Dive into the complexities of concrete masonry unit (CMU) wall intersections with other materials and assemblies, including their interaction with joists and beams.
Key Insights
- Learn how CMU walls interact with other materials and assemblies, including stair landings and slab on metal deck. The author also explains how CMU walls interact with joists and beams, involving complex angles and continuous supports.
- A comprehensive explanation is given on site retaining wall schedules, serving as an essential structural element in landscaping and hardscaping of buildings. The article emphasizes the importance of these retaining walls in supporting large bodies of soil and managing significant height differentials.
- The importance and engineering of drainage systems within retaining walls are discussed, highlighting the detrimental effects of water freeze and thaw cycles on concrete. The article underscores the role of corrugated drainage piping and weeping systems in preventing water buildup and subsequent damage to concrete structures.
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Several details on blueprint sheets focus on how concrete masonry unit walls meet other materials and assemblies throughout the building. These include conditions such as stair landings where slab on metal deck sits on top of beams and ties into the CMU walls, as well as conditions where joists or beams run up to the face of a CMU wall.
Detail C specifically addresses the joist-to-CMU wall condition. Where joists or beams bear against or terminate at a CMU wall, a continuous steel angle is used to tie the top of the wall into the surrounding structure. This angle provides a reliable connection point that holds the wall in place relative to the structural frame above it.
Details D, E, and F cover additional CMU support conditions, primarily showing how reinforcement is handled adjacent to openings in the wall. These are standard details that will appear repeatedly wherever CMU walls include door or window openings that interrupt the normal coursing of the masonry.
Site Retaining Walls: Purpose and Function
Beyond the structural steel and metal decking inside the building, the structural drawings also cover site retaining walls. These are exterior elements located within the landscaping and hardscaping around the building, and they serve a critical structural function: holding back large volumes of soil where there is a significant grade change from one side of the wall to the other.
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The typical condition shown has soil bearing against the full height of one side of the wall, while the opposite side drops down to a much lower finished grade. The retaining wall must resist all of the lateral pressure that this soil exerts, which is what drives both the wall geometry and the reinforcement strategy.
Reading the Retaining Wall Schedule
A retaining wall schedule in the bottom right corner of the detail sheet identifies the different wall types used on the project. Each type is labeled with a designation and assigned its own set of dimensions across several columns: T1, T2, D1, B1, and B2. These columns define the specific proportions of each wall type such as stem thickness, footing width, and footing depth.
The general detail drawing shows how all of these dimensions relate to each other in cross section, illustrating the stem, the footing, the rebar layout, and the surrounding soil conditions. Reading the schedule alongside the detail gives a complete picture of what each wall type looks like and how it is built.
Footing Design and Soil Pressure Resistance
One notable feature of the retaining wall detail is the depth of the footing. Rather than a shallow footing, the base of these walls extends well down into the ground. This is intentional: the deeper the footing, the more resistance it provides against the tendency of the retained soil to push the wall laterally and rotate it at its base.
The rebar layout reinforces this further. Bars are placed in an L-shape configuration to tie the stem and footing together, and additional bars extend horizontally back into the soil zone of the footing. Together, these create a reinforced assembly that can handle the sustained lateral load of the retained soil over the life of the building.
Rebar Specifications from the Schedule
The retaining wall schedule specifies the bar size and spacing for each zone of the wall. As an example, a vertical bar specification might call for number 8 bars at 8 inches on center, while other zones use number 7 bars at different spacings. Each wall type in the schedule carries its own bar specifications, so it is important to read the schedule carefully for the specific wall type being constructed rather than assuming a single reinforcement layout applies to all conditions.
Concrete Specifications
The notes on this sheet call out two different concrete strengths depending on the portion of the wall:
- Footing: 4,000 PSI concrete
- Stem: 5,000 PSI concrete, air entrained, Class F2
The higher strength and air entrainment specified for the stem reflects its exposure to weather and freeze-thaw cycles above grade. Air entrained concrete contains microscopic air bubbles that give water room to expand when it freezes, reducing the risk of cracking and surface deterioration over time.
Control, Construction, and Expansion Joints
The notes also establish joint spacing requirements for the stem:
- Control and construction joints at a maximum of 25 feet on center
- Expansion joints at a maximum of 100 feet on center
These joints are essential for managing the natural movement of concrete due to temperature changes, moisture, and shrinkage. Without them, uncontrolled cracking is likely to develop in long continuous walls over time.
Drainage, Weeps, and the Risk of Water Buildup
The retaining wall assembly includes corrugated drainage piping behind the stem to collect and redirect water that infiltrates the soil side of the wall. Weep holes through the wall face allow that drainage pipe to discharge water out of the assembly rather than letting it accumulate.
This drainage strategy addresses two distinct risks. First, standing water exerts hydrostatic pressure on the wall, and if that pressure builds without relief it can eventually push or displace the structure. Second, water trapped in cracks or voids within the concrete is subject to freeze-thaw expansion. As water freezes, it expands in volume, widening existing cracks. Repeated freeze-thaw cycles progressively enlarge those cracks and can lead to serious deterioration of the concrete over the building's lifespan. Proper drainage is one of the most important long-term durability measures in any below-grade or earth-retaining assembly.
Coordination Notes
Several notes on this sheet require coordination with other drawing disciplines before the retaining walls are built:
- Note 5: The top-of-wall elevation must be confirmed against the landscaping plans.
- Note 6: Where underground piping crosses through the wall, the footing must be stepped to accommodate it. This requires coordination with the mechanical, plumbing, or civil utility drawings.
- Note 7: Any fencing above the retaining wall must be coordinated with the landscape drawing set.
These notes are a reminder that structural drawings do not exist in isolation. The retaining walls must be built to the correct elevation, must not conflict with underground utilities, and must be properly prepared to receive any site elements that bear on or attach to them. Reviewing all relevant drawing disciplines before finalizing the retaining wall layout is essential to avoiding conflicts in the field.
