Concrete Formwork: Reading Structural Drawings for Pour Planning

What Structural Drawings Tell You About Formwork

The structural engineer's drawings are the formwork contractor's blueprint. They specify:

• Slab thickness and extent of each pour zone
• Beam sizes, depths, and spacing (which determines formwork support density)
• Column locations and heights (which affect prop placement and loads)
• Drop panels, haunches, and openings (which require custom forms)
• Construction joints and cold joint locations (which affect pour sequencing)
• Shoring and reshoring requirements and duration

If the structural drawings are unclear or incomplete about these details, the formwork contractor must submit RFIs before committing to a pour schedule.

Reading Slab Edges and Pour Boundaries

Identifying Perimeter Edges

Structural plans show slab edges with heavy solid lines where concrete meets the building perimeter:

• Outer edge of slab (edge of overhanging portion) is typically a thicker line
• Interior walls or drop panels are shown with solid lines and noted as "drop panel" or dimensioned with depth
• Changes in slab thickness are marked by thicker lines or callout notes

Understanding Drop Panels

Drop panels are thickened areas of slab around columns (for flat-slab construction). On the plan:

• Shown as a square or rectangular area around each column
• Dimensions given: e.g., "Drop panel 12' x 12' x 3" thick" means the drop extends 12 feet on each side with 3 inches of additional depth
• Formwork must change elevation within the drop—requires custom forms at the step
• The border of the drop panel is a "step" where elevation changes; mark this line carefully on the formwork layout

Openings in Slabs

Slab openings for mechanical equipment, stairs, or light wells require forms to be built around them:

• Openings are shown as hollow rectangles or circles on the plan view
• Dimensions and location are critical—measure twice, build once
• Some openings have pans (metal or fiberglass inserts) that go into the formwork; detail views show these
• The structural drawing must show openings clearly; if missing, this is an RFI before forming starts

Understanding Beam and Joist Configuration

Reading Beam Depth and Spacing

Beams are the "ribs" of the floor structure and dictate formwork support requirements:

• Beam size is shown on the structural plan (e.g., "B1 24" x 18"") or in a legend
• Beam depth determines the elevation of the deck—mark this on your forms
• Beam spacing (distance center-to-center) determines how many props and beams you need
• Wide beam spacing = fewer support points = heavier props; narrow spacing = lighter, more props

Joists and Ribbed Decks

Some floors use joists (smaller secondary beams) or joist systems (one-way or two-way ribbed slabs):

• Joists are shown as parallel lines at regular intervals
• Joist spacing is small (16"–30" typical), so the formwork is more like a grid than isolated beams
• Ribbed deck formwork is specialized—often uses standard dome or pan forms that are reusable
• The structural plan specifies joist depth and spacing; formwork must match exactly

Haunches and Thickened Beams

Some beams widen or deepen at specific locations (haunches) for structural reasons:

• Shown on detail views or the section (profile) of the beam
• Typically used at beam-column connections or mid-span reinforcement concentrations
• Formwork must step at the haunch—look at the detail view for exact dimensions and location

Decoding Construction Joints and Cold Joints

Construction Joints vs. Cold Joints

A construction joint is a planned break in the concrete pour. A cold joint is where one pour ends and the next begins (which always creates a construction joint). Understanding where they occur is essential for formwork planning.

How They Appear on Drawings

Construction joints are marked with dashed or solid lines with a label like:

• "CJ" or "CONST JT" (construction joint)
• Sometimes shown as a thick dashed line parallel to beams or around perimeter
• Dimensional reference: "Construction joint at Grid 3" or "Construction joint 40' from west wall"

Why Location Matters for Formwork

The formwork contractor must:

• Build a temporary "wall" (bulkhead) at each construction joint to hold the concrete on each side
• Ensure the bulkhead is rigid and won't leak (no concrete bleed-out)
• Plan the pour sequence around the joints—each zone between joints is a separate pour

Pour Strip Locations

The structural drawing specifies where pours start and stop. On a typical floor with long pours:

• Grid line or column line (easy to identify and form against)
• Mid-span (rare but sometimes used for long beam spans)
• Often every other bay or every third bay, depending on concrete volume and equipment

Shore and Reshore Requirements

What Are Shores and Reshores?

Shores: Vertical props that support the weight of wet concrete and formwork during a pour. Installed before concrete arrives and removed after concrete has gained strength (typically 7–28 days, per engineer).

Reshores: Props left in place (often at reduced load) after initial shores are removed. Reshores support the concrete slab above while allowing the next floor to be formed and poured below.

Where Shores Are Shown

Shoring requirements appear in:

• Shoring plan (separate drawing showing prop locations, spacing, and load ratings)
• General notes on the structural plan (e.g., "Shore all beams at 4'–0" centers, remove shores after 14 days")
• Section views or details showing how shores connect to deck and support structure

Reading Shoring Notes

Typical shoring notes specify:

• Spacing: "Props at 4' x 4' grid" or "One prop per beam"
• Load capacity: "Shores rated 15,000 lbs each"
• Duration: "Remove shores after 7 days" or "Shores required until concrete reaches 3,000 psi"
• Connection: "Shore bases bear on slab below; bearing plates 12" x 12" minimum"

Reshore Duration and Load Carrying

Reshores are left in place longer and at lighter loads:

• May support 20–50% of the slab load (engineer specifies)
• Typically remain for full building height (removed only after construction is complete)
• Spacing may be reduced compared to initial shores

Shoring Failure Risk

Improper shoring has caused floor collapses. Common errors:

• Removing shores too early (before concrete has gained strength)
• Under-spacing props (loads exceed capacity)
• Removing all reshores at once (the slab below isn't strong enough to carry full load)
• Not tightening or re-tightening shores as concrete cures and slab deflects

Column Pour Heights and Sequencing

Understanding Column Heights

Structural plans show column heights in two ways:

• Floor-to-floor height: The vertical distance between finished floor elevations. Used to plan overall framing but not for forming.
• Column pour height: The vertical distance the column formwork must reach (from the floor below to where the next floor beam sits). This is what the formwork crew needs.

Finding Column Pour Heights on Drawings

Look at:

• Elevation views or sections showing exact heights of columns relative to floor slabs
• Detail views at column-to-beam connections showing the pour height and where beam seats
• Notes on the structural plan specifying column heights (e.g., "Column 12'–3" tall")

Column Sequencing with Slabs

Typical concrete sequencing:

1. Form and pour columns (or piers) for a story
2. Place beam and joist forms on top of the columns
3. Pour the beams and deck as one pour (or in strips)
4. Remove beam/deck forms after concrete gains strength
5. Use reshores if the next story forms immediately below

Some projects pour columns and beams separately; others combine them. The structural drawing should clarify this in the notes.

Step-by-Step Approach to Reading Formwork Requirements

1. Identify the slab extent and thickness. Mark on the plan view which areas are concrete and at what elevation.
2. Mark all drop panels, beams, and changes in elevation. Use a colored pencil or digital markup to trace these boundaries.
3. Locate all openings. Check the plan for any voids in the slab (mechanical penetrations, stair wells). Verify dimensions against detail views.
4. Find construction joints. Locate bulkhead locations on the plan and note them on your forming layout.
5. Determine beam spacing and deck load. Calculate the total weight of concrete and formwork; this determines prop spacing and size.
6. Review shoring notes and the shoring plan. Confirm shore spacing, load capacity, and removal/reshore timing.
7. Check detail views for edge conditions, beam seat locations, and embed details. Some items (like stone set in slab) must be formwork considerations.
8. Cross-check with architectural and MEP drawings. Verify that penetrations and embeds shown on other trades' drawings don't conflict with your forming plans.
9. Identify any ambiguities and submit RFIs. Before committing to a pour schedule, clarify any unclear dimensions, joint locations, or shore requirements.

Common Formwork Issues and How to Avoid Them

Misread Elevation or Slab Thickness

Setting forms at the wrong elevation wastes concrete and creates a structurally deficient pour. Always dimension your forms off the drawing and double-check elevations with a laser level on site.

Forgotten or Incorrectly Sized Opening Forms

Slab openings poured solid or oversized/undersized cause rework. Carry a dimensions sheet when building forms and check each opening against the drawing.

Under-Shoring or Over-Spacing Props

Insufficient shoring can collapse the slab during the pour. Follow the shoring plan exactly and verify that all props are load-rated for the concrete weight.

Removing Forms Too Early

Concrete must reach sufficient strength (typically 50–75% of design strength, confirmed by concrete test cylinders) before forms are removed. The structural notes specify the wait time; confirm with the engineer if in doubt.