Building Construction

The collapse of the Interstate 35W bridge over the Mississippi River had done major damages in Minneapolis, Minnesota. Many assumptions and speculations about the causes of the collapse of the bridge system had appeared in the public. The public was seemingly confused about the real cause of the incidents and it is their right to be informed about the state of the investigation. The closest and very logical of the causes indicated in some of the investigations are stress or fatigue failure and lack of redundancy.

Environment, Design, and Description of the I-35W bridge The I-35W bridge supports a total of eight lanes (four lanes on each direction). The average daily traffic (ADT) is given as 15,000 in each direction, with ten percent trucks. Constructed in 1967, the 581 meter long bridge has 14 ps. The main p is consist of a steel deck truss. The south approach ps are steel multi-beam. The north approach ps include both steel multieam and concrete slab p. There are two steel deck trusses. Builtup plates mostly composed the truss members.

Rolled I-beams comprised the diagonal and vertical members. The truss members undergo poor welding details with the connections as mainly riveted and bolted. According to recent evaluation and inspection before the collapse of the bridge, corrosion at the floorbeam exists and rust are forming between connection plates. The two main trussses have an 11. 6-meter cantilever at the north and south ends. Twenty-seven floor trusses spaced at 11. 6 meters are also present. These floor trusses were framed into the vertical members of the main truss.

The floor trusses consist of WF-shape members and have a 4. 97- meter cantilever at each end. The design specifications used in the bridge was the 1961 American Association of State Highway Officials (AASHTO) Specifications. During that time, most of the design uses unconservative fatigue design provisions. According to the fatigue evaluation report provided by the University of Minnesota’s Center for Transportation Studies in 2001, the approach ps had exhibited several fatigue problems promarily due to the distortion of the girders.

The bridge truss and the floor truss system also exhibited poor fatigue details. Lack of redundancy in the main truss system was also present in the design. It is stated in the evaluation report of the University of Minnesota that cracking due to fatigue cause by a future increase in loading will first appear on the floor truss. According to them these future cracks is detectable since the floor truss are easy to inspect. In the incidence that cracks are not detected, the bridge could still hold the bridge system without the entire collapse of the system.

In the report, the failure of the two main trusses of the bridge will definitely take much effect on the bridge system. Fatigue Resistance The Standard Specification and the Load and Resistance Factor Design provided by the American Association of State Highway Officials (AASHTO) contain similar provisions for the fatigue design of welded details on steel ridges. These details are designed ased on the nominal stress which can be calculated using standard design equations and does not include the effects of welds and attachments.

Since fatigue is usually present during sevice load application, the design parameters are only applied during service load conditions. Cracks due to fatigue have insignificant effect on the structures in compression but have tremendous effect on structures that experience tension. With this idea, the assessment on the cracks that propagate on such a bridge as the I-35W should only be consider to elements in tension. Structural Redundancy In all the design criteria of any structural system, loads existed in variety of paths should be significantly considered.

The strength and reliability of the system can be ensured by the existence of redundant paths or elements. Without the existence of this redundant system of elements, the failure of the entire system is much possible. Past survey of the Committee on Redundancy of Flexural System on steel highway ad railroad bridges. The report summarized that a total of 96 structures were suffering some distress. It was also taken into account that most of the failures were related to connections that were mainly welded.

The report had also collected data that indicates that few steel bridges collapse if redundancy is present. Bridge systems with no redundancy were reported to have large number. In another research conducted by Ressler and Daniels, they found that the number of fatigue sensitive details present in the structure significantly affected the bridges with no redundant elements. Theoretical and Actual Bridge Response Many studies have shown that the simplified calculations used to predict the stresses provide a much higher value compared to the actual service stresses.

Though the design calculations and load models provide appropriate results, it has great uncertainty in the maximum life of a bridge system. However, it is still beneficial to have an accurate estimate of the typical everyday stress ranges. In a large bridge, 20 Mpa is the typical value of the service live-load stress ranges. The stress ranges are typically governed by dead loads and strength design specifications. This is the reason why the stress ranges are small. Since the strength design must account for a single case loading scenario over the life of the bridge, conservative load models are used.

In addition to load conservative models, the assumptions provided in the analysis of the design can also be the cause of the large difference of the predicted stress and actual stress. A great example of the effect of the assumptions is the case of US Highway 69 in Oklahoma. Fatigue damage was said to be present upon the welding that had been used in the widening of the bridge. The design computations of the bridge illustrated that the allowable stress ranges could be exceeded at over 100 locations on the bridge.

However, when the bridge was inspected, it appeared that the measure stress ranges was only 27 percent of the allowable stress ranges. This only shows the great effect of the assumptions used in the design of a certain structural system. Moreover, another study that indicates fatigue failure to be caused by the considerable amount of corrosion takes into account. This is the case of the Bridge 4654 in Minnesota where measured stress ranges ranged from 65 to 85 percent of the calculated analysis.

These differences are to point out to the fact that analytical methods provide assumptions that neglect ways in which the structure resists loads. For example, in the study conducted y Brudette et al., more than 50 years of bridge test data were collected and examined to determine specific load-resisiting mechanisms that are ignored in the design of the system. The study concluded that lower stress ranges in a structure can be due to unintended composite action, contribution from non-structural elements, unintended partial fixity at abutments and direct transfer of load through the slab to the supports.

In another study of the Ministry of Transportation of Ontario, they conducted a program of bridge testing that included more than 225 bridges over a period of 15 years. The study noted that much of the bridges can sustain much larger loads than their estimated capacities. Observations were also made regarding the behavior of the steel truss bridge. The observations are as follows:

1. the stringer of the floor system share a large tensile force thus reducing the strains experienced by the chord in contact with the floor system and
2. Composite action in non-composite system was shown to exist

Statistics indicate that about two hundred individuals die every year as a result of explosions and accidents in the workplace. Additionally, these same accidents injure about five thousand workers in the workplace. Businesses can spend as much as five billion dollars on fire related accidents. This shocking statistics is actually an indication of what all business in the US spent on rectifying these accidents in the year 1999. The means of egress standards are a code of practice that are designed to ensure maximum [protection in case of fire emergencies in buildings.

The standards were based on the National Fire Protection Authority (NFPA 101). The means of egress can be found in the NFPA 101 in the subsection E part. 3 parts of the means of egress The means of the egress are a pathway from a building to a public area. This pathway must be continuous in nature. In encompasses three main sections that include;

• The route of exit access
• The exit itself
• A way of exit discharge

It should be noted that all structures and buildings designed to host human beings must have a means of egress. The main purpose and principle behind these rules is to ensure that there are fewer instances of danger to the worker or the occupant in the case of a fire. The means of egress also protect occupants from the resulting fumes or smoke that emanate from the fire and they also ensure safety as result of panic from the occurrence of the fire. It should be noted that three means of egress only apply to protection and has nothing to do with property protection. There are a total of seven requirements that buildings must conform to in relation to the latter 3 sections.

First of all, buildings should be positioned such that all the exits facilitate free movement. Additionally, the means of egress must not obstructed by any materials, persons and activities for that matter. The egress must be a key component in all parts of the building and must allow occupation at all times. The second aspect about these three main components of egress is insurance of visibility. Buildings should be arranged in such a way that the routes to all exits are conspicuously located. The third aspect refers to the lighting aspect. Buildings should be such that they provide adequate illumination at all the exit areas.

The fourth requirement is about fire alarms. The means of egress standards require that fire alarms be put in place in order to warn their occupants of the fire. This ought to occur in the event that the fire in itself does not provide adequate warning to the residents of the facility. The fifth requirement is in regard to how the exit routes are arranged. In the event that one means of egress is blocked by the resulting smoke or fire, then buildings ought to have two means of egress. These means of egress should not be adjacent to one another as these will beat the whole purpose of the contingency plan.

The two means of egress should be located as a far way from each other or as far as the building design can allow. Additionally, constructions ought to be done in such a way that one means of egress does not block the other. This means that when fire blocks one route, then the other should be easily accessible to the building’s occupants. The sixth egress requirement refers to means of safety during building construction. Employees ought to be protected during the construction process by denying them access into the building until all the aggress means have been installed into the building and they can be utilized.

In the event that a building has to be modified or reconstructed, then all the exit routes must be usable and available in case of a fire emergency. If this is not possible, then the constructors must provide resident occupants with an alternative route. The sixth requirement also protects constructors themselves; it requires that there should be no flammable substances or explosive substances introduced into the building when there are some occupants within. There should be fire permits, hot zones and fire watch precautions whenever a construction is ongoing.

There are a number of unique egress components that building designs should adhere to. In this case, egress routes should be constructed using materials that are fire rated. Additionally, the egress building materials should be protected from all other parts of the building. In case a building has a one to three stories, then its egress must be made up of one hour fire resistant material. In case a building will be made up of four stories or more, then it must have two-hour-rated egress material. Additionally, the fire doors making up the egress must be self-closing

After conforming to the material specifications, egress designs should fall within the following dimensions; the width of the egress is largely determined by the angle of inclination of the egress itself. All egress routes must have a minimum of thirty degrees elevation. This angle applies to all the sections that make up the egress system. During construction, the egress’ width should not be limited by any nearby doors. Additionally, exit doors should open in the direction of the exit direction.

The means of egress should also be designed in such a way that it can accommodate occupant load where the latter term refers to the sum of persons in a particular building at any given time. Means of egress should also incorporate the Floor area to occupant load factor (OLF) ratio. For instance, if the occupant load factor within a building is ten square feet per person and if the floor area is four hundred and fifty square feet, then the occupant load is forty-five people. Means of egress should be designed such that there are two means of exist at each location.

Additionally, they should be placed far away from one another. Building designs should be made in such a way that the storage rooms, lockable rooms and bathrooms are nowhere near the means of egress. The means of egress should also not be placed in an area that requires passage near a hazardous area. Exits must be made as accessible as possible this means that there should be a good arrangement on where the exit is. Additionally, there must be no mirrors or hanging drapes at the exit routes. The exterior section of the egress must be arranged in such a manner that there is a roof if it is likely to snow or rain in that area.

Also, there should be no obstructions at the exterior egress. They should be made up of solid floors which are smooth in nature. The exterior egress should also have some guards on its sides; these sides should be left undisclosed. The exit discharge itself should be constructed in such a way that it gives access to a safe public area. These may include a court, yard or the street. The discharge are must be constructed in such a way that its width is adequate enough to accommodate occupants. Additionally, if there are any stairs there, they must give a direction to the streets very clearly.

The egress headroom should have a minimum projection of six feet nine inches and a minimum height of seven feet six inches. A ramp or a staircase should negotiate any changes to the egress elevation. The egress must be maintained constantly through adequate checks. In this regard, the doors ramps, stairs passages must be reliable. There should be no gaps in the stairs and care must be taken to ensure that the concrete does not crumble. Occupants should avoid placing decorations near the egress. Additionally, there should be no furniture or explosive material in the egress.

Additionally, occupants must endeavor to conduct frequent inspections and tests of the sprinkler systems in the egress. All exits must be marked using colors, sizes and designs that can be easily read by observers. The ones indicating exist must contrast with existing backgrounds or interior finishes. Care must be taken to ensure that the signs have not been covered in any way or that they are no other signs near the exit sign itself. Illumination should be reliable enough in that the total level should not be less than the amount of light emanated by five candles.

It is better to use internally illuminated exit signs. Exit signs must have the word ‘exit’ indicated. They must be six inches long (or more). They must not be less three quarters of an inch. Evolution of fire code history and current functions related to this In the past means of egress have been designed depending on just a few requirements. Most of the time, they used to relate to the nature of occupancy within the building. However, with time the means of egress began incorporating other aspects such as the number of persons exposed to the fire, the nature of fire protection preparations.

Additionally, its design also incorporates other issues such as the nature of the building construction and how tall the specific building is. Elements of code administration, inspection practices and appeals process in code enforcement The means of egress provisions are found either in the International Building Code or the Uniform Building Code. The latter two codes are somewhat similar in nature and they provide the ways of administering these means of egress. In the Uniform Building Code the exit discharge consists of the balconies and exit staircase.

However, the International Building Codes considers the exterior balconies as the exit access. On the other hand, this code defines the exterior stairways as part of the exit. It should also be noted that both of these codes describe the means of egress in a similar format. However, the major difference comes about in the manner of arrangement. The International Building Code has more sections than the Uniform Building Code. Additionally, three is a provision for guards in the IBC. However, the same cannot be said of the UBC. Conclusion

A means of egress is a pathway that facilities the safe exit of occupants from a building and does not encompass property safety. The Code gives specifications on the most appropriate material for an agrees, its dimensions and the other features that must be incorporated to make it safe and usable. Some of these features include the sign and the exit discharge. The Code also gives direction about maintenance practices of the egress. In this case, the egress must be well illuminated at all times, it must not be obstructed and fire response facilities such as alarms must be working in proper order.