Concrete Piled Slab Design 2020

Posted on July 29th, 2020 in Analysis

Summary

The provided excerpts detail the design process of a suspended piled slab for an industrial building that includes warehousing and office space. The design involves several steps, covering loading definition, analysis, reinforcement detailing for sagging and hogging moments, and resolving punching shear issues.

Initial Setup and Loading

The initial steps involved defining the structure and its loads.

• Loading: Loading was considered straightforward, including gravity loading (roof and floor loads) and wind loading.
• Mesh Creation and Pattern Loading: A small mesh of members was created for the ground slab. The primary reason for this mesh was to facilitate pattern loading. Strips were created through the building to allow for alternate load patterns (max min, max min). This technique helps determine maximum sagging combined with minimum sagging on specific spans, and vice versa. This pattern loading process was applied in both directions using strips.
• Other Loads: The analysis also accounted for superstructure loading and line loads located around the structure.

Slab Design and Reinforcement Strategy

After analysis, which covered different alternate loading cases in both directions for serviceability and ultimate states, the concrete slab design commenced.

• Basic Parameters: The design used 30 top cover and 50 bottom cover.
• Basic Reinforcement: The basic reinforcement was set based on experience at 12s at 150. While a typical start might be 10s at 200, 12s at 150 was chosen as the basic requirement for the slab.
• Detailing Preferences: For design settings, peaks were set to utilize one zone, and strips were preferably set to use replacement bars perpendicular to the line. The minimum size/spacing preference was 150, with additional bars set at 300.

Addressing Failures (Sagging and Hogging)

Initial results showed warnings and failures across the slab. The design process involved adding additional reinforcement strips and peak areas to resolve these unity value failures.

Sagging (Bottom Steel) Fixes:

Sagging failures were identified in specific strips in the X-axis (horizontal steel) and YY direction.

1. X-Axis Sagging: For the outer strips failing in X-axis sagging, an additional strip of steel was added, replacing the basic reinforcement with 16s at 150. This adjustment was made for strips on both sides of the structure.
2. YY Sagging: For the two problems identified in the YY sagging direction, a third strip was added. The basic reinforcement (10s) was replaced with 16s at 150. This strip was extended over the full panel width to simplify detailing.

Hogging (Top Steel) Fixes:

Top steel failures (overstressing) typically occurred over the column pile heads.

1. Peak Addition: These failures were addressed by using peaks as additional reinforcement over the pile heads.
2. Reinforcement Size: Initially, 12s at 150 in both directions were attempted, which resulted in failures that were close (within 1%) and accepted. To ensure reliability across all pile heads, all peak areas were ultimately set to use 16s at 150 in both directions (Direction 1 and Direction 2). These standard one-meter square pile heads were then confirmed to be working.
3. Adjustments: Some peak areas were enlarged or adjusted symmetrically (e.g., to 2 by 1) to fully cover minor failure zones.

Punching Shear Resolution

The final design step addressed punching shear.

• Shear Rails Option: Some points required shear rails, which could be placed radially at 150 centres (e.g., three shear rails at 150).
• Drop Head Alternative: Instead of shear rails, the punching shear issue was resolved by using a drop projection (or drop head). This involved adding a 100 mm thickening of the slab around the column head, typically covering a 1-meter square area (or 350 projection). This modification is noted as quite easy to implement in a piled slab.

The completion of these steps finalizes the design of the suspended piled slab. The design results, including basic strips, peaks, and punching shear details, can be exported as rebar drawings (DWG) . The final reinforcement included strips of 16s and other areas using 12s.

Summary with Timestamps

The following table provides a summary of the piled slab design process, organized chronologically by timestamp, drawing directly from the provided sources: