Tension Member Design: Gross and Net Area

The design of steel tension members must consider two potential failure modes: yielding of the gross cross-section along the length of the member, and fracture (rupture) of the net cross-section at the connections where holes are drilled for bolts.

Example

An A36 steel plate (Fy=36F_y = 36 ksi, Fu=58F_u = 58 ksi) is used as a tension member. The plate has a width of 8 inches and a thickness of 0.5 inches. It is connected to a gusset plate using a single line of three 3/4-inch diameter bolts.
Calculate the nominal tensile strength based on gross yielding (PnyP_{ny}) and net section fracture (PnfP_{nf}). According to AISC specifications, the effective hole diameter for calculating the net area is the nominal bolt diameter plus 1/8 inch (to account for the standard hole clearance and damage from punching/drilling).

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Compression Member Design: Euler Buckling

Columns fail in compression not by yielding (crushing) the material, but by instability known as buckling. The Euler buckling load (PcrP_{cr}) determines the critical load at which a long, slender column will suddenly bow sideways.

Example

A W8x31 wide-flange steel shape is used as a column in a single-story building. The column is 15 feet long and is pinned at both ends (K=1.0K = 1.0).
The properties of the W8x31 section are: Gross Area (AgA_g) = 9.13 in2in^2 Moment of Inertia about the strong axis (IxI_x) = 110 in4in^4 Moment of Inertia about the weak axis (IyI_y) = 37.1 in4in^4 Modulus of Elasticity for structural steel (EE) = 29,000 ksi.
Determine the critical Euler buckling load (PcrP_{cr}) for this column. Does it buckle about its strong axis (xxx-x) or weak axis (yyy-y)?

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Case Study: Bearing Failure in Bolted Connections

In a bolted lap splice, forces are transferred from one plate to another through the bolts via shear. However, the bolts also push intensely against the thin edges of the holes in the steel plates. This can cause the plate material to crush or tear out, known as bearing failure.

Example

Two 3/8-inch thick steel plates (Fy=50F_y = 50 ksi, Fu=65F_u = 65 ksi) are connected by a single 7/8-inch diameter high-strength bolt. The bolt is located exactly 1.5 inches from the edge of the plate (measured from the center of the hole to the edge).
Determine the nominal bearing strength (RnR_n) of the plate at the bolt hole. The AISC specification states that if deformation at the bolt hole at service load is a design consideration, the bearing strength is given by: Rn=1.2LctFu2.4dtFuR_n = 1.2 \cdot L_c \cdot t \cdot F_u \le 2.4 \cdot d \cdot t \cdot F_u
Where LcL_c is the clear distance (in inches) between the edge of the hole and the edge of the adjacent material, dd is the nominal bolt diameter, and tt is the plate thickness.

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