Falsework Design Studio

Falsework is the temporary structure that holds a permanent structure in place until it can carry itself. On a cast-in-place bridge it carries the fresh concrete, the reinforcing steel, the forms, the crew and their equipment. The Caltrans definition covers the steel or timber beams, girders, posts, foundations and any other members that support the permanent work during construction.

Formwork is the mold that gives concrete its shape: plywood soffits, side forms, bulkheads and the studs and walers that stiffen them. Falsework holds the formwork up. Shoring is the general word for vertical supports, most often used for building slabs. Reshoring is shoring placed after the original forms are stripped, so the load of new floors above is shared with the floors below.

The contractor is responsible for the design, construction and safety of falsework. A licensed engineer prepares the drawings and calculations when the contract requires it. The owner's engineer reviews them for compliance with the contract criteria. Review does not move design responsibility to the owner. Changes in the field need revised drawings from the falsework engineer before concrete is placed.

Check the governing contract first. For IDOT work, start with Standard Specifications Section 503 (Concrete Structures) and the special provisions. Caltrans work follows Standard Specifications Section 48-2 and the Falsework Manual. Cal/OSHA Title 8 §1717 sets its own requirements for falsework and vertical shoring.

Falsework drawings should show

• Design criteria and loads: unit weights, live loads, minimum loads, horizontal and wind loads.
• Member sizes, grades, species and spacing at every level, with splice locations.
• Bracing layout and connection details in both directions.
• Foundation type, pad and sill sizes, and the allowable soil bearing used.
• Concrete placement sequence and any stage loading.
• Falsework camber and grade adjustment method (wedges, jacks, sand jacks).
• Traffic openings: clear widths, vertical clearance, protection and lighting.
• Removal sequence and the earliest release criteria.
• Stress and deflection calculations for every load-carrying member.

The three criteria sets built into the studio agree on the big numbers but differ in detail. Always confirm against the edition named in your contract.

The vertical design load is dead load plus live load, but never less than the minimum total load. Dead load is the concrete, reinforcing steel, forms and the self-weight of each falsework member. Live load covers workers, tools and equipment. The minimum covers small loads nobody wrote down: material staging, a pile of rebar, a buggy.

Because concrete thickness changes across a section, the design pressure changes too. Under a box girder web the falsework sees the full girder depth; under a cell it sees only the top and bottom slabs.

Falsework must be braced for a horizontal load in any direction. Caltrans uses the sum of actual horizontal loads plus wind, but not less than 2% of the total dead load at the location, applied at the top of the posts. The check is made with the falsework both loaded and unloaded. ACI 347 uses the greater of 100 plf along the slab edge or 2% of the dead load.

For timber posts and other conventional falsework, Caltrans wind pressure is a velocity coefficient for each height zone times a drag factor Q = 1 + 0.2W (not more than 10), where W is the width of the falsework in the wind direction. Wind on each height zone gives an overturning moment, and dividing by the height to the top of the posts gives the design wind load at that level.

Timber post and beam bents

The common highway system: timber posts on sills and pads, a timber or steel cap, timber or steel stringers and timber joists. Grade is set with hardwood wedges at the top or bottom of the posts. It is cheap and flexible but labor-heavy, and posts are limited by slenderness.

Pipe post bents with steel caps

Steel pipe columns carry heavier loads over taller heights. A braced pipe bent behaves as a rigid frame with top and bottom caps. Grade adjustment is made at the top or bottom with wedges or jacks.

Heavy-duty steel shoring towers

Proprietary towers carry up to about 100 kips per leg and permit long falsework spans. Capacity comes from the manufacturer's tested values for the actual extension, bracing and load eccentricity. Phase 2 adds tower design.

Spans over traffic

Steel beams span openings over live traffic on bents protected by temporary barrier. Caltrans increases post and tower loads to 150% next to traffic and requires positive connections that resist impact. Phase 3 adds openings, clearances and crash-protection checks.

Building shoring and reshoring

Floors are formed on joists and stringers carried by adjustable steel or aluminum shores. After stripping, reshores share the weight of the next floor with the floors below. The number of levels of reshores depends on cycle time and slab strength.

Load moves down through a stack of members, and each interface is a bearing check. The studio follows the same order: sheathing, joists, stringers, caps, posts, then sills and pads.

Plywood soffit forms are designed per foot of width using the APA Plyform properties: effective section modulus KS, moment of inertia I and rolling shear constant Ib/Q. Panels usually run across three or more joists, so the continuous three-span coefficients apply. Bending uses the center-to-center span, shear uses the clear span, and deflection uses the clear span plus a small allowance (1/4 in. for 2-in. lumber, 5/8 in. for 4-in. lumber).

Current Caltrans practice designs timber with the NDS. Each reference design value is multiplied by adjustment factors: load duration CD = 1.25 for falsework loads (1.6 for wind), wet service CM when moisture content exceeds 19%, size factor CF, beam stability CL, and repetitive member Cr for closely spaced dimension lumber.

Steel stringers and caps are usually rolled W-shapes. Caltrans limits flexural compression stress to 12,000,000/(Ld/bt) psi, where L is the unbraced length of the compression flange, d the depth, b the flange width and t the flange thickness. The limit is 22,000 psi for unidentified or A36 steel and 0.6Fy for other identified grades. The AISC 360 option uses the lateral-torsional buckling rules of Chapter F with Ω = 1.67.

Wherever one member sits on a timber member, the contact area is checked against Fc⊥. The load duration factor does not apply to Fc⊥. When the bearing length is under 6 in. and at least 3 in. from the end of the member, the bearing area factor Cb raises the allowable value.

Posts are checked as columns. For timber, the NDS column stability factor Cp depends on the slenderness Le/d, which may not exceed 50. Steel pipe posts use AISC 360 Chapter E, or the Caltrans formula 16,000 − 0.38(L/r)² psi for unidentified steel with L/r ≤ 120. Manufactured shores and tower legs use the manufacturer's allowable load for the actual extension and bracing.

Before the pour, the soffit is set high so it settles onto the design profile. The adjustment adds the elastic deflection of the stringers, caps and joists under concrete weight, the shortening of the posts, joint take-up at each wood contact, and expected settlement of the pads. The bridge plan camber for permanent dead load is added on top.

A cast-in-place box girder is usually placed in two pours: the bottom slab and stems first, then the deck. Falsework is still designed for the full section, because the first pour is not yet structural when the deck goes on. Caltrans allows the stems to be treated as self-supporting between bents only when the bent spacing does not exceed four times the first-pour stem depth and the deck is placed more than 5 days after the stems.

For T-beams, the stems concentrate load the same way box webs do. Place stringers under the stems where you can, and check the stringer under each stem with its full tributary load.

Walls, columns and pier cap sides are loaded by the lateral pressure of fresh concrete. ACI 347 gives the maximum pressure from the rate of placement R (ft/hr), concrete temperature T (°F), a unit weight coefficient Cw and a chemistry coefficient Cc. Phase 3 adds full form and tie design.

• Columns, and walls with R ≤ 7 ft/hr and placement height ≤ 14 ft: p = Cw·Cc·[150 + 9,000R/T]
• Walls with R ≤ 7 ft/hr above 14 ft, or 7 < R ≤ 15 ft/hr: p = Cw·Cc·[150 + 43,400/T + 2,800R/T]
• Not less than 600Cw psf, and not more than full liquid head wh. Upper limits of 3,000Cw (columns) and 2,000Cw (walls). Use full liquid head for R > 15 ft/hr, self-consolidating concrete or pumping from the base.

• Missing or weak bracing. A braced bent is a frame; an unbraced one is a row of columns waiting to sway. Bracing connections fail before the braces do.
• Foundation failure. Pads on soft fill, saturated ground, frozen ground that thaws, and washouts under sills all cause sudden settlement.
• Crushing at bearings. Small contact areas on timber caps and sills crush perpendicular to grain, so grades drop during the pour.
• Steel beam web and flange failures. Concentrated reactions at caps and posts cause web yielding, crippling and flange bending when stiffeners are missing.
• Unauthorized changes. Substituted members, moved posts or removed braces without revised drawings.
• Unbalanced or out-of-sequence placement. Concrete placed against the planned sequence loads one side or one span first.
• Premature removal. Releasing falsework before the concrete reaches the specified strength, or before post-tensioning when required.
• Impact. Vehicles or equipment striking posts at openings or on the jobsite.
• Wind during erection. Partially erected falsework, steel girders and forms are most vulnerable before they are braced.

• Caltrans, Falsework Manual, Chapter 3: Loads
• Caltrans, Falsework Manual, Chapter 5: Analysis
• Caltrans, Falsework Manual, Chapter 4: Design Considerations
• Caltrans Standard Specifications, Section 48-2 Falsework
• AASHTO, Guide Design Specifications for Bridge Temporary Works
• ACI 347R, Guide to Formwork for Concrete; ACI 347.2R, Guide for Shoring/Reshoring of Concrete Multistory Buildings
• ASCE/SEI 37, Design Loads on Structures During Construction
• AWC, National Design Specification (NDS) for Wood Construction and Supplement
• AISC 360, Specification for Structural Steel Buildings
• APA, Concrete Forming Design/Construction Guide (Plyform)
• OSHA 29 CFR 1926 Subpart Q; Cal/OSHA Title 8 §1717
• IDOT Standard Specifications for Road and Bridge Construction, Section 503
• INDOT/Purdue research summary, Structural Impact of Construction Loads

Limitations. This studio is a design-and-check aid. Library section properties and reference design values are typical published values and should be verified against the editions in your contract. Final falsework drawings and calculations remain the responsibility of the falsework engineer.

FALSEWORK DESIGN STUDIO · ENG. AHMAD SAFI, PE