The arched truss design of the upper dome is a hybrid of the Polonceau truss and the three-hinged arch. It borrows the internal system of struts and ties from the Polonceau truss, and the shape of the top chord from the arch, thus combining the positive characteristics of both systems.
1. Polonceau truss (Wiegmann's, Russian)
The Polonceau truss consists of a rectilinear top chord, intermediate posts, and bottom ties. Under vertical loads (gravity, snow) the bending stresses of the upper chord are compensated by compressed posts, which are stabilized by tensile ropes. In order to compensate the overall thrust of the system, the lower ends of the posts are connected by a horizontal tie, in large spans suspended from the ridge of the truss by means of an additional rope.
Compared to rafter systems, arched structures are generally lighter need less material to bridge compatible spans. However, the main problem of all arched structures is the thrust, which can be absorbed by the massive supporting walls or by the horizontal tie. Another weak point of arches is their buckling under uneven load (snow or wind). The structure can be stabilized by increasing the cross-section or by installing additional diagonal ties.
3. Combined Museum's dome system
When developing the system of the dome of 24 m span, possibly based on the climatic conditions of St. Petersburg, the architect created a design that would combine the favorable qualities of the two above-mentioned systems. The design of the upper chord in the form of a circular arch allows to reduce the bending moments from the vertical load, and the combination with the system of struts and ties increases the rigidity of the arch, prone to bulging under unequal loads.