Concrete masonry unit walls look indestructible. That's the problem.

A CMU block wall — the kind found in thousands of commercial buildings, mixed-use properties, parking structures, and older multifamily buildings across Los Angeles — projects permanence. Solid. Heavy. Immovable. To the untrained eye, a building wrapped in concrete block looks like the last thing that would fail in an earthquake.

To a structural engineer, it looks like a liability that hasn't been tested yet.

Unreinforced and under-reinforced CMU walls are among the most dangerous structural conditions in the existing LA building stock. They don't fail gradually. They don't give warning signs. When seismic demand exceeds their capacity, they fail suddenly, catastrophically, and in ways that are almost impossible to survive if you're nearby. And because they look solid right up until the moment they don't, owners routinely defer assessment and retrofitting — right up until a seismic event removes the option entirely.

Here's what CMU walls actually do under seismic loading, where they fail, and what engineering-led retrofitting looks like when it's done correctly.

The Basics: What CMU Construction Is and Why It Was Used

Concrete masonry unit construction — commonly called CMU, cinder block, or concrete block — became a dominant building material in Los Angeles from the 1940s through the 1980s. It was cheap, fast, fire-resistant, and readily available. For commercial and light industrial construction, it was the default.

A CMU wall is built from hollow or solid concrete blocks, stacked in a running bond pattern and mortared at the joints. In reinforced CMU construction, vertical and horizontal steel rebar is placed within the block cavities, which are then grouted solid — creating a composite masonry-and-steel system with meaningful tensile and shear capacity.

In unreinforced CMU construction — which describes a substantial portion of LA's older building stock — there is no rebar. The wall's strength comes entirely from the compressive capacity of the block and the bond strength of the mortar joints. Under gravity loads, this is adequate. Under seismic lateral loads, it is frequently not.

What CMU Walls Are Actually Supposed to Do in an Earthquake

To understand CMU failure, you first need to understand what a CMU wall is engineered to accomplish under seismic loading — when it's properly designed.

A reinforced CMU shear wall serves two primary structural functions during an earthquake.

The first is lateral force resistance. When ground motion moves the building horizontally, the shear wall absorbs that lateral energy and transfers it through the wall plane down to the foundation. The rebar and grout give the wall the tensile capacity to resist the forces trying to tear it apart — because seismic lateral loads don't just push, they pull, they twist, and they reverse direction multiple times per second.

The second function is diaphragm connection. The CMU wall must be positively connected to the floor and roof diaphragms above it — the horizontal structural planes that distribute seismic loads to the vertical elements. Without a positive, engineered connection between the wall and the diaphragm, the wall and the building can separate under seismic loading. The wall goes one direction. The roof goes another.

Both functions depend entirely on the presence, placement, and continuity of reinforcing steel. Remove the steel — or never install it in the first place — and the CMU wall's seismic performance drops to near zero in the critical failure modes.

The Three Ways CMU Walls Fail in Earthquakes

Not all CMU failures look the same. Understanding the failure mode matters because it determines the retrofit strategy.

Out-of-Plane Collapse

This is the failure mode that kills people.

An out-of-plane failure occurs when a CMU wall is subjected to seismic forces perpendicular to its face — forces pushing or pulling the wall away from the building rather than along it. In unreinforced CMU, the mortar joints have virtually no tensile capacity to resist this loading. The wall cracks horizontally at a mortar joint, loses its vertical continuity, and falls outward — or inward, toward occupants.

Out-of-plane collapse of unreinforced masonry walls was the primary cause of earthquake fatalities in the 1971 Sylmar earthquake, the 1987 Whittier Narrows earthquake, and the 1994 Northridge earthquake. It is a well-documented, well-understood failure mode. It is also entirely preventable with proper anchorage and reinforcement.

The triggering condition: an unreinforced CMU wall that is not adequately anchored to the floor and roof diaphragms. Without that connection, the wall has no out-of-plane restraint. It behaves as a freestanding panel under lateral loading — and freestanding CMU panels fall.

In-Plane Shear Failure

In-plane shear failure occurs when a CMU wall is loaded along its face — the direction it's theoretically designed to resist. Under high seismic demand, an under-reinforced CMU wall develops diagonal tension cracking, typically running at 45 degrees through the mortar joints or through the block units themselves. Once diagonal cracking initiates, the wall loses a significant portion of its shear capacity, and the damage progresses rapidly.

In-plane shear failure is less immediately life-threatening than out-of-plane collapse, but it can lead to building collapse when the shear walls are the primary lateral system and they fail simultaneously. It also produces the characteristic X-shaped cracking pattern visible on CMU buildings after seismic events — which is the visual signature of a wall that absorbed more seismic demand than it was reinforced to handle.

Diaphragm Separation

The third failure mode is the least visible and often the most consequential for building survival.

When the connection between the CMU wall and the roof or floor diaphragm is inadequate — under-designed anchor bolts, corroded connections, missing ledger angles, or no connection detail at all — the diaphragm can separate from the wall under seismic loading. Once the diaphragm separates, the lateral load path is broken. The roof or floor above loses its lateral support and can collapse independently of the wall.

This failure mode is particularly common in older tilt-up and CMU warehouse and commercial buildings, where the original construction used minimal diaphragm-to-wall anchorage by modern standards. It is also invisible during a visual inspection — the separation point is typically at the top of the wall, concealed by roofing material and parapet construction.

The Unreinforced Masonry Problem in Los Angeles

Los Angeles has an active Unreinforced Masonry Building ordinance — one of the first in the country, adopted after the 1971 Sylmar earthquake. Under this ordinance, buildings with unreinforced masonry bearing walls were required to be retrofitted or demolished. Most of the buildings subject to that original ordinance have been addressed.

The ongoing problem is the large inventory of CMU buildings that are technically reinforced — but reinforced to standards that predate the post-Northridge updates to the California Building Code. These buildings have some rebar. They have some grouting. But the reinforcement spacing, the development lengths, the connection details, and the diaphragm anchorage were designed to a seismic standard that the engineering community has significantly revised upward over the past 30 years.

A building that passed inspection in 1975 with minimal CMU reinforcement is not a safe building by 2025 standards. It is a building that hasn't been tested yet.

What Engineering-Led CMU Retrofitting Actually Looks Like

There is no universal CMU retrofit. The appropriate intervention depends on the specific failure mode risk, the building's configuration, the existing reinforcement condition, and the occupancy and risk tolerance of the owner. This is why CMU retrofitting requires a licensed structural engineer — not a general contractor working from a template.

The most common retrofit interventions for CMU buildings include:

Diaphragm anchor installation — Adding positive mechanical anchorage between the CMU wall and the roof or floor diaphragm, typically through steel anchor bolts, embedded angles, or through-bolt connections. This directly addresses out-of-plane collapse risk and diaphragm separation.

Shotcrete overlay — Applying reinforced shotcrete (pneumatically placed concrete) to the face of an existing CMU wall, bonded to new rebar and connected to the existing structure. This dramatically increases the wall's in-plane shear capacity and out-of-plane resistance without requiring demolition of the existing wall.

Reinforced concrete pilasters — Adding cast-in-place concrete pilasters at regular intervals along a CMU wall, connected to the foundation and diaphragm above, to reduce the unsupported span of the wall and provide out-of-plane restraint.

Carbon fiber reinforced polymer (CFRP) strapping — For certain out-of-plane retrofit applications, CFRP straps bonded to the face of the CMU wall provide tensile reinforcement that significantly increases out-of-plane capacity with minimal impact on the building's interior or exterior finish.

New shear wall additions — In buildings where the existing CMU lateral system is fundamentally inadequate, new reinforced concrete or steel shear walls may be added to supplement or bypass the existing CMU system entirely.

The choice between these approaches — and the combination thereof — is an engineering judgment that requires analysis of the existing structure, calculation of the seismic demand at the site, and design of a retrofit that meets current code requirements for the building's occupancy category.

This is not a job for a contractor who retrofits by intuition. It is a job for an engineer who designs by calculation.

Why In-House Engineering Is Non-Negotiable for CMU Retrofit Work

CMU retrofit projects have a higher-than-average rate of scope change once construction begins. Walls that appear reinforced on the original drawings turn out to have incomplete grouting. Diaphragm connections that should exist aren't there. Rebar that was specified was never installed, or was installed at incorrect spacing.

When a general contractor discovers these conditions mid-project and has to call an outside engineer for guidance, the project stops. The engineer has to mobilize, assess the condition, revise the design, resubmit to the city if required, and issue new instructions. Days become weeks. The retrofit scope expands without a clear cost structure because nobody priced the contingency.

When SKS's in-house engineer is on-site — which is standard on every structural retrofit project we take — conditions are assessed and design decisions are made in real time. No mobilization. No delay. No ambiguity about who owns the problem.

Thirty-nine years of design-build construction in Los Angeles has taught us that the gap between what drawings show and what walls contain is never zero. Our process is built for that reality.

SKS and Structural Retrofits: Engineering-Led From Assessment to Sign-Off

SKS Construction has been structural retrofitting in Los Angeles since 1987. Our CMU and structural retrofit work is led by in-house licensed structural engineers who own the project from initial assessment through city sign-off. We handle structural analysis, retrofit design, permit preparation, LADBS submission, construction, and final inspection under one contract.

Fixed-price bids. No subject-to-change clauses. Direct owner access to Shahab and Sam Shaolian on every project. Over 3,000 completed projects. 80% repeat clients. One firm that has been engineering and building in this city long enough to have retrofitted buildings that were new when we started.

CMU walls that look indestructible deserve to actually be indestructible. That requires engineering — not assumption.

Get a FREE Structural Assessment for Your CMU Building

If your property includes CMU construction — whether it's a commercial building, a mixed-use property, a parking structure, or a multifamily building with masonry walls — SKS Construction offers FREE structural assessments for owners across Los Angeles County.