Title:
Design of A Grade Beam to Resist Torsion due to Wind loading on CMU Wall
Author(s):
Gary G. Greene, Jr. and David L. Hartmann
Publication:
Symposium Paper
Volume:
344
Issue:
Appears on pages(s):
183-194
Keywords:
grade beam; reinforced concrete; shear; torsion
DOI:
10.14359/51728297
Date:
10/1/2020
Abstract:
The Joint ACI-ASCE Committee 445 published a document titled Report on Torsion in Structural
Concrete that contained an in-depth review of historical theory development, design models, and simplified design
procedures for the effect of torsion in concrete structures. That document contained three design examples that were
relatively simple. An important goal of this ACI Special Publication is to provide more realistic design examples
that are usable by design professionals. This paper satisfies that goal by showing a detailed solution to a realistic
example that has been encountered on several occasions by one of the authors. Another goal of the ACI Special
Publication is to show applications where torsion is combined with flexure and shear. In this example, the torsional
effects are combined with biaxial flexure and biaxial shear forces. This example includes a check of the new
provisions in ACI 318-19 for bi-axial shear effects.
This paper shows a detailed solution for the design of a reinforced concrete grade beam subjected to torsional effects
combined with biaxial shear and biaxial flexure. The grade beam is a portion of a structural screen wall system. A 25 psf (1.20 kPa) strength level wind pressure acts on a 20 ft (6.10 m) tall CMU wall supported by a continuous grade beam. The 21 in (533 mm) wide by 18 in (457 mm) deep grade beam is isolated from an expansive soil and is supported by drilled shafts 21 ft (6.40 m) on center. The wind load and gravity loads induce torsion, biaxial bending moments, and biaxial shear forces in the grade beam. This example shows how to calculate the internal forces in the grade beam at the critical section and design the required longitudinal and shear reinforcement according to the ACI 318-19 code.
The design of the grade beam includes closed stirrups of #4 (Ø 12) bars spaced at 5.5 in (140 mm), five #8 (Ø 25) bars used near the top and bottom faces and one #6 (Ø 16) bar used at mid-height near the side faces.