w w=PSF*Trib

Typical Beam, Isolated View

w, Linear Force

L, Beam Length

Design Equations

(Real) Design Equations• Strength:– Bending caused by Force, B = wL²/8– Bending Capacity of Beam, Bcap = SFb

• Deflection:– Maximum Deflection Allowed, Δmax = L/360– Actual Deflection caused by Force, Δ = 5wL⁴/384EI

• Solve for S and I and then choose Beam!

• NOTE: Be careful of units (ft vs. in and lbs vs k)

Design SummaryDESIGN GIVENS• Force, PSF = 55.00 psf• Length, L = 39.5 ft• Tributary Width, Trib= 6.0 ft• Allowable Bending Stress, Fb = 33.0 ksi• Young’s Modulus, E = 29,000 ksi

DESIGN EQUATIONS• w = PSF*Trib/1000• B = 12*wL²/8 • Bcap = SFb • Δmax = 12*L/360 • Δ = 1728*5wL⁴/384EI

DESIGN PROCEDURE• Solve for w• Solve for B• Equate B to Bcap and solve for S• Choose a beam based on strength• Solve for Δmax • Equate Δmax to Δ and solve for I• Choose a beam based on deflection• Select controlling beam shape

Selection Of Beam

• Sreq = 23.4 in³• Ireq = 480.0 in⁴

• W18x35• S = 57.6 in³ > 23.4 in³• I = 510 in⁴ > 480.0 in⁴• Note that W18x35 is the lightest shape that

meets these requirements therefore it is chosen because it costs the least.

Site Safety

What to look for?

Beams vs. GirdersWide Flange vs. Tubes

Steel Shear StudsAngles and other Miscellaneous Steel Shapes

Column Type and OrientationColumn Baseplates

X-BracesRebar Density

Concrete Slab Control JointsConcrete Foundations

Any Questions?