Tuesday, January 29, 2013

Design Example: Tied Portal Frame Base.

The pad bases for a single-bay portal frame are to be joined by a horizontal tie to take out the horizontal thrusts from the portal legs. The portal is similar to the one which was designed as an untied portal in section 11.3.4. Loads and dimensions are shown in Fig. 12.4.

From section 11.3.4,

vertical superstructure load, P = (dead load)
                                                + (imposed load)
                                               = G + Q
                                               = 175 + 225
                                               = 400 kN

Q as a percentage of P is 100 Q/P = (100 × 225)/400 = 56%.

From Fig. 10.20, the combined partial factor for dead and imposed loads is γP = 1.51.

Horizontal thrust, H = 50 kN

The horizontal thrust H arises from vertical loads G and Q, and will therefore have the same combined partial load  factor γP = 1.51.
Size of base
From section 11.3.4, the net allowable bearing pressure, na = 300 kN/m2.

On the basis that the horizontal thrust will be taken out by the tie joining the portal feet, the minimum area of foundation required is

A base 1.2 m × 1.2 m will therefore be chosen. Comparison with the example in section 11.3.4 shows that the introduction of the horizontal tie has reduced the base size.

Design of horizontal tie
The tie will be a mild steel bar, as shown in Fig. 12.5, encased in concrete for durability.

Ultimate tensile force in bar, HuPH
                                               = 1.51 × 50
                                                = 76 kN

From BS 8110, the characteristic tensile stress fy = 250 N/mm2 for hot rolled mild steel. The partial material factor γs = 1.05.

The required cross-sectional area of bar is

Provide one number 25 mm diameter mild steel bar (area  = 491 mm2 ) to act as the tie. This will need to be  adequately anchored into the pad base as shown in Fig. 12.5.

To prevent possible foundation spread from lack of fit, the tie will incorporate a turn-buckle, to take up any slack prior to steel erection.

Tied base design example – tie rod detail.
Fig. 12.5 Tied base design example – tie rod detail.

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