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
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, Hu =γPH
= 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
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.
|Fig. 12.5 Tied base design example – tie rod detail.|