We do not assign a life to the structure. We can just say that there is a 95% probability of structure standing for 100 years provided we have taken corresponding values of wind, earthquake, loadinds etc. in design. But again there is 5% risk that such events(e.g. earthquake) may happen very next year.
In is a probabilistic age, like life expectancy but we can not guarantee any life period.
Does designing a structure to the 100 year wind load or the 100 year flood or the 100 year earthquake, mean its' intended useful life is 100 years (taking into consideration flame's proviso of 95% probability and maintence).
I don't recall seeing anything in any code discussions regarding relating the design load return period to the useful life of the structure. A pre-engineered metal building with a useful life of 25 years is designed to the same loading conditions (say snow load for example) as a bank that may have a useful life of 75 years. The same question arises with bridges.
I do realize that the code authorities use a statistical analysis when setting design live loads, but I've never seen this related to useful life of the structure.
So if we are following latest codes, then we can say that the designed structure will stand for another 50 years at least, taking into account the maintenance and other unseen factors?
What about the life of structure if after 20 years new code revisions suggests that the building is no longer safe to carry the loading?
In other words if we analyse a structure today(which was designed 30 years ago) based on current code provisions and finds that the building is not safe then what happened to the life of structure predicted 30years ago?
A 100 year wind load is a statistical maneuver where there is a 90% chance that this 100 year wind speed will be exceeded within any random 100 year period.
Even if it is exceeded, you won't necessarily have a collapse or loss of the structure.
Building's life is based more upon the maintenance and usefullness of the building to the owner, not the loading.
Guys, We have all contributed immensely to this topic and i must commend you all but, why don't we consider the behaviour pattern of the materials involved i.e. concrete of steel and we can draw conclusive inferences from the behaviour under stess in service.
the maajor area i will want for us to look at is the behaviour of the materials under creep load and the plastic state to breakpoint in the stress strain relationship in the lifrcycle of a material
Unless you can guarantee the viability of a society.
If you have societal breakdown all bets are off.
Aluminum siding has an outstanding lifespan, but not if a homeless person has a shopping cart, a crowbar, and opportunity to pry that siding off the building.
The anticipated life may influence the design actions in accordance with the probabilistic approach mentioned above however the actual life will depend mainly upon the inspection and maintenance regime.
That said, one can design structures for greater durability, i.e.: corrosion allowance on steel elements; quality of concrete constituents; curing of concrete; concrete cover to reinforcement; protective coatings; etc.
One of the things that is rather interesting is that everyone seems to be jumping on the LEED bandwagon and the whole green building thing. Correct me if I'm wrong, but I don't believe that the current US LEED program offers any additional points for the design of buildings that would be expected to be around say 100 years later versus some cheaply built structure that will rot or rust away in a much shorter time period.
To establish the relative life expectancy of a new structure one needs to consider the type of materials used and the environment in which the building will be placed. Our office is in a building that is over 100 years old, with a brick exterior, beautiful hardwood floors, cast iron columns, decorative tin ceilings and big wood timber beams. The building was a sewing factory until the 1980's and now it is functioning as an office building.
Unfortunately in today's day and age, many owners want to slap up a building as cheaply as possible and then rake in the dough...(i.e. condos)
The life span of a building IS based on probability as described in earlier posts. Simplistically this means that we design for the worst probable combination of forces we anticipate the building we experience in its ‘design life’. The probability of a larger combination of forces within the design life is always present and could happen at any time. We justify this as an acceptable risk limit and this is usually incorporated into your design codes.
There is a second issue of design life and this is maintenance and durability. All buildings need regular maintenance to achieve their anticipated life. The quantity of maintenance activity is controlled, in part, by the durability of the materials used. It is also controlled, to a greater extent, by the design details and environment.
Material suppliers can often supply information on ‘period to first maintenance’ for their products in a range of typical environments. This is especially useful for cladding materials and exposed paint finishes.
It is effectively impossible to define an actual life of a building as there are too many variables. I’ll stick my neck out now – If properly designed, detailed and maintained; Steel and Concrete framed buildings can have an almost infinite life span in ‘normal’ environments. The life consideration then comes down to choosing the right load combination and ensuring that appropriate details are used.
Good comments pba - one other aspect of building life is useability. Buildings are designed to accommodate a particular human function applicable at the time of their construction.
As the years go by many things change:
Demographics
Building Codes
Fire Safety Codes
Building purpose
We had a project some years ago where an old, 5 story wood and brick building was evaluated as to whether to renovate or replace. It was used as a "mother house" for a convent of nuns....essentially, their residence. A number of problems reared up such as:
The desired spaces required for the apartments did not line up well with the existing window spacing.
The wood floors violated building codes without a heroic effort to fireproof.
No ADA accessibility features.
Questionable brick quality in many areas.
No elevator - and adding an elevator would further decrease the floor plan efficiency.
With all of the above, the choice was to tear down and replace....thus the life of this building was limited by many sources. In fact, you could argue that the building's effective life was already long gone, just that the owners had to wait for the right financial opportunity to resolve it.
OK. Let's look at this from the investor's viewpoint.
Let's say that if you spend:
$100/sf you get a building that will last 25 years
$150/sf you get a building that will last 40 years
$200/sf you get a building that will last 50 years
$300/sf you get a building that will last 100 years,
and so on and so forth.
Now, you are asked to estimate the useful life of a structure in today's dollars. The structure originally cost $140/sf in today's dollars, was built in 1990 and appears to be in good shape.
How would you go about estimating the remaining useful life of the structure?
