Steel and concrete are considered the safest materials by experts in tornado mitigation. An ideal building envelope would be 30cm thick reinforced concrete shell or 5cm maraging steel armour. While both of these are conceivable in theory, construction is not always economically viable.
Incorporating tornado resistance in buildings is a multi-pronged approach. The correct choice of building material, form and technique determines their performance in the event of a tornado. For this article, let’s approach specifically from the point of view of materials.
Gusts affect the entire super structure of the house starting from the plinth, all the way to the roof. It is imperative for us to understand how and where it is affected in order to arrive at effective solutions.
What do tornados do to buildings?
Buildings receive damage during tornados primarily in two ways:
- Force of winds travelling at speeds ranging anything from 85mph to more than 200mph.
Based on the EF** scale the sheer force of wind itself can dislodge features like windows and cladding. Stronger winds can even blow away everything from the roof to the whole house as a single unit. The key to protecting components of the building envelope from the direct force of wind is “bonding, bracing and reinforcing.” We can divulge into the meaning and significance of this in the following sections.
2. Impact of debris and other air borne objects.
Tornados displace debris and objects like vehicles which become hazards to property and human life. Winds of speeds exceeding 90mph are even known to uproot trees. These objects can collide with houses damaging them severely. Well bonded and reinforced building envelopes are to be built to weaken the effects of impact.
|WIND SPEED||EF SCALE||TYPICAL DAMAGE|
|65 – 85mph||0||Peels surface off some roofs, some damage to gutters or siding|
|86 – 110mph||1||Roof severely stripped, mobile homes overturned or badly damaged, loss of exterior doors, windows and other glass broken|
|111 – 135mph||2||Roofs torn off well-constructed homes; foundations of frame homes shifted; mobile homes completely destroyed|
|136 – 165mph||3||Entire stories of well-constructed homes destroyed; severe damage to large buildings such as shopping malls|
|166 – 200mph||4||Well-constructed houses and whole-frame homes completely levelled|
|200+ mph||5||Strong frame houses levelled off foundations and swept away; high-rise buildings have significant structural deformation.|
Table 1: Enhanced Fujita Scale showing strength of Tornados and their effects (Source: www.wunderground.com)
Conventional materials used in tornado prone areas:
Before moving into this section, please be advised that the materials discussed here only help to mitigate the effects of a tornado. They are not a hundred percent damage proof themselves. Wind force and debris will still affect them while keeping occupants safe and secure.
1. Reinforced cement concrete
As described earlier, an RCC structure would be one among the safest spaces to ride out a tornado. The concrete – TMT bar combination works in tandem to resist both tension and compression forces during a gust. They are also strong enough to resist penetration by flying debris.
The downside to constructing a complete house with RCC is always the enormous cost. According to FEMA P-320*, the construction of even a ‘safe room’ of size 8′ x 8′ can cost anywhere between 9,400$ to 13,100$; and a 14′ x 14′ can cost around 18,900$ to 25,500$.
RCC can be incorporated into construction of any building component. They are used for floors, walls, ceilings and even windows!
Cast-in-place rcc structures
In this method, concrete of sufficient strength is poured into forms reinforced with TMT bars. These forms can be made up of aluminium sheets or plywood boards. After the concrete is set, the form work is removed to expose the finished RCC structure.
The advantage of this method is that the floor, roof and ceiling can be made out of one form and cast together. This creates a single monolithic structure with no joints or unintended gaps. This ensures a stronger, hardier and resilient structure.
Some examples of spaces constructed with this method are nuclear bunkers, doomsday vaults, missile silos etc.
Precast concrete structures
These are similar to manufactured homes in that they are constructed in a factory elsewhere. They are then transported to the destination site and assembled.
Just as cast-in-place, forms are reinforced with TMT bards as different modules for walls, ceilings and floors. Concrete is poured into these forms and allowed to set. The different modules can be put together in any formation on site as per need.
The different modules have to be “braced” as firmly together as possible with devices like steel angles and bolts. A precast modular structure is as hardy as a cast-in-place one. Yet, special care must be taken while anchoring components to each other to achieve comparable safety.
Important advantages of precast concrete structures are that:
- they are economical because they are mass produced
- Strict quality control is ensured as they are created in factory conditions.
2. Reinforced masonry
This material technique is used solely for the construction of walls. In this method, masonry units with adequate impact resistance are reinforced with beams and TMT bars to increase strength. Although all masonry units can be reinforced; solidly grouted hollow concrete blocks are preferred for tornado resistance.
A horizontal RCC band is laid along the masonry walls at an average height of every 2m which helps in tying the walls together like a belt. Apart from wind resistance, this is also an effective earthquake mitigation technique.
Conventional lintel and plinth beams have a depth of 15cm. Primary beams are usually given a depth of 1/10th the corresponding room dimension. These are values calculated by rule of thumb. More realistic details of structural members depend on a variety of factors including expected force of winds.
Hollow blocks can be vertically reinforced with M16 bars placed at around 1.2m spacing. The cavities that contain the bars are then filled with concrete mix strengthening the wall in both compression and tension. It is mandatory to provide vertical reinforcement bars at corners and junctions of walls for adequate strength.
All courses of the concrete blocks have to be solidly grouted in order to achieve strong bonding. It goes without saying that the grout mix must be up to code and properly cured for the system to attain maximum strength. This converts the entire wall envelope into one solid (comparatively) impervious shell.
A house made of reinforced masonry may not be as resistant to tornados as RCC. Yet, it is more feasible mainly due to reduced material and construction costs.
3. Wood frame with steel sheathing
This material technique is meant for storm safe rooms rather than an entire house. This is simply because of the sheer cost of procuring and installing huge amounts of 14 gauge (2mm) steel sheets.
It is made by attaching two layers of 18mm plywood to one layer of 2mm steel sheet held on to a 30cm spaced wood frame. The key to constructing a successful structure in this manner lies in bracing. This is done by devices commonly referred to as ‘hurricane ties/straps’ and anchor bolts.
These ties firmly hold different components of the structure together. They help in safely transferring wind loads from the frame to the foundation.
New-age and experimental materials used in tornado proofing
Rising costs, inefficiency of existing practices and their environmental impacts have pushed researchers to experiment with new materials and techniques. A few stronger, more robust systems have been developed which are currently being tested in real world scenarios.
3d cementitious sandwich panel
This is an innovative product developed by RSG 3d which is hurricane & earthquake resistant and fire proof all rolled into one.
The core of the panel is a three dimensional space frame like structure. This frame is welded to a grid mesh on either face. The frame is then filled with fire retardant foam (modified expanded polystyrene).
These panels are assembled on site sprayed with concrete to a thickness ranging from 30 to 40mm. This renders the finished envelope system into an ultra strong monolith like box.
This technology has already been in use for decades for building spacecrafts owing to its strength; as well as construction in disaster prone areas.
Insulated concrete form
If the 3d sandwich panels discussed in the previous section are a precast technique: ICF is a cast in place method for tornado resistance.
This is basically a strong insulating form system for Reinforced cement concrete. The form is made up of individual units that are stacked as any conventional building block. As in other cast-in-place RCC form systems, reinforcement bars are embedded within the ICFs and filled with concrete.
Once the concrete sets, it turns into a solid monolith like envelope which has been tested to resist direct winds of 200mph. It can even withstand impacts of air borne debris travelling at speeds of 100mph.
ICF is a sustainable, environment friendly building material that is fire resistant. It is heat and noise insulating as well which makes it an ideal material for use in tornado proof construction.
US building codes and local regulations do not sanction houses made of bamboo. But, it is one of the most sought after materials for tornado and wind resistance in Central and South East Asian countries. Bamboo houses have been even recorded to withstand hurricanes with winds clocking 173mph.
Restrictions in US building codes today do not encourage the use of novel, innovative methods and materials. The process of testing and rating these should be streamlined. Governmental agencies should promote the use of new technology in creating sustainable and cost-effective tornado resistant houses.
As mentioned in the beginning of this article, tornado resistance cannot be achieved by materials alone. Factors like construction techniques, building shape; site planning strategies, off-site features etc. play important roles in building resistance and resilience.
Wise and informed design approaches incorporating all this information can prevent the loss of life, property and infrastructure worth billions.
*FEMA P320 – Federal Emergency Management Agency (Safe Room Guide)
**EF Scale – Enhanced Fujita Scale (Tornado Strength)
3d Panel Building System. (n.d.). Retrieved from RSG 3D: https://www.rsg3d.com/3-d-panel-building-system/
FEMA. (2021). Taking Shelter From The Storm – FEMA P320.
ICF: Frequently Asked Questions. (2011). Retrieved from Concrete Buildings: http://www.concretebuildings.org/icf/faq.html
Komali Kantamaneni, I. A. (2017). Cost vs. safety: A novel design for tornado proof homes. HBRC Journal , 223-232.
NCMA. (2008). CONCRETE MASONRY HURRICANE AND TORNADO SHELTERS. Retrieved from National Concrete Masonry Association: https://ncma.org/resource/concrete-masonry-hurricane-and-tornado-shelters/
Tilford, A. E. (2021, August 13). How to reduce the impact of tornadoes. Retrieved from insure.com: https://www.insure.com/home-insurance/tornado-building.html