Tiêu đề 48 Bearing and Base Plates.pdf ✅

PSAD 43: Bearing and Base Plates 1. Column-Base Plates Column-base plate connections allow for the transfer of loads from the main steel structure above to the foundation below. These connections mainly consist of a base plate where the steel column is welded/bolted to, anchor rods which primarily resist and transfer uplift and horizontal forces, and a concrete support where the anchor rods are embedded for effective load transfer. Column-base plate connections, depending on the intended idealization for the structural model, can act as pinned (can transfer axial and shear loads only), or fixed (can transfer axial loads, shear loads, and bending moments). For the design of pinned column-base plates under compressive load, the limit states of Bearing on Concrete and Base Plate Yielding are primarily considered. 1.1 Bearing on Concrete The base plate, as it presses onto the concrete support, should have sufficient area to allow the compressive load from the steel column to be distributed uniformly without exceeding the concrete bearing strength. From NSCP 2015: 510.8 Nominal Bearing Strength 510.8.1. On the full area of a concrete support: P௣ = 0.85f௖ ᇱAଵ 510.8.2. On less than the full area of a concrete support: P௣ = 0.85f௖ ᇱAଵඥAଶ/Aଵ ≤ 1.7f௖ ᇱAଵ Where: Aଵ = area of steel concentrically bearing on a concrete support Aଶ = maximum area of the portion of the supporting surface that is geometrically similar to and concentric with the loaded area φ௖ = 0.65 (LRFD) Ω௖ = 2.31 (ASD)


Formula Derivation: For rectangular cross section of width b and height h, the plastic section modulus is taken as: Z = bhଶ 4 For the cantilever strips of 1 mm unit width and height equal to base plate thickness t௣, Z = (1)t௣ ଶ 4 Plastic bending moment now becomes: M௣ = F௬Z = F௬ t௣ ଶ 4 For the cantilever strip of maximum length l, the maximum bending moment at the face of the equivalent rigid area due to the bearing pressure is: M = f௣ (1)l ଶ 2 = f௣l ଶ 2 Considering LRFD: M௨ ≤ φ௕M௣ f௣l ଶ 2 ≤ φ௕F௬ t௣ ଶ 4 tp ≥ lඨ 2 fp φbFy Considering ASD: M ≤ M௣ Ω௕ f௣l ଶ 2 ≤ F௬ Ω௕ t௣ ଶ 4 tp ≥ lඨ Ωb 2 fp Fy Required Minimum Thickness of the Base Plate (Based on Elastic Moment Capacity) t௣,௠௜௡ = lඨ 3 f௣ F௕ Where: F௕ = Allowable bending stress Formula Derivation: For rectangular cross section of width b and height h, the elastic section modulus is taken as: S = bhଶ 6 For the cantilever strips of 1 mm unit width and height equal to base plate thickness t௣, S = (1)t௣ ଶ 6