Tutorial 34: Parapets, Control Joints & Pour Strips

Three features that have nothing in common except timing. A parapet is panel that continues past the roof. A control joint decides where the concrete cracks. A pour strip is concrete placed after the panels are already standing. Each one is a decision about what happens to the panel after it leaves the bed.

Feature

What it is

Standard

Parapet

A panel extension above the roof line

Architectural practice

Control joint

A tooled or scored joint that controls crack location

ACI / TCA

Pour strip / closure strip

A cast-in closure strip for a pour-back between panels

TCA Detail Library

Two planned features — and one you can already express

ConstructiVision models the roof line today (Tutorial 2: Panel Options), which is where the parapet begins, and it already treats the closure strip as a real thing: slab-dowel projections are measured into it, so Step 4 is something you can do in your build now. The parapet and the control joint are not modeled. This lesson records the intended design for those two; steps describing behaviour not in your build are marked (planned).

Note

A browser version covers part of this lesson. Open it here — it lists exactly which steps it does not cover, so you know what to come back to AutoCAD for.

What you will learn

  • Why a parapet is not simply “more panel” — and what changes about the panel when it has one.

  • Why a control joint is the one feature in the whole catalog whose job is to make the panel fail on purpose.

  • What a pour strip is, why concrete placed after construction is a different animal from concrete placed in the bed, and how to size the slab dowels that reach into it.

Before you start

Watch the lesson

Warning

What this recording does and does not demonstrate. It places the one part of this lesson that is built today: the slab-dowel row of Step 4 on TP08-12, at Elevation −3”, Max Spacing 1’-6”, Length 2’-0” and Inset 9”. The video zooms in on the redrawn row: it runs along Finished Floor with its note “2’-0” Dowels inset 9” @ 1’-6” o.c. Max.” and an elevation marker. The parapet, the control joint and the pour strip itself are described in the video, not drawn: they are not in the product yet.

Lesson

Step 1 — Parapet (planned)

A parapet is the part of the panel that continues above the roof. It hides rooftop equipment, it carries the roof edge detail, and — because it is cantilevered above the last point of lateral support — it is the part of the panel most exposed to wind.

Three things change once a panel has a parapet, and all three matter before the pour:

  1. The panel is taller and heavier, so the pick points move. A pick layout calculated on the roof-line height is wrong.

  2. The centre of gravity rises, which changes the lift and the bracing.

  3. The roof line stops being the panel top, so anything previously derived from “the top of the panel” has to be re-read as “the roof line” or “the parapet top”, and the two are now different numbers.

Intended behaviour: a parapet height on the panel, measured above the roof line, so both elevations exist independently and neither has to be inferred. The drawing dimensions both, and every downstream consumer — weight, pick points, elevations — reads the correct one rather than assuming they are the same. Where the parapet steps or slopes, it takes a start and end height.

Tip

A sloped parapet top is common and easy to get wrong on a drawing, because in elevation the slope is visible and in section it is not. Dimension both ends.

Step 2 — Control joint (planned)

Concrete cracks. That is not a defect; it is what concrete does as it cures and as the building moves. A control joint is a deliberate weakness — a tooled or scored line — that decides where.

It is the only feature in the catalog whose purpose is to make the panel fail in a chosen place, and that inverts the usual reasoning. A control joint that is too shallow does not work: the crack goes somewhere else, and now there is a crack and a joint. Depth is the whole feature.

Intended behaviour: a control-joint row taking position, orientation and depth, drawn as a scored line and dimensioned like one. It shares geometry with a reveal (Tutorial 29: Feature Strips & Reveals) and is not the same feature, for the same reason a reglet is not: a reveal’s depth is chosen for shadow, a control joint’s depth is chosen so the concrete cracks there and not elsewhere.

Where the depth comes from

Control-joint depth is a specified value — it comes from the project specification or the standard the job is built to, as a proportion of the panel thickness. ConstructiVision records the depth you enter and draws the joint at it. It does not select the depth and does not verify it is sufficient.

Control joints are often placed to line up with reveals so the joint disappears into the articulation. When they do, the two features are at the same position with different depths — which is exactly why they need to be separate rows rather than one row with a flag.

Step 3 — Distinguish a control joint from an expansion gap

They both sound like “a gap that lets the building move”, and they are not the same thing:

Control joint

Expansion gap

Where

within one panel

between two panels

What moves

nothing — it directs a crack

the panels, relative to each other

Set by

specification, as a fraction of thickness

the joint detail and the panel layout

Covered in

this lesson

Tutorial 9: Brace Points & Expansion Gaps

If a drawing shows a control joint where an expansion gap belongs, the panels have nowhere to move and the joint sealant fails. This is worth being pedantic about.

Step 4 — Pour strip / closure strip

A pour strip is concrete placed after the panels are standing, closing a gap between them — at a corner, at a column, or where the panel layout could not close cleanly. It is a pour-back: forms are built in place against the standing panels, and concrete goes in vertically instead of flat.

That timing is what makes it its own feature rather than a wider joint:

  • It is not cast in the bed, so it is not part of any panel’s weight, and including it in a panel weight overstates the pick.

  • It needs something to tie to — usually bent bars projecting from the adjacent panels, placed while those panels were still flat, months earlier.

  • It has its own concrete quantity, placed by a different crew with a different method.

The closure strip is already in your model — as the thing the slab dowels reach into. Slab dowels are the J-shaped bars along the panel base: they protrude from below Finished Floor and extend into the slab, anchoring the panel to the slab and to other floors after the closure strip is poured. Their Length is the distance they extend from the near face of the panel into the closure strip; a negative value raises a warning. The strip is not drawn, but it is the datum that number is measured against, and it is why that number is what it is.

So the strip already governs a value you enter today:

  1. Open Slab Dowels. Click Slab Dowels on the toolbar, or check Slab Dowels in the Main Panel Details feature checklist (Tutorial 2: Panel Options) — either route opens the same dialog. Six rows, one per dowel row.

  2. Enable a row and set its Elevation — from Finished Floor to the centre of the row. As in every feature dialog, an unchecked row keeps its values and stays out of the drawing.

  3. Set Length to the reach you need across the strip. This is the projection past the near face of the panel, so it is the closure detail that decides it — a dowel sized for a panel-to-slab edge is not a dowel that crosses a wide pour-back.

  4. Set Maximum Spacing and Inset, and choose what the row does where it crosses an opening — Trim, Offset, Extend from Top or Extend from Bottom.

Check the drawing: the length, spacing and inset are written as text along the dowel row, and an elevation marker dimensions the centre of the row. That text is what the yard reads, and it is the one place the closure strip shows up on a panel drawing today.

Still planned: the strip itself as an entity between two panels, taking a width and a height and reported as its own line in the materials list — separate from panel concrete, because it is a separate pour. Right now the strip is only implied by a dowel projection: it is not drawn, it carries no concrete quantity of its own, and it is not tied to the two panels that form it. Nor does anything flag those panels as requiring projecting reinforcement, so the bent bars that cross the joint are still entered panel by panel from the detail rather than derived from the strip.

The failure this prevents

The classic pour-strip failure is not structural — it is that the bent bars were never cast into the adjacent panels, because nothing on the panel drawing said they were needed. By the time the gap is being formed, the panels are up and the bars cannot be added. The slab-dowel row is not that check: it anchors one panel down at its base, not two panels to each other across the joint. Until the strip is an entity that knows its neighbours, the check is yours — read the closure-joint detail and place the bent bars on both panels yourself.

Check your result

You should now be able to:

  • Name the three things that change about a panel when it gains a parapet.

  • Say why the roof line and the panel top become two different numbers, and what breaks if they are conflated.

  • Explain what a control joint is for, and why its depth is the whole feature.

  • State the difference between a control joint and an expansion gap in one sentence.

  • Say why a pour strip is not part of any panel’s weight, and what it needs from its neighbours.

  • Set a slab-dowel Length that reaches across the closure strip, and say what on the panel drawing tells the yard that number.

Where next