STRUCTURE
NCSEA | CASE | SEI SEPTEMBER 2024
concrete:
Where City and
Nature Take Form
INSIDE: One River North Project 40
Spiderweb Cracking in Two-Way Slabs 13
What Does a Dam Engineer Do? 32
Intuit Dome Is Nothing But Net 46
structural DESIGN
Protective Design Strategy of
Blast-Resistant Structures
Reducing blast demands is an important aim achieved through reducing deformations.
By Dr. Ibrahim M. Metwally, Ph.D, PE
T he main strategy for blast-resistance structures design is to reduce
blast demands, which inherently is achieved by reducing the
deformations in structural and non-structural building components.
shapes and layouts of buildings when designing for blast loading.
This is accomplished through various techniques: Proper Selection of Structural System
According to FEMA 427 Guidelines
Increase Standoff Distance Not many code resources are available when designing for blast
loading. However, FEMA 427 does provide guidelines. The general
Providing sufficient protection by increasing protected standoff design recommendations are not that different from what has been
distances against external attacks. The most cost-effective solution for discussed here, in that blast resistant building designs should use simple
mitigating explosive effects to a building is to ensure the explosions geometries without sharp re-entrant corners and be placed on a project
occur as far away from the building as possible (increased standoff). site as far from the lot perimeter as practical. The following discusses
Therefore, the site selection for new construction and site protection FEMA 427 recommendations for different types of framing systems:
in existing structures is important in mitigation blast risk.
Frame System
Use of Protective Barriers Walls In frame structures, column spacing should be limited. Large column
spacing decreases the likelihood that the structure will be able to redistribute
Many types of barriers are designed to resist the impact of a vehicle load in the event of column failure (Figure 3).
explosive. Among them are massive concrete barriers (Kontek 2008), In frame structures, the exterior columns should be designed to resist
concrete enclosed with steel plates (Crawford and Lan 2006), and the direct effects of the specified blast.
soil filled corrugated metal (Crawford and Lan 2006). Each barrier The frame structures system should be designed to resist the likely progres-
is designed to absorb the large amounts of energy from an impact or sive collapse. In case of occurrence any localized failure must be considered.
blast with minimal effect on the facilities it is protecting. It is not desirable to use transfer girders in design. Loss of a transfer girder
or one of its supports can destabilize a significant area of the building.
If transfer girders are required, it must be to add extra transfer systems.
Proper Selection of Building Layout
Bearing-Wall Systems
The building shape and layout should be selected to minimize the
effects of blast loading. Re-entrant corners and overhangs are likely to In bearing-wall systems that rely primarily on interior cross-walls,
trap shockwaves and amplify blast effects. The reflected pressure on interior longitudinal walls should be spaced to enhance stability and
the surface of a circular building is less intense than on a flat building. to control the lateral progression of damage. In bearing-wall systems
When curved surfaces are used, convex shapes are preferred over con- that rely on exterior walls, perpendicular walls should be provided at a
cave shapes. Figures 1-2 present desirable and undesirable structural regular spacing to control the amount of wall that is likely to be affected.
Fig. 1. Desirable structures when designing for blast loading incorporate arches, domes (right), and single-
story layouts (top).
20 STRUCTURE magazine
Fig. 2. Less than desirable building features
for blast loading are multiple levels of
stories, complex designs, projecting roofs or
floors, and U-shaped buildings.
Roof System shear failures. Steel systems have inherent ductility but are locally
vulnerable open sections and connections. A combination of steel
The primary loading on the roof is the downward air-blast pressure. and concrete is ideal.
The preferred system is cast-in place reinforced concrete with beams in
two directions. If this system is used, beams should have continuous top
and bottom reinforcement with tension lap splices. Stirrups to develop Ductile Detailing of Structural Elements
the bending capacity of the beams closely spaced along the entire span
are recommended. Finally, use two-way floor and roof systems. Blast-resistant design philosophy allows structural elements to
undergo large inelastic (plastic) deformations under blast loading. A
ductile structure that undergoes large deformations without failure
Proper Selection of Structural Material can absorb much more energy than a brittle structure of the same
strength. Tensile reinforcement between 0.5% and 2% of the cross-
Cast-in-place reinforced concrete is the structural system preferred sectional area of the concrete element will usually ensure ductile
for blast-resistant construction. This is the material and structural behavior while providing the required strength.
type used for military bunkers. The military has performed extensive Compression steel in flexural members serves two purposes. After
research and testing of its performance. Concrete has significant mass, a structural member is deflected by blast loads, it attempts to spring
which improves response to explosions. back or rebound. Dynamic rebound causes load reversal and, under
Generally, simple geometries and minimal ornamentation (which certain conditions, can result in catastrophic failure.
may become flying debris during an explosion) are recommended.
If ornamentation is used, it is preferable to use lightweight materials
such as timber or plastic, which are less likely than brick, stone, or Acceptable Damage Levels
metal to become lethal projectiles in the event of an explosion.
Ultra high performance concrete (UHPC) is known for its superior Minor: Non-structural failure of building elements such as windows,
mechanical properties; compressive strength can reach up to 200 MPa doors, and cladding.
(29,000 psi) and tensile strength up to 40 MPa (5800 psi). Also, the Moderate: Structural damage is confined to a localized area and is
crack propagation can be well controlled due to inclusion of steel usually repairable. Structural failure is limited to secondary structural
fibers in its cement matrix, leading to a higher ductility and energy elements, such as beams, slabs, and non-loading bearing walls. However,
absorbing capacity so as to make it an ideal material for structural if the building has been designed for loss of primary members, local-
members that are exposed to the constant threat of blast attacks. Previous ized loss of columns may occur without initiating progressive collapse.
experimental work conducted by Mao et Major: Loss of primary structural compo-
al. and Wu et al., Barnett et al., Ibrahim nents such as columns or transfer girders leads
Metwally, Schleyer et al., and Melançon to loss of additional elements that are adjacent
confirmed the superior blast resistance of to or above/below the lost member. In this
UHPC structures under high loading rate case, the building is usually not repairable.
conditions such as explosion and impact
compared to traditional normal and high- Full references are included in
strength concrete. the online version of the article at
STRUCTUREmag.org.
Increase the Capacity of
Ibrahim M. Metwally, PhD, PE, is a professor of
the Ground Floor Columns concrete structures at the Concrete Structures Research
Concrete-filled steel columns have high Institute at the Housing and Building National Research
ductility and very good blast resistance Center, Giza, Egypt. He is licensed by the Wyoming
[Peyman, et al., Ibrahim Metwally and Board of Professional Engineers in the U.S. and
Fig. 3. Example of detonation and destruction of Level 1 column
Zhang, et al.). Concrete systems have (top) explosive location; (bottom) failure result of transfer girder and registered as a senior structural consultant at DRSO of
significant inertia but are susceptible to secondary columns. the Ministry of Housing of Egypt.
SEPTEMBER 2024 21
