Four-story frame building

The design is for a four-story frame building with overall dimensions of 21 by 42 meters. Interfloor slabs are located at the 4.2-meter mark. Standard live loads were used as the basis for the calculation: full load of 6.9 kPa, with a sustained load component of 3.6 kPa. Materials for the supporting structures include grade B25 concrete (upgraded to B30 for columns) and grade A400 hot-rolled rebar.

2. Selection and justification of the structural design

The space-planning solution was developed based on a comparison of two structural frame layout options. Calculations revealed that the minimum slab thickness is achieved with a uniform column grid of 7 x 7 m. The adopted solution involves placing the main load-bearing beams transversely across the building, with secondary beams spaced at 1.75 m intervals.

Justification of the cross-section dimensions:

The monolithic slab thickness is assumed to be 50 mm.

Secondary beams are designed with dimensions of 0.2 x 0.4 m.

The main beams have an enlarged cross-section of 0.3 x 0.6 m.

The square cross-section of the columns is 0.45 x 0.45 m.

3. Design of the slab section

The slab calculation model is assumed to be a multi-span continuous beam 1.0 m wide. Loads were calculated taking into account the weight of the cement screed, the leveling layer, and the slab's own weight. Based on the static analysis results, the maximum bending moments in the spans and above the supports were determined. The slab is reinforced with welded rolled mesh, using 6 mm diameter bars (with a 150 mm pitch) as the main reinforcement, and in areas of high stress (the first intermediate supports), the pitch is increased to 125 mm.

4. Floor Beam Design

Secondary beams are designed as continuous structures with force redistribution. To accommodate positive moments, the cross-section is assumed to be T-shaped (due to the slab's behavior in the compressed zone). The design reinforcement in the spans is provided with 22 mm diameter bars, and in the support zones, with 18 mm diameter bars. Transverse reinforcement is provided by 8 mm diameter stirrups with a variable pitch (from 250 to 350 mm) depending on the magnitude of the transverse force.

The main beams operate using a beam-and-span design with pinned supports. Concentrated loads from secondary beams are summed, taking into account the rib's own weight. Following the static analysis, the support moments were redistributed (reduced by 30%) to ensure crack resistance. The selection of longitudinal reinforcement revealed the need for ∅28 mm reinforcement in the end spans and ∅25 mm reinforcement at intermediate supports. To support shear forces, ∅14 mm stirrups were installed at 250 mm intervals in the support zones.

5. Design of vertical load-bearing elements

The columns were designed as conventionally centrically compressed elements, accounting for random eccentricity. Reinforcement of 4∅16 mm was assigned based on bearing capacity and flexibility. The column cantilever was designed to support the beams; its overhang and height were determined based on local concrete compression conditions. The cantilever was reinforced with horizontal stirrups and bent bars, and the longitudinal reinforcement was ∅32 mm.

6. Foundation Base

A monolithic columnar foundation was designed for the columns. The footing size (3.0 x 3.0 m) was determined based on the estimated soil pressure (280 kPa) and a foundation depth of 1.2 m. The punching shear test was performed using a classic pyramidal design; the strength requirement was met. The footing reinforcement is made with a mesh of ∅14 A400 bars spaced to accommodate bending moments in the cross-sections along the edges of the steps and columns.