Issue link: https://cp.revolio.com/i/288390
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We Can Help! enters the building. It also holds the deck together, helping to prevent roof deck fail- ures and limiting progressive failure in the case of partial failure of the roof deck. PICKING A WIND-RESISTANCE CRITERIA How much wind resistance to design for can be considered variable depending on circumstances and cost. Generally, hav- ing enough wind resistance to be meaning- fully higher than the local competition is enough. When competing against struc- tures that are designed to meet code crite- ria (generally 90mph in non-coastal loca- tions), 125mph wind-resistance design is a cost-effective criterion. Some enterprise users may desire wind resistance of up to 150–200mph, but they may be willing to settle for (and able to justify internally) the "best available" in the marketplace. ROOF MEMBRANE WIND RESISTANCE Increasing the wind resistance of a roof membrane is accomplished simply by attaching the membrane more securely to the overlay/insulation (and attaching the overlay/insulation more securely to the roof deck) to resist the upli created by wind passing over the building. For adhered systems this means more adhesive, and for mechanically attached systems, more at- tachment points. For the field (center area) of the roof, this approach may have little or no impact. Upli is created by low pressure when the wind hitting a building is forced up and over it. At the perimeter, and par- ticularly behind parapets, a low-pressure area is formed, and it tries to "suck" the roof off of the structure. Corner conditions are the most critical and have the high- est attachment requirements; edges of the building are next highest, and the field of the roof has the lowest criteria. Because additional attachment is relatively inexpensive, high wind resistance for the roof membrane isn't costly and is easily achievable regardless of the type of membrane selected. ROOF MEMBRANE PROJECTILE RESISTANCE In addition to the direct effect of wind on the roof membrane, wind-born projectiles can cause significant damage and can create failure points that make the roof more susceptible to direct wind action. Protection from wind-born projectiles is achieved by installing a protective layer above the roof membrane. Concrete is the most effective and durable material for this purpose, but good protection can be achieved from the insulation and overlay board provided in a double roof system. STEEL STRUCTURE—JOISTS Many existing buildings that are con- verted to data centers have roof structures built with steel joists. ese joists are the most economical alternative for relatively long spans because they have the lowest weight-to-carrying-capacity ratio. Top and bottom members are steel angles, and web members are steel rods. Metal deck is used to span between the joists and is welded to the joists to create a diaphragm that gives the overall structure rigidity. Joists span one direction and bear on similar, but heavier, joist girders, which are in turn supported by steel columns. e controlling factor in the design of joist systems is generally the gravity loads pulling down on the structure. ese grav- ity loads put the top members in compres- sion and the bottom members in tension. Since the top members are supplemented and laterally braced by the roof deck, they are naturally resistant to bending or buckling as they are compressed. e bot- tom members stay straight because they are in tension and are continuously "pulled straight." Under high wind loading, the low pressure above the roof can pull the structure up and reverse the loading on the joists, putting the bottom members in compression instead of tension. Since they do not have the roof deck to give them lateral support, they are much less resistant to compressive forces and tend to buckle and bend, resulting in failure. Existing Buildings ese structures are generally de- signed to support code-required loads and no more. Because they are fabricated from many small pieces, it is difficult and expen- sive to substantially increase their capacity, since that would involve "beefing up" each of the many individual components. Some increase in capacity can oen be achieved by adding additional bracing/ bridging to the bottom chords of the joists to help them resist the buckling associated with the upward forces noted above. New Buildings Because joists are optimized for light- weight and lightly loaded structures, they are typically not used for the more highly loaded data center structures. For the right spans and loading situations, however, joists may be a valid choice. STEEL STRUCTURE—ROLLED SECTIONS AND HEAVY TRUSSES Some more-substantial buildings have roof structures built with rolled sec- tions or trusses constructed from angles or other rolled sections. ese structures can have significant load-carrying capacity and create very resilient structures. Individual members are robust, and because the con- nections are usually very substantial bolted or welded moment connections designed to resist bending, the entire structure is tied together in a way that resists failure very effectively. Composite systems that tie the steel members solidly to concrete decks oen add additional resiliency. Although the controlling factor in the design of heavier steel systems is still the gravity loads pulling down on the structure, under high wind loading when the low pressure above the roof reverses the loading on the rolled sections, putting the bottom members in compression, they are more able to naturally resist buckling thanks to the substantial bottom flange of the rolled sections. Existing Buildings ese structures are more oen used when higher than code-required loads are to be supported. Because they are