Reinforced Concrete Tilt-up Wall Panels
Tilt-up is a form of construction with increasing popularity owing to its flexibility and economics. Tilt-up concrete is essentially a precast concrete that is site cast instead of traditional factory cast concrete members.
The construction of tilt-up walls involves pouring the walls horizontally on the building's floor slab at the job site. A crane hoists the panels into a place where steel braces temporarily secure the panels until workers can weld permanent fasteners into the panel's joints, footing, and roofline.
The design guide for tilti-up concrete panels, ACI 551, states that tilt-up concrete walls can be analyzed using the provisions of ACI 318 - Alternative Analysis (Design) Method. Most walls, and especially slender walls, are widely evaluated using this method. The method is applicable when the conditions summarized below are met:
- The cross section shall be constant over the height of the wall
- The wall can be designed as simple supported
- Maximum moments and deflections occurring at midspan
- The wall must be axially loaded
- The wall must be subjected to an out-of-plane uniform lateral load
- The wall shall be tension-controlled
- The reinforcement shall provide design strength greater than cracking strength
ACI 318 provides the alternative design method as a simple and accurate option for analysis and design of simple walls meeting the method conditions. ACI 551 allows the use of this method for some cases even though some of the method limitations are not satisfied as long as the results obtained are still within acceptable ranges. However, some walls with complex geometries (multi-span continuous walls with the presence of openings) bring a lot of challenges accompanied with the use of the alternative analysis (design) method.
To understand the wall behavior and adequately address strength and stability requirement, other methods such as finite element analysis, utilized in spWall, can be used. Many other issues arise with panels not meeting the method limitations (continuous and contilevered walls, variable thickness and width, wall with openings, non-standard boundary conditions, walls with high compressive loads, in-plane lateral loads, non-standard concentraded load position from attachments of piping, racking etc., concentrated out of plane loads).
spWall uses the Finite Element Method for the structural modeling, analysis, and design of slender and non-slender reinforced concrete walls (including tilti-up walls) subjected to static loading conditions. The wall is idealized as a mesh of rectangular plate elements and straight line stiffener elements. The program calculates the required amount of reinforcement in the plate elements and stiffener elements based on the design code selected by the user. For solid walls, spWall can also compare cross sectional shear forces with calculated in-plane and out-of plane shear strength provided by concrete.
