Designing Temporary Shoring Systems for Complex Removals
Temporary Shoring Fundamentals
Temporary shoring systems represent the critical interface between existing structures carrying loads and the new support configuration that will support those loads after demolition. Design failures in temporary shoring lead directly to collapse.
Unlike permanent structures where you design to specific loading conditions you can predict and control, temporary shoring must account for construction variables you can't perfectly predict: incomplete connections, temporary loading patterns, variable ground conditions, and dynamic loading from demolition activities.
Load Calculation for Temporary Shoring
Before designing shoring, accurately determine the loads it must support:
Tributary Area Analysis: Identify the area of floor or roof that will load the shoring. This isn't always obvious—loads might concentrate at unexpected points, particularly in older buildings with non-standard framing.
Dead Load Determination: Calculate weight of floors, roofs, walls, and any other permanent load. Demolition often uncovers hidden structural elements affecting dead load. When in doubt, use conservative estimates.
Live Load Assessment: Construction live load is typically 50 psf minimum, but some situations require higher assumptions. A floor being used to stage demolition equipment might experience concentrated loads that exceed design assumptions.
Temporary Load Concentration: Where will equipment sit on the floor being supported? Where will demolition materials be staged? Identify concentrated loads and their locations.
Impact and Dynamic Effects: Demolition creates vibration and dynamic loading as structures fail or are forcefully removed. Increase static loads by 25-50% to account for dynamic effects, depending on the specific demolition activities.
Lateral Loads: Don't forget lateral loads. Wind, personnel movement, and equipment vibration create lateral forces that temporary shoring must resist.
Soil and Foundation Conditions
Temporary shoring transfers loads to the ground. Ground capacity must support the loads:
Site Investigation: Understand soil conditions where shoring will bear. Soft clay, fill, or saturated soils might have insufficient bearing capacity for concentrated shoring loads. Sometimes site investigation reveals soil conditions that make specific shoring approaches impractical.
Bearing Capacity Verification: Calculate required bearing area and verify it against soil bearing capacity. Underpredicting soil strength creates settlement risk. Conservative soil bearing capacity (1,500-2,000 psf) is safer than optimistic assumptions.
Settlement Analysis: Calculate expected settlement under design loads. Will settlement create problems? In some cases, acceptable settlement under temporary loading might create unacceptable damage to adjacent structures or create unplanned load redistribution.
Groundwater: High groundwater creates bearing capacity and settlement complications. Stabilizing soil or using alternative approaches might be necessary.
Post and Column Design
Most temporary shoring uses vertical posts (typically steel pipe or adjustable steel shores) transmitting loads to base plates:
Post Capacity: Select post size based on:
- Axial load from floors and equipment above
- Buckling length (unsupported length of the post)
- Material strength
- Required safety factor (minimum 2.0 for temporary structures, often higher)
Buckling is the critical consideration for tall shoring posts. A post that's adequate for axial strength might fail by buckling under the same load if the unsupported length is too great. This is why shoring posts are typically limited to 12-15 feet without lateral bracing.
Lateral Bracing: Posts subjected to lateral loads (wind, vibration, personnel contact) must have lateral bracing. Diagonal bracing or connections to adjacent structural elements provide this bracing. Lateral bracing spacing typically shouldn't exceed 12 feet.
Connection Details: Posts must be securely connected to the floor above (to prevent toppling) and to base plates (to prevent slipping). Welds, bolts, or bearing connections all work, but connections must be detailed and installed per specifications. Field-improvised connections create failure risk.
Base Plate Design
Base plates distribute concentrated post loads to the soil:
Bearing Area: Calculate required bearing area based on post load and soil bearing capacity. Typical base plates might be 12" × 12" to 24" × 24", depending on loads and soil strength.
Plate Thickness: Base plates must be stiff enough not to bend under load. Thin plates allow the concrete or soil beneath to yield in a cone pattern, reducing bearing capacity. Thicker plates distribute loads more uniformly.
Leveling Plates: Uneven ground creates bearing problems. Leveling plates (grout or steel plates) under base plates ensure uniform bearing. This is often overlooked but critically important.
Anchoring: Base plates must resist sliding. Friction alone might not provide sufficient resistance. Consider anchor bolts if there's risk of lateral movement.
System Design Approaches
Single-Column Shoring: Individual posts supporting a floor directly above. Simple but requires many posts if large areas must be supported.
Temporary Beams with Posts: Installing temporary beams (steel, composite, or heavy wood) that span between posts allows larger spacing between columns. This reduces the total number of posts but requires beam design and adequate bearing at posts.
Adjustable Shores: Commercially available adjustable shores (hydraulic or threaded) allow height adjustment during installation. These simplify field installation but must be designed and loaded per manufacturer specifications.
Walls or Grid Systems: Where shoring must support very large areas or irregular floor configurations, temporary wall systems or post-grid systems with bracing might be most practical.
Integration with Demolition Sequence
Temporary shoring design must integrate with the demolition sequence:
Installation Sequence: When is shoring installed? Before, during, or after removal of existing structure? The sequence affects load calculations and temporary structure design.
Progressive Installation: In some cases, you'll install shoring progressively as loads are incrementally transferred. This requires careful planning and monitoring.
Removal Sequence: When is temporary shoring removed? This must be planned just as carefully as installation. Incomplete shoring removal leaves hazardous structures.
Interference with Other Work: Does shoring interfere with MEP contractor work? With material removal? Good temporary shoring design accounts for operational constraints.
Safety and Quality Control
Construction Inspection: Temporary shoring should be inspected by the design engineer before application of design loads. Field conditions often differ from assumptions—inadequate soil bearing, uneven foundations, or incorrect installation requires correction before load application.
Quality Control: Ensure that materials installed match design specifications (post sizes, connection types, base plate details). Field improvisation creates failure risk.
Load Testing: Before applying full design loads, consider test loading to verify that the shoring system responds as predicted. Large discrepancies might indicate design or construction problems.
Monitoring: During demolition, monitor the temporary shoring system for settlement, lateral movement, or damage. Stop work if conditions indicate problems.
Common Design Mistakes
Undersizing for Dynamic Effects: Demolition creates dynamic loading that static calculations underestimate. Conservative dynamic load factors are essential.
Inadequate Lateral Bracing: Posts without proper lateral bracing fail at loads far below their axial capacity. This is one of the most common shoring failures.
Neglecting Bearing Capacity: Assuming adequate soil bearing without investigation creates settlement risk. Soft or fill soils might require base plate enlargement or alternative approaches.
Field Improvisation: Temporary shoring that doesn't match design specifications shouldn't be installed. Field changes require re-design and approval.
Inadequate Monitoring: Temporary shoring should be monitored just as carefully as permanent structures. Unexpected settlement or movement should trigger investigation and corrective action.
The Professional Standard
Temporary shoring system design should be performed by or under the supervision of a licensed structural engineer. The complexity of load paths, soil conditions, and construction sequencing justifies professional engineering input. This also provides liability protection and ensures code compliance.
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