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4. Steel Moment Base Plate Design Example

# Steel Moment Base Plate Design Example Figure 1: The user interface of skyCiv Base Plate

Structural engineers often encounter the design of moment resisting in the Steel Base Plate. Steel Base Plate Design provides these features. Figure 2: Moment-resisting column base plates

Steel Base Plates are frequently designed to resist axial and bending moment loads. Axial load causes the compression between a base plate and the concrete support (column, slab, footing, etc.). Moment load contributes to the compression and uplift pressure at the steel base plate.

$$e = \frac{M}{P}$$

When the eccentricity value is sufficiently large that the resultant falls outside the middle third of the plate, there will be an uplift pressure on the other side of the column.

Anchor bolts will counteract as a tension force when an uplift force exists.

This section provides a detailed comparison of the design example between Textbook and Steel Base Plate Design results.

## Design Example (Structural Steel Design, Maccormack, 2012).

Design a moment-resisting base plate to support W14 x 120 column with an axial Pu of 620k and a bending moment Mu of 225 ft-k. Use A36 steel with Fy = 36 ksi and a concrete footing f’c = 3.0 ksi. As illustrated in Figure 3, Steel Base Plate Design provides rendered 3D modeling. The applied loads can be visually seen in the columns based on the given examples.

based on AISC Database, W14 x 120 dimension are as follows:

• d = 14.5 in
• tw = 0.590in
• bf = 14.70 in
• tf = 0.940 in

This succeeding section discussed manual computation and Steel Base Plate Design in each parameters checks:

### Computation of Eccentricity

This computation of eccentricity play important in designing of Steel Base Plate Design, whereas we can determine the type of behavior such small or large moment.

$$e = \frac{M}{P}$$

$$e = \frac{12 \times 225}{620} = 4.35 in$$

$$\therefore \text{The results fall between the column flanges and within the middle third of the plate.}$$

### Computation of Base Plate Dimensions

From the textbook example, the author initially assumes base plate dimension of 20 x 28 inches. In skyCiv Steel Base Plate Design this can be inputted on the module inputted fields.

$$f = -\frac{Pu}{A} \pm \frac{ Pu \times ec }{I}$$

$$f = -\frac{620}{20 \times 28} \pm \frac{ 620 \times 4.35 \times 14 }{ \frac{1}{12} \times 20 \times 28^{3} }$$

$$f = -1.107 \pm 1.032 \rightarrow \frac{-2.139 < \phi P_{n} = 3.32 ksi }{ -0.075 ksi \text{still compression} }$$

### Computation of Base Plate Thickness

The author manual computation stated that “Taking moment to right at center of right flange” to calculated the Moment at plate where will be used for the thickness computations.

$$M = 1.606 \times 7.22 \times \frac{7.22}{2} + (2.139 – 1.606)\times 7.32 \times (\frac{2}{3} \times 7.22 ) = 51.12 in-k$$

$$t = \sqrt{ \frac{6 \times Mu }{ \phi _{b} \times F_{y} } }$$

$$t = \sqrt{ \frac{6 \times 51.12 }{ 0.90 \times 36 } }$$

$$t = 3.08 in$$