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What is structure? What are Structures? Why do some structures fail? What do structures do? Can you think of some Famous Structures? Classifying Structures Types of Structures Understanding Building Construction Types |
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What is structure? Construction or framework of identifiable elements (components, entities, factors, members, parts, steps, etc.) which gives form and stability, and resists stresses and strains. Structures have defined boundaries within which (1) each element is physically or functionally connected to the other elements, and (2) the elements themselves and their interrelationships are taken to be either fixed (permanent) or changing only occasionally or slowly. What are Structures? Structures are things that have a definite size and shape, which serve a definite purpose or function. To perform its function, every part of the structure must resist forces (stresses such as pushes or pulls) that could damage its shape or size. Classifying Structures Natural - not made by people - occur naturally in the environment Natural Mount Everest Tree Iceberg Manufactured - built by people - many are modelled after natural structures Man-Made Buildings Bridges. Chair Table Famous Structures Structures can also be classified by their Design Mass Structures Can be made by, piling up or forming similar materials into a particular shape or design. - Mountains, coral reefs are natural mass structures - Sand castles, dams and brick walls are manufactured mass structures) Advantages: held in place by its own weight, losing small parts often has little effect on the overall strength of the structure A Layered Look - mass structures are not always solid, but are layered and have hollowed out areas for specific functions. - a power dam and the Great pyramids of Egypt are a good examples Sandbag Wall Structure to prevent Flooding (4 Key Elements) to avoid failure - must be heavy enough to stay in place - must not be too heavy to compact the earth unevenly below it - must be thick enough so it cannot be pushed out of place - must be anchored firmly Frame Structures Have a skeleton of strong materials, which is then filled and covered with other materials, supporting the overall structure. Most of the inside part of the structure is empty space. - Load-Bearing Walls: these are the walls that support the load of the the building. - Partition Walls: these are the walls that divide up the space inside the building. - because they are relatively easy to design and build, and inexpensive to manufacture, the frame structure is the most common construction choice. A Bicycle frame supports the load it carries on the seat. Identify: Rigid Joint Mobile Joint Brace Rigid Shape Type of Material Support All frames, whether simple or complex must overcome similar problems. To solve these problems joints, type of material, bracing, anchoring and design all must be considered in the overall structural frame construction. Golf Ball Bridge Shell Structures Structures, which keep their shape and support loads, even without a frame, or solid mass material inside, are called shell structures. These structures use a thin, carefully shaped, outer layer of material, to provide their strength and rigidity. The shape of a shell structure spreads forces throughout the whole structure, which means every part of the structure supports only a small part of the load, giving it its strength. Examples include: igloos, egg cartons, turtle shell, food or pop cans, or, even bubbles in foam and cream puffs. Flexible structures, like parachutes, balloons and different types of clothing are a different type of shell. Shell structures have two very useful features: - they are completely empty, so they make great containers - their thin outside layer means they use very little material Problems in building shell structures include: - A tiny weakness or imperfection on the covering can cause the whole structure to fail. - When the shell is formed from hot or moist materials, uneven cooling can cause some parts to weaken other parts by pushing or pulling on nearby sections. - Flat materials are difficult to form into the rounded shell shape. - Assembly of flexible materials is very precise, so that seams are strong where the pieces are joined. Mix and Match Some structures are combinations of different types of structures: - Football helmets are shell structures - to protect the head, with a frame structure attached in front - to protect the face. - Hydro-electric dams are mass structures, with frame structures inside to house the generators - Airplanes are frame structures, with a 'skin' that acts like a shell - giving it the added strength to resist stresses and making it lightweight and flexible. - Domed buildings combine shell and frame construction - Warehouses are often built with columns to support the roof (frame) and concrete blocks, (mass structures) which stay in place because of their weight. Types of Structures Frame structure Mass structure Shell structure Suspension structure Trusses Vaulted structure A dam is an example of a mass structure. Load-bearing walls hold up a frame structure, while partition walls simply divide rooms. The dome of the Taj Mahal (one of the most famous shell structures in the world.) Structure - Types of Structures - Definition & Classification Advertisements Definition: A combination of members connected together in such a way to serve a useful purpose is called structure. Types of Structure Rigid Frame Its is that type of structure in which the members are joined together by rigid joints e.g. welded joints. Truss (Pin connected joints) A type of structure formed by members in triangular form, the resulting figure is called a truss. In truss joints are pin connected and loads are applied at joints. No shear force & bending moment are produced. Only axial compression and axial tension is to be determined while analyzing a truss Structural Members Those members that are interconnected in such a way so as to constitute a structure are called structural members. Beam Beam is a flexure member of the structure. It is subjected to transverse loading such as vertical loads, and gravity loads. These loads create shear and bending within the beam. Columns A long vertical member mostly subjected to compressive loads is called column Strut A compressive member of a structure is called strut. Beam-Column A structural member subjected to compression as well as flexure is called beam column {loadposition articlemid} Grid A network of beam intersecting each other at right angles and subjected to vertical loads is called grid. Cables and Arches Cables are usually suspended at their ends and are allowed to sag. The forces are then pure tension and are directed along the axis of the cable. Arches are similar to cables except hath they are inverted. They carry compressive loads that are directed along the axis of the arch. Plates and Slabs Plates are three dimensional flat structural components usually made of metal that are often found in floors and roofs of structures. Slabs are similar to plates except that they are usually made of concrete. Understanding Building Construction Types Buildings are broken down into five categories (Types 1–5), ranging from the stoutest of construction to that which will most likely fail rapidly when under fire conditions. Type 1: Fire-Resistive Type 1 structures are high-rises, and they’re the stoutest of all construction types when exposed to fire. High-rises are usually defined as buildings more than 75 feet tall, with some agencies making amendments for buildings that are 35–55 feet tall. Type 1 structures are constructed of concrete and protected steel (steel coated with a fire-resistant material, most often a concrete mixture), and are designed to hold fire for an extended amount of time in order to keep the fire at bay in the room and/or floor of origin. As far as the typical ventilation operation of getting on the roof and cutting a hole, that’s not really an option when dealing with Type 1 construction. Even horizontal ventilation becomes challenging, as the windows are thick, tempered glass and may not be an efficient way to ventilate the structure. _______ must be aggressive in securing the stairwells for both firefighters and victims evacuating the structure. If the structure meets building code, it will be equipped with self-pressurizing stairwells and have HVAC systems that will aid in air movement. If necessary, the _________ may need to mechanically pressurize the stairwells using a series of fans strategically placed at the base of the stairwell and every 10–12 floors depending on the effectiveness of the fans. The fire protection and fire-related systems in these buildings are overwhelming, so crews should make it a priority to locate a maintenance worker and keep them close throughout the incident. Type 1 structures are easy to identify on height alone. It’s important for firefighters to know their city’s building codes, as this may affect which features are found inside the structures. Ladder crews should frequent Type 1 buildings in their area and be familiar with the systems that they may encounter (elevators, HVAC, fire pumps, etc). Finally, they must not forget to maintain good working relationships with the maintenance workers at these buildings. Type 2: Non-Combustible Type 2 construction is typically found in new buildings and remodels of commercial structures. The walls and roofs are constructed of non-combustible materials. Specifically, walls are usually reinforced masonry or tilt slab, while roofs have metal structural members and decking. The top of these roofs are often covered with lightweight concrete, foam, an insulated membrane or a combination of these materials. Because most of these buildings are newer builds, they’re usually up to code and include fire suppression systems. And because metal roofs may fail with heat—not just from direct fire—expect early collapse, especially in some of the bigger buildings that have a substantial fire load. Firefighters should suspect Type 2 construction in newer commercial structures (both big box buildings and strip malls). A good habit to practice: sounding the walls to determine whether they’re made of a combustible material. When on the roof, ladder crews should cut an inspection hole to identify the decking material. Once a metal roof has been confirmed, the rooftop crew should consider opening skylights or resorting to natural ventilation in the form of large roll-up doors that are often found in the rear of the big-box structures. Common ventilation tools (chainsaws and circular saws) may simply not be efficient enough for cutting large holes on the roof to support ventilation for interior crews, as a circular saw will often cut through only small areas of metal and “gum up” with insulation, or the blade will wear out quickly. Type 3: Ordinary Type 3 buildings can be of either new or old construction, and they have non-combustible walls and a wood roof. Older construction buildings may consist of unreinforced masonry and have a conventionally framed roof, while newer buildings will have lightweight roof systems supported by reinforced masonry or tilt slab. The most common types of roof systems in a commercial setting of Type 3 construction include parallel cord truss and panelized roof systems. To identify if a building is of an older style, firefighters should look for clues, such as collar ties, king’s rows and arched lintels. If operating on one of these buildings, firefighters should be suspect of conventionally framed materials that may be weathered, built-up roofs or roof-on-top-of-roof systems. If it is determined that the roof is tenable, a ______ should be able to effectively use chainsaws to ventilate the building and make the appropriate cuts based on the type of roof system. If approaching a building with no signs of unreinforced masonry, firefighters should sound the walls to determine wall type before going to the roof. Once on top, they should be able to identify the roof system and make an aggressive ventilation operation using saws. Newer construction uses truss systems in both panelized and parallel cord truss roof types that are known to fail rapidly and unexpectedly with direct fire impingement. As such, _______ should sacrifice some property for time and make vent holes over smoke, not fire. Whether conventional or lightweight, vertical ventilation on Type 3 construction is feasible and can be very effective. But safety is paramount; crews should always remain on ledger walls or structural members. Sounding and diagnostic cuts are effective ways to not only ensure the location of the structural members but also to allow the ventilation crew to monitor the roof conditions and act accordingly. Type 4: Heavy Timber Type 4 construction is found in older buildings and utilizes large dimensional lumber for structural members and interior elements. These buildings hold up well under fire conditions, but it’s critical that firefighters not feel a false sense of security, as these buildings are often poorly maintained, or have termites and/or weathering issues that can contribute to an earlier-than-expected collapse. Firefighters can identify these buildings by the large lumber used for walls and the long distance of roof spans. These buildings were most commonly built before 1960, when bolts and metal plates were used as connectors. Vertical ventilation may be achieved on these buildings, but sawyers may encounter thicker-than-expected decking that may make for a longer completion of a ventilation hole. Type 5: Wood-Framed Type 5 construction is found in many modern homes. The walls and roofs are made of combustible materials—most commonly wood. If the walls are wood-framed, the roof usually is as well. Rooftops are ceramic tile or asphalt shingles placed over lightweight trusses and OSB. Both UL and NIST studies have found that lightweight construction will fail within minutes of direct fire impingement. Firefighters should sound the walls prior to going to the rooftop. Whether operating on tile or asphalt rooftops, alternatives to rooftop ventilation should be considered if there is heavy attic involvement. If fire is isolated to a room, flashover (not collapse) is the main concern, and aggressive ventilation is beneficial. Because the roofs are made of wood, ventilation can occur through the asphalt shingles, but tiles should be removed first if encountered. Positive-pressure attack is another tool that may prove beneficial for Type 5 construction. Here are further guidelines. |