A building does not have to collapse when it is first hit to be structurally dangerous. Damage from a blast can weaken columns, beams, slabs, walls and the connections between them while leaving enough of the structure standing to support its own weight. If the remaining structure has lost critical load-carrying capacity, a later failure can spread from one damaged element to others and produce a partial or total collapse. Engineering guidance describes this process as progressive collapse.
The risk is particularly consequential in Gaza, where a building damaged during the war can later become occupied because families have few alternatives. On September 16, a previously damaged residential building in Gaza City collapsed while displaced families were sheltering inside, killing at least 20 people in Reuters’ latest account. The UN said about 100 people, including roughly 50 children, had been living in the building.
The important distinction: damage is not the same as collapse
When a building is struck, the visible damage may tell only part of the story.
A blast can break concrete, bend or fracture reinforcing steel, damage structural connections, displace walls or remove part of a load-bearing system. Some damage is immediately obvious. Other damage can be concealed inside structural elements.
A reinforced-concrete building is designed so that different components work together to carry gravity and lateral loads. If one component is damaged, the forces it previously carried can be redistributed through other parts of the structure.
That redistribution can allow a damaged building to remain standing. But it can also leave the surviving components carrying loads for which they were not originally intended.
Research into blast-damaged reinforced-concrete structures specifically examines how initial local damage can reduce a building’s resistance to progressive collapse.
How a delayed collapse can develop
There is no single mechanism behind every delayed building collapse. But several structural processes can be important.
| Mechanism | What can happen | Why it matters |
|---|---|---|
| Local structural damage | Columns, beams, slabs, walls or connections are weakened or destroyed | The remaining structure must carry redistributed loads |
| Load redistribution | Loads move through alternative structural paths | Undamaged components may become more heavily loaded |
| Progressive failure | One failed component transfers additional demand to others | Local damage can develop into a much larger collapse |
| Deterioration | Cracking, exposed reinforcement, corrosion or water penetration can reduce capacity | The margin of structural safety can decrease over time |
| Additional loading or damage | Further impacts, construction activity or changes in occupancy can add stress | A weakened structure may have less capacity to absorb another event |
The important point is that time itself does not necessarily cause a damaged building to collapse. A delayed collapse generally requires a structural condition or subsequent event that pushes the remaining system beyond its available capacity.
What is progressive collapse?
Engineers use the term progressive collapse for a failure in which an initial, localized structural failure spreads to other parts of a building, producing a disproportionately large collapse. The U.S. National Institute of Standards and Technology uses essentially this definition in its structural investigations.
Imagine a simplified multi-storey building in which a critical column is badly damaged.
The floors above it still have to be supported. If neighboring columns and beams can redistribute the load, the building may remain stable. If they cannot, those elements can become overloaded. Their failure increases the load on other components, potentially creating a chain of failures.
This is why the condition of the remaining structure matters as much as the part that was initially damaged.
FEMA guidance on blast-damaged buildings similarly distinguishes direct blast damage from progressive collapse, noting that localized failure can spread depending on the building’s design and construction.
Why a building can remain standing after serious damage
Modern buildings are not simply collections of independent columns and floors.
They contain multiple structural elements that interact. When one component is damaged, other components can sometimes provide alternative load paths.
That structural redundancy can be lifesaving because it prevents a localized failure from immediately becoming a global one.
But redundancy has limits.
If several important components are damaged simultaneously, or if a critical connection fails, the remaining load paths can become insufficient. Research on blast-damaged reinforced-concrete structures has found that pre-existing damage needs to be considered when assessing progressive-collapse behavior rather than assuming an otherwise undamaged building.
This helps explain why the same type of visible damage can have very different consequences in different buildings.
Cracks are not all equal
A common mistake is to judge structural safety simply by looking for cracks.
Cracking is common in concrete structures and does not automatically mean that a building is about to collapse. Conversely, a building can have serious structural problems without displaying an obvious warning sign to an untrained observer.
Engineers need to consider questions such as:
- Which structural elements were damaged?
- How much of their cross-section or reinforcement was affected?
- Are columns still carrying their intended loads?
- Have beams or slabs lost their connections?
- Has the structure shifted, tilted or undergone excessive deformation?
- Can remaining components safely carry redistributed loads?
- Has water or corrosion affected exposed reinforcement?
- Has additional damage occurred since the original strike?
The answers require technical inspection and, in serious cases, structural analysis.
Why corrosion and water can matter
Damage can expose steel reinforcement inside reinforced concrete.
Normally, concrete provides protection to embedded steel. When reinforcement becomes exposed to moisture and aggressive environmental conditions, corrosion can occur. Corrosion can reduce the effective steel section and weaken the bond between reinforcement and surrounding concrete.
Research published in 2025 found that corrosion reduced the collapse resistance of tested reinforced-concrete substructures and could significantly affect their ultimate load capacity. The experiments were not conducted on Gaza buildings, so they cannot be used to calculate the risk for any particular Gaza structure. They do, however, illustrate a general structural-engineering mechanism relevant to damaged reinforced-concrete buildings.
In other words, deterioration is one reason why structural condition can change after the original damaging event.
Why occupation makes the problem more dangerous
A damaged building that is empty presents one type of risk.
A damaged building containing dozens of people presents another.
When families move into a structurally compromised building, ordinary activities add occupancy loads and people are exposed if part of the structure fails. More importantly, residents may have little ability to determine whether a building has suffered critical structural damage.
That is the dilemma now facing parts of Gaza.
The UN said people are living in damaged structures assessed as unsafe because they lack better options. Its humanitarian coordinator called for safer alternatives following the September 16 collapse.
The latest collapse involved a building that had previously been damaged and was subsequently occupied by displaced families. Reuters reported that approximately 2,000 damaged buildings are being used as accommodation across Gaza.
Gaza’s structural problem is much larger than one building
The September collapse occurred within an enormous landscape of damaged infrastructure.
A UNOSAT assessment published in 2025 estimated that more than 320,000 housing units in Gaza had been damaged by October 2025.
More recent UN reporting has documented extensive additional damage to Gaza’s built environment.
These figures should not be interpreted as a count of buildings that are all in danger of immediate collapse. Damage assessments and structural-safety assessments are different exercises.
Satellite imagery can help determine whether a structure appears destroyed or damaged. Establishing whether a surviving building can safely support its occupants generally requires closer engineering inspection.
Why the September 16 collapse is significant
The Gaza City collapse demonstrates the danger created when three conditions overlap:
severe previous structural damage, continued occupation and a shortage of safer accommodation.
According to the UN, approximately 100 people were living in the building when it collapsed. Rescue crews continued searching for survivors, but the operation was hampered by limited heavy equipment. Egypt provided machinery, while UN agencies and other responders supported the emergency response.
The National reported that Civil Defence teams were initially working with basic tools and digging by hand, while UNDP’s Gaza chief said approximately 400 buildings were considered at risk of collapse.
Those numbers should not be interpreted as a prediction that 400 buildings will necessarily collapse. They indicate a reported population of structures considered sufficiently vulnerable to warrant concern.
Can engineers predict which building will collapse?
Not with certainty from a headline figure or a photograph.
Structural assessment is inherently building-specific. Engineers need information about the original design, construction materials, extent and location of damage, deformation, remaining load paths and any subsequent deterioration.
The uncertainty is one reason authorities generally treat visibly or suspected structurally damaged buildings cautiously.
FEMA’s blast-damage guidance notes that the precise failure sequence depends on factors including the building’s construction and the location and nature of the damage. Two nearby structural elements can respond differently to the same blast depending on how they are affected.
This also means that the phrase “damaged building” should not automatically be translated into “building about to collapse.”
The opposite mistake is equally dangerous: assuming that a structure is safe simply because it remained standing after the original strike.
What can reduce the risk?
The most effective approach is not to wait for a building to collapse.
Where conditions permit, structural engineers can assess damaged buildings and classify them according to their condition. Depending on the findings, a structure may require restricted access, temporary stabilization, repair, partial demolition or complete demolition.
The process also requires safe alternatives for occupants.
That is especially important in Gaza, where the UN says families are already living in damaged buildings because they lack safer options. Humanitarian agencies are therefore working on both sides of the problem: emergency rescue capacity for buildings that fail and alternative shelter for people who need to leave unsafe structures.
Why the risk is likely to remain a long-term issue
The danger from war-damaged buildings does not end when active fighting decreases.
Thousands of damaged structures still need assessment. Some can potentially be repaired; others may require major stabilization or removal. Debris must also be cleared, access restored and reconstruction undertaken.
That makes structural safety one part of a much longer recovery process.
For Gaza’s residents, the most important distinction is therefore between a building that survived an attack and a building that has been demonstrated to be safe after an attack. They are not the same thing.
The September 16 collapse is a reminder of that difference. A structure can remain standing after severe damage, accommodate families for months, and still possess insufficient structural capacity to withstand the next failure mechanism that reaches it. The unresolved challenge is determining which buildings remain safely occupiable—and doing so quickly enough that families are not forced to discover the answer when the structure comes down.