Concrete Formwork Design

By Patrick McLaughlin, MCE, SE | Project Engineer | Nishkian Dean

Concrete formwork is the temporary structure built to support and confine concrete until it hardens and it is commonly broken into two categories: formwork and shoring.  Formwork refers to vertical forms used to form walls and columns whereas shoring refers to horizontal formwork to support slabs and beams.

Forms must be designed to resist all vertical and lateral loads exposed onto the formwork during transport and in-use.  Forms can be either pre-engineered panels or custom-built for the job.  The advantage of pre-engineered panels is the speed of assembly and the ease of reconfiguring the forms to cycle to multiple pour locations.  The disadvantages are fixed panel and tie dimensions that limit their architectural applications and allowable design loads that may limit their use for certain applications.  Custom-built forms are designed to maximize the efficiency for each application but they are not as easy to reconfigure for other pour locations.  Custom forms can be built to accommodate any architectural consideration or loading condition.

Wall forms are typically assembled lying flat on the ground and must be lifted vertically and flown into place.  For tall panels, lifting the panel to a vertical position subjects the form to large bending moments, so the pick points must be carefully designed so that the panel components are not overloaded in this process.  Once the panels are flown into place they are secured with bracing to plumb the forms and resist wind forces.  Wind forces should be designed per ASCE-7: Minimum Design Loads for Buildings and Other Structures with the adjustments outlined in ASCE 37: Design Loads on Structures during Construction to account for the short duration of the construction project.  For a typical construction project with a duration ranging from 6 weeks to 1 year, the adjustments in ASCE 37 account for a 36% decrease in design wind pressures.  ASCE 37 does not specify a minimum wind pressure, however ACI 347: Guide to Formwork for Concrete specifies a minimum pressure of 15 psf which often controls.

Forms are designed for a lateral concrete pressure that is dependent upon many factors including concrete type, pour height, pour rate, temperature, and the use of superplasticizers and retarding admixtures.  The design concrete pressure should be discussed early on during the design of the formwork since that will influence the contractor’s allowable pour rate.  If a slow setting concrete and/or a fast pour rate is planned, the forms might need to be designed for higher concrete pressures than normally practiced.

For concrete with a slump less than or equal to 7 in and vibrated with internal vibrators to a depth of 4 ft or less from the top of the forms, ACI 347 section 2.2.2.1 provides the following equations for estimating the lateral concrete pressure:

For all columns and for walls with a placement rate less than 7 ft/hr and form height less than or equal to 14 ft

p_max=Cw*Cc[150+9000*R/T] ≥ 600 psf

For walls greater than 14 ft tall or for placement rates between 7 to 15 ft/hr: p_max=Cw*Cc[150+43,400/T + 2800*R/T] ≥ 600 psf

Cw=unit weight coefficient per ACI 347, table 2.1

Cc=Chemistry coefficient per ACI 347, table 2.2

T=temperature of concrete during placement, degrees Fahrenheit

R=rate of placement, ft/hr

Due to the finishing advantages of self-consolidating concrete (SCC), some contractors prefer SCC concrete mixtures versus traditional concrete mixes.  However the concrete pressures resulting from these mixes are considerably higher than traditional mixes.  ACI code lacks a procedure for estimating concrete pressures for SCC, so ACI recommends the design pressure should be based upon the hydrostatic pressure given by the equation:

p=w*h

w=unit weight of concrete

h=depth of fluid concrete from top of placement

Designing to the full liquid pressure of the form is a conservative approach, but this method proves costly and difficult to achieve for tall walls.  Researchers are working to better understand the effects of various concrete parameters to provide a method for accurately estimating SCC pressures.

Shoring systems are designed to support the full dead load and construction live loads of the surface being formed.  ASCE 37 specifies design live loads corresponding to four classifications of the working surface: very light duty, light duty, medium duty, and heavy duty.  Descriptions of these classes and their corresponding live load are summarized in the following table.

Note ACI 347 recommends that formwork should be designed for a minimum live load of 50 psf and combined dead and live loads not less than 100 psf.

Shoring systems are also designed for minimum specified horizontal loads.  ASCE 37 and ACI 347 specify a horizontal load equal to 2% of the vertical load applied in any direction.  In addition ACI 347 recommends this load should not be less than 100 lbs/ft of floor edge.  These horizontal forces are independent load cases and are not applied concurrently with wind or seismic. 

Due to the short duration of construction, the probability of a seismic event occurring during construction is incredibly low.  Therefore unless specifically required by the governmental authority, owner, or engineer of record, seismic forces are typically not considered.  Since the formwork system is already designed to resist lateral forces due to wind and minimum specified horizontal loads, it is reasonable to neglect the lateral forces due to seismic.

In construction the two most important objectives are ensuring the safety of personnel and keeping the project on or ahead of schedule.  A well designed formwork system helps the contractor meet both objectives and allows them to focus on the critical challenges of the project.  Nishkian Dean’s staff is experienced in the construction of a wide range of project types, ranging from multistory buildings to rocket launch facilities.  If you need assistance with your formwork design or have a different construction challenge, set up a meeting with one of our team members.

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