Independent Consulting Engineers NV (ICE) is an engineering company which has as one of its main activities the engineering design of building structures: engineering of the structure that makes sure a building stands tall and does not collapse or suffer unacceptable damage in case a major disaster strikes, as an earthquake, a major hurricane, fire, in some cases an explosion or collision by a heavy vehicle and so on. This is called the structural engineering of a building or structure.
On September 6, our island was hit by a major hurricane, Irma, the strongest ever recorded in the Atlantic Basin, with extensive damage as a consequence. As we emerged from our hidey-holes and began to look around, this question inevitably came into our minds: Why have some buildings been severely damaged and others came through with very little damage?
Weekender posed this question to ICE’s founding member and vice president Jan Vanden Eynde. He explained that besides false or exaggerated claims about the performance of a building or building element (“rated for…”), there are several factors that affect a structure’s ability to withstand extreme climate such as a hurricane, even a category 5+ hurricane. Vanden Eynde discussed the problem of engineering by inexperienced or unqualified engineers, the lack of a proper building code, and the specifications for “secondary elements of a building such as sliding doors, windows, roof sheets and fastenings. Failure of these secondary elements often causes buildings to sustain massive damage or even become completely unusable.
Vanden Eynde: “Independent Consulting Engineers inspected a large number of buildings since the passing of Irma, and have found that the same reasons for severe damage come back again and again.”
One hears quite often statements like: “It is impossible to design for this kind of wind,” or “You cannot build hurricane resistant with such and such materials.” But the engineer stated categorically, “These statements are utter nonsense: In most cases, with proper engineering, all of the causes for damage could have been eliminated.”
The following text was delivered from Vanden Eynde in an effort to explain the different types of damages and the reasons they occur, and at the same time identify measures that can be taken to avoid such damages.
The building code
First of all, one has to realize that Dutch Sint Maarten has no real building code in effect, except a document dated 1935 and several more recent documents that deal mostly with urbanistic issues, such as building height and distance to boundaries.
No real up-to-date and detailed code exists for strictly technical issues such as hurricane or earthquake resistance. Some loose and relatively vague guidelines are used by governmental departments. This means that projects are engineered (or not engineered) in accordance with widely varying standards. The effects of this were made quite clear on September 6.
Engineering for wind
Here we deal with aspects of damage due to hurricane winds. Earthquake aspects are not further considered here, but should receive the same amount of attention since we are in an area where earthquakes can and do occur. (On the French side of the island, structures have to be engineered to withstand the heaviest category of earthquakes.)
As a result of our inspections, we found that the main structures of buildings which were engineered by qualified engineers (engineers who have experience designing for hurricane conditions) and for the appropriate wind loads (as ICE has adopted since Hurricane Luis in 1995), these structures have come through the hurricane with little or no significant damage.
We suspect, however, that in most cases, safety factors have been used to the max. If this storm would have moved at a slower speed, most probably more building structure damage would have occurred. In 1995, ICE adopted the Uniform Building Code of the USA, using a design wind speed of 135 mph. This code includes a large number of additional safety factors which depend on the height of the building, terrain configuration, importance of the building, etc. When we did a check for one of our larger projects, we found that in that specific case, the weakest link in the building would fail at a wind speed quite a bit above 215 mph. The result? The building sustained no damage to its structure.
On the other hand, we have seen quite a few structures, ordered directly from a manufacturer, with a so-called rating for 160 or 180 mph, of which only the foundations are still there. One has to realize that a manufacturer located in the USA or Europe, who is asked to provide a quotation for a building structure without an engineering design and dimensions imposed upon him by appropriate specifications, makes every effort to lower his price as much as possible, and has his engineers work with that as the main objective. Very often, the engineers involved are poorly qualified – or even interns.
Furthermore, rated for 160 mph basically means that the building will fail when that wind speed is exceeded. There is no uniform definition of the term “rated for…” Especially in the USA, a number of building codes exist, and each defines and designates wind speed in a different way. Comparison becomes very difficult without in-depth knowledge of each of those codes. Sales people will use the terminology which helps them best to sell the building: they will not be around when that major storm hits, and good luck suing the manufacturer for damages.
Also, we often see significant weaknesses in the engineering design. The design will work in theory, but one forgets that a hurricane creates dynamic conditions: the building shakes and deforms. What works in theory in static conditions all of a sudden does not work anymore in reality. The lesson here for owners, who wish to construct a building ordered from a manufacturer or supplier, is to have the building design verified by a qualified, and hurricane-experienced engineering firm before ordering. In most cases, the difference between hurricane resistant and less resistant is a few percent in cost, not more.
We see that a very large percentage of damage to buildings is caused by construction elements that are considered secondary and strictly “architectural” sliding windows, doors, windows, outside wall finishes, hurricane shutters.
Very often, these items are purchased and installed without real verification as to how well they will stand up to hurricane conditions. However, when a sliding window or door blows out, the damage to the interior is enormous. In most cases, it will make the space behind completely unusable and in need of very extensive repairs, far larger than the value of the element that failed.
- Very frequently, we have seen failure of windows and sliding doors: The above causes immediate failure of the window pane, causing total destruction of everything inside the room behind, and often failure of the roof above due to the shockwave caused by the failure.
- Simple under-dimensioning of the window elements (frames/ profiles not suited for any kind of hurricane winds), causing plain failure of the window.
- Insufficient anchoring of the window: Insufficient number of screws, insufficient screw lengths, causing the window to “blow out of the wall.”
- Use of simple glass instead of laminated glass, causing breakage by an object hitting the glass. (Note that windows sold on the French side as hurricane resistant have solid, non-laminated glass, which can withstand a certain pressure, but will be completely destroyed if something hits the glass.)
- Insufficient profile depth for the holding of the glass.
- We see outside doors with inset panels that are not properly fastened, or too lightly designed. This again causes immediate failure and total destruction of the rooms behind.
- Rolling shutters sometimes suffer from flexing of the horizontal elements which can then pop out of guiderails.
- Especially with larger commercial buildings, we see façade elements (panels, built-up composite on the outer skin of the building, and/or metal sheeting) which are very often fastened to properly engineered elements but not sufficiently strong themselves.
- Failure of these elements causes blow-out of the area behind and severe damage to the interior of the building
Metal Roof Sheeting
- Very often, insufficient sheet thickness, insufficient caps to prevent screws being ripped out of the sheet, insufficient number of screws, especially around roof edges (where higher wind loads occur).
Most of the above could have been prevented if the element in question was checked by a qualified engineer on its performance in hurricane conditions.
- In general, buildings (load bearing structures) that have been properly engineered with the “135 mph design wind speed of UBC building code” conditions have come through the storm virtually unharmed, with very few exceptions. However, buildings or load bearing structures that were designed for lesser wind loads have, in many cases, sustained significant damages.
Verification of the wind ratings offered by different codes and manufacturers is complicated. It would simplify matters significantly if our government could be convinced to adopt a uniform code, like the Eurocode for example, as is also applied on the French side. We suggest that those wind speeds be adjusted, however; to an even higher level than those permitted on the French side.
- A very large portion of damages to buildings have been caused by failure of “architectural finishing elements.” These failures have caused enormous collateral damages to the interiors of the buildings since they allow wind and water freely into the building, very often with significant destruction of the interior of the building as a consequence .
Failures of this kind have caused the bulk of the damages to hotels and businesses on the island and will have a disproportionate effect on the island’s economy for the months to come.
For smaller buildings, the main reasons for failure are in the elements of roof and windows/doors.
- We have suggested that, for smaller buildings, on short term, a simplified building code be adopted, which would in first instance only deal with the elements described above, and which would cover all principal elements of construction which protect residents in case of a similar event as the passing of Hurricane Irma: Minimum rafter sizes for certain spans, minimum number and types of screws, minimum thickness for metal roof sheeting, minimal number and length of screws for fastening door and window frames, etc. Such a simplified code would be very well usable also for smaller contractors and architectural offices and would provide a clear guideline as to how this simplified code could at a later date and after more elaborate study be extended to a full blown code covering all aspects of building construction in St. Maarten.
Jan Vanden Eynde has been with Independent Consulting Engineers NV since its inception in 1982. ICE has permanently on staff in Sint Maarten a number of highly qualified professionals, with years of experience in hurricane and earthquake resistant engineering. In ICE’s 37 years of existence, several major natural disasters have hit the island, and ICE is proud to be able to say that of all the projects they have done over the years, virtually all have performed very well through these disasters.
ICE is available for any type of engineering project, from the smallest (“ I want to make an opening in this wall in my house, can I do that or do I have to reinforce something?”) to the largest projects on the island (Porto Cupecoy, Atrium, Aqua Marina Towers). ICE can be reached at Zaegersgut Road 13, telephone 542-2421, e-mail firstname.lastname@example.org