Design and Structural Behavior of Nested Long-Span Double Beam-String Steel Bracing System

To address the increased in-plane depth of excavation-side bracing， restricted working space along the pit perimeter， and limited interchangeability of structural members resulting from increased bracing spans in large-scale foundation pits， a long-span nested double beam-string steel bracing system is proposed. The bracing system consists of inner and outer beam-string units， with the crown beam or waler serving as their common upper-chord beam-column member. The beam-string units provide zoned support， with the bracing span adjusted by varying the horizontal tension-rod lengths while keeping the strut lengths and in-plane installation heights unchanged， thereby facilitating modular and standardized member design. Compared with the ZXL36 single beam-string bracing system with the same span of 36 m， the proposed system reduces the maximum in-plane depth from 9.113 m to 8.000 m， a reduction of 12.2%， while reducing the numbers of both struts and active displacement-control devices from five to four. A three-dimensional finite element model of a 40 m-span nested double beam-string steel bracing system was developed to investigate its structural response to prestressing and equivalent earth pressure. The effects of solar radiation and uniform temperature variation on the structural behavior of the bracing system were examined. For the 40 m-span system subjected to a resultant equivalent earth-pressure force of 25000 kN， prestressing established an initial tension-compression state， with the steel tension rods in tension and the struts in compression. Prestressing also produced an active in-plane pre-deformation of 63.6 mm toward the outside of the foundation pit at the midspan of the common upper-chord member. As the earth pressure was applied， the in-plane displacement increment toward the inside of the foundation pit at the midspan of this member reached 67.1 mm. The outward pre-deformation induced by prestressing offset most of the earth-pressure-induced displacement， resulting in a final inward in-plane displacement of only 3.5 mm. The axial forces in the inclined tension rods， horizontal tension rods， and compression struts of the outer beam-string unit were 14125 kN， 9987 kN， and -11066 kN， respectively， whereas the corresponding values in the inner unit were 7467 kN， 5618 kN， and -4947 kN. The axial-force levels in the outer unit were higher than those in the inner unit. The maximum cross-sectional von Mises stresses of the members ranged from 157 to 409 MPa. The outer inclined tension rod exhibited the highest stress of 409 MPa and was therefore the strength-governing member under this loading condition. Under solar radiation， the absolute axial forces in the main members of the inner and outer beam-string units decreased by 10.2%-10.3% and approximately 25.9%， respectively， relative to those under the equivalent earth-pressure condition at ambient temperature； the maximum cross-sectional Mises stresses of these members decreased by 2.5%-10.1% and 18.5%-27.1%， respectively.Nonuniform heating produced a pronounced stress gradient across the strut sections， while the thermal elongation of the steel tension rods caused a loss of effective prestress. Consequently， the inward in-plane displacement at the midspan of the common upper-chord member increased from 3.5 mm to 76.5 mm， exceeding the allowable limit of 30 mm. In engineering applications， the in-plane displacement of the bracing system and the prestress levels in its members should therefore be monitored， and the prestress should be adjusted promptly based on the monitoring results to satisfy the displacement-control requirements. Following a uniform temperature decrease of 15 ℃， the midspan displacement of the common upper-chord member changed from an inward displacement of 3.5 mm to an outward displacement of 1.8 mm. The absolute axial forces in the main members of the inner and outer beam-string units increased by 1.2%-1.3% and 1.5%-1.6%， respectively， while the maximum cross-sectional Mises stresses increased by 2.1%-3.7%. These results indicated that the uniform temperature decrease had only a limited effect on the internal forces and stresses of the structural members.