ECB5204 Lecture 1: Construction Methods

• Method and equipment for construction is detailed in ECB5204 Lecture 1, and includes design and construction of formwork systems

Course Data ECB5204

• Requires ECB3202 as a prerequisite
• Includes design and construction of formwork systems for horizontal and vertical applications
• Covers concrete technology, mixing, batching, transporting, placing, and compacting concrete
• Includes design and construction of shoring systems, continuous piles, secant piles, and diaphragm walls
• Discusses excavating equipment, draglines, clamshells, derrick cranes, mobile cranes, and tower cranes

Formwork for Concrete

• Essential Textbooks:
  • Construction Methods and Management by S.W. Nunnally
  • Formwork for Concrete by M.K. Hurd, 6th edition, American Concrete Institute, Detroit, Michigan, 1995
  • Handbook of Temporary Structures in Construction by Robert T. Ratay, 2nd edition, McGraw Hill, New York, 1996
• Formwork is a temporary structure erected quickly, heavily loaded briefly for concrete placement, and disassembled soon after
• Connections, braces, tie anchorages, and adjustment devices are classic temporary components
• Forms, tie hardware, and accessories are used repeatedly, which needing high durability, maintainability, and productivity-maximizing design

History of Formwork for Concrete

• Lumber was popular, but now plywood, metal, and plastics are more common
• Formwork used to be built in place and destroyed after single use
• High labor costs are driving prefabrication, assembly in large units, mechanical erection, and form reuse in the U.S.
• In 1908, the American Concrete Institute (ACI) discussed wood vs. steel formwork
• Steel forms for paving were commercially produced by 1910 in field applications
• Today, modular panel forming is the norm

Objectives of Form Building

• Molds concrete to the required size and shape, controlling position and alignment
• Acts as a temporary structure
• Supports its own weight, freshly placed concrete, and construction live loads

Basic Form Building Objectives

• Quality: strength, rigidity, position, and dimensions
• Safety: for workers and the concrete structure
• Economy: cost-effectiveness while maintaining quality and safety
• Collaboration between engineers/ architects, and contractors is essential

Economic Considerations

• Formwork costs can constitute up to 60% of the total project cost
• Contractors should aim for the maximum economy without neglecting quality or safety

How Formwork Affects Concrete Quality

• Accurate form construction ensures correct size, shape, and alignment of slabs, beams, and other concrete structural elements
• Forms should be rigid enough under construction loads to maintain the designed shape of the concrete and stable enough to keep large members aligned
• Forms should allow for reuse without losing their dimensional integrity
• Formwork must stay in place until the concrete can support its weight to prevent damage

Causes of Formwork Failure

• Formwork failures often occur during concrete construction, especially when fresh concrete is placed
• Unexpected events can cause a member to fail, overloading others and collapsing the formwork structure
• Main causes of formwork failure:
  • Wrong stripping and shore removal
  • Unsuitable bracing
  • Vibration
  • Unstable soil under mudsills and shores not plumb
  • Inadequate concrete placement control
  • Lack of attention to formwork details

Formwork Terminology

• Mudsill: a plank, frame, or small footing as a base for a shore or post

Wrong Stripping and Shore Removal

• Premature removal of forms/shores and poor reshoring creates catastrophic results
• Case Study: Bailey's Crossroads, Virginia (1972)
  • A 26-story apartment building collapsed due to early shore removal
  • Forms were supported by floors 7+ days old
  • The 24th floor failed because it was shored to a 5-day-old 23rd floor

Unsuitable Bracing

• Lateral force components or displacement of supporting members are frequent causes of formwork failure
• Inadequate cross and horizontal bracing of shores is a prevalent factor
• Diagonal bracing can prevent accidents, saving significantly on potential damages

Failure: Vibration

• Forms collapse when supporting shores or jacks are displaced by vibration
• Common causes of forms collapsing are passing traffic, movement of workers and equipment, and vibrating concrete to consolidate it
• Diagonal bracing helps prevent failure

Failure: Unstable Soil under Mudsills, Shoring Not Plumb

• Adequately braced formwork should safely transfer loads through vertical members to solid ground
• Shores must be plumb, and the ground must bear the load without settling
• Shores and mudsills must not rest on frozen ground since thawing can cause settling and shifting
• Adequate site drainage is a requirement so it can prevent soil washout supporting the mudsills

Failure: Inadequate Control of Concrete Placement

• The temperature and rate of vertical concrete placement can affect lateral pressure
• Concrete placement should be slowed if temperatures drop during construction, or else this will lead to formwork failure
• Improper order/rate of placing concrete on horizontal surfaces or curved roofs causes unbalanced loads and formwork failures

Failure: Lack of Attention to Formwork Details

• Minor assembly differences can cause local weakness or overstress, leading to failure
• Other issues that may cause failure:
  • Inadequate rotation prevention in beam forms where slabs frame into them
  • Lack of anchorage against uplift for sloping form faces
  • Insufficient bracing/tying of corners, bulkheads, or other areas with unequal pressure

Planning for Safety

• Comply with OSHA, ACI, and local code requirements
• Essential elements:
  • Supervision and Inspection
  • Platform and Access for Workers
  • Control of Concreting Practices
  • Improving Soil Bearing and Bracing
  • Shoring and Reshoring
  • Relationship of Architect, Engineer, and Contractor
  • Maintaining and Coordinating Tolerances
  • Preparing a Formwork Specification

Lecture 1 of ECB5204 details construction methods and equipment. This includes the design and construction of formwork systems for horizontal and vertical applications. The lecture also covers concrete technology and the design of shoring systems.