Vibrations And Vibration Control

Rotational movement of an unbalanced mass can generate vibrations transmitted to the surrounding construction elements through the bearing points of the load

Managing Acoustics and Vibrations

Noise and vibration may arise from sources inside or outside a health care facility. Internal sources may include HVAC equipment, electrical transformers, emergency generators, and plumbing equipment, while external sources might include adjacent building equipment and traffic. Careful planning and anticipation in the early stages of design may help avoid expensive modification, such as acoustic and vibration isolation.

For example, simply distancing mechanical and electrical rooms, public corridors, lavatories, and other sources of sound and vibration from sensitive spaces can minimize need for structural premiums to control these effects. If isolation of a sensitive space is required, however, the building's structural design plays an important role. Using thicker floor slabs can protect against airborne noise, while providing structural stiffening elements and damping devices (to dissipate kinetic energy) can minimize the effect of vibrations. Acoustic isolation slabs may also be used. Structural discontinuities, such as simple joints in the floor slab, can also prevent transmission of sound and vibrations through the structure. The structural designer must work closely with planners, as well as acoustic and vibration consultants, to develop the most desirable environment in the most cost-effective manner.

Not only is lead the most impervious of all common metals to X-rays and gamma radiation, it also resists attack by many corrosive chemicals, most types of soil, and marine and industrial environments. Although lead is one of the heaviest metals, only a few applications are based primarily on its high density. Main reasons for using lead often include low melting temperature, ease of casting and forming, good sound and vibration absorption, and ease of salvaging from scrap.

With its high internal damping characteristics, lead is one of the most efficient sound attenuators for industrial, commercial, and residential applications. Sheet lead, lead-loaded vinyls, lead composites, and lead-containing laminates are used to reduce machinery noise. Lead sheet with asbestos or rubber sandwich pads are commonly used in vibration control.

The natural lubricity and wear resistance of lead make the metal suitable, in alloys, for heavy-duty bearing applications such as railroad-car journal bearings and piston-engine crank bearings. Lead is also widely used as a constituent in solders. Most common solders are the lead-tin alloys; melting temperature can be as low as 361°F.

In its unalloyed form (defined by ASTM B29 as 99.85% minimum lead), lead is soft and weak; it requires support for mechanical applications. This "chemical lead" is used primarily in corrosive chemical-handling applications such as tank linings.

"Hard lead" — lead alloyed with 1 to 13% antimony — has sufficient tensile strength, fatigue resistance, and hardness for many mechanical applications. These alloys can be cast, rolled, or extruded and are especially suited for castings requiring good detail and moderate strength. Rolled antimonial alloys are harder and stronger than the cast alloys. Battery-plate lead contains 7 to 12% antimony.

Calcium (0.03 to 0.12%) forms another series of mechanically suitable alloys with lead. These alloys age harden naturally at room temperature — usually for 30 to 60 days — after being cast or worked. Properties of wrought Pb-Ca alloys are somewhat directional, being greater in the longitudinal direction. Uses include cable sheathing and grids in storage batteries.

Tin, added to Pb-Ca alloys in amounts to about 1.5%, raises tensile strength and stress-rupture resistance but increases aging time to 180 days. Tin is also used to reduce coefficient of friction for bearing applications. Higher tin-bearing alloys are primarily used in solders, which normally contain from 40 to 60% tin.

Lead alloys can be cast using most methods. Intricate details can be reproduced, and the surface can be readily painted. Lead alloys can be extruded into pipes, bars, channels, and rods. Cold-rolled lead sheet is available in thicknesses ranging from foil to 2 in., in widths to 11 ft, and in lengths to 60 ft. Sheet thicknesses are specified by weight: Generally, each 1/64 in. of thickness corresponds to 1 lb/ft².

Lead often requires support by other materials. For example, lead sheets can be clad or fastened (mechanically or with adhesives) to plastic or steel panels and pipe. Lead-clad steel, in a range of thicknesses, is produced by cold-rolling lead sheet on terne-coated steel. Other clad combinations are produced by spot welding, spraying, hot dipping, and electroplating. These laminates are particularly useful in noise and vibration-absorption applications.

Lead Extrusion Damper

Lead Extrusion Damper for damping the motion of a seismically isolated structure and structural vibrations. In some buildings flexibility is provided structurally by long columns while the damping is provided by lead extrusion dampers. The lead extrusion damper acts as a coulomb damper or plastic damper and is particularly suitable for very large displacements.

Forces — 1 kN to 1 MN, Displacements — up to 1 m.