ROOF DRAINAGE BASICS AND TUTORIALS

ROOF DRAINAGE BASIC INFORMATION
How To Design Basic Roof Drainage?


Many roof failures have been caused by excessive water accumulation. In most cases, the overload that caused failure was not anticipated in design of those roofs, because the designers expected rainwater to run off the roof.

But because of inadequate drainage, the water ponded instead.

On flat roofs, ponding of rainwater causes structural members to deflect. The resulting bowing of the roof surface permits more rainwater to accumulate, and the additional weight of this water causes additional bowing and collection of even more water.

This process can lead to roof collapse. Similar conditions also can occur in the valleys of sloping roofs.

To avoid water accumulation, roofs should be sloped toward drains and pipes that have adequate capacity to conduct water away from the roofs, in accordance with local plumbing codes.

 Minimum roof slope for drainage should be at least 1⁄4 in / ft, but larger slopes are advisable.

The primary drainage system should be supplemented by a secondary drainage system at a higher level to prevent ponding on the roof above that level.

The overflow drains should be at least as large as the primary drains and should be connected to drain pipes independent of the primary system or scuppers through the parapets.

The roof and its structural members should be capable of sustaining the weight of all rainwater that could accumulate on the roof if part or all of the primary drainage system should become blocked.

ASEISMIC DESIGN OF CIVIL ENGINEERING STRUCTURES BASICS AND TUTORIALS

ASEISMIC DESIGN OF CIVIL ENGINEERING STRUCTURES BASIC INFORMATION
What Are Aseismic Design Of Civil Engineering Structures?


The basic methods for providing wind resistance—shear walls, diagonal bracing, and rigid frames are also suitable for resisting seismic loads. Ductile rigid frames, however, are preferred because of large energy-absorbing capacity.

Building codes encourage their use by permitting them to be designed for smaller seismic loads than those required for shear walls and diagonal bracing. (Ductility is a property that enables a structural member to undergo considerable deformation without failing.

The more a member deforms, the more energy it can absorb and therefore the greater is the resistance it can offer to dynamic loads.) For tall, slender buildings, use of the basic methods alone in limiting drift to an acceptable level may not be cost-effective.

In such cases, improved response to the dynamic loads may be improved by installation of heavy masses near the roof, with their movements restricted by damping devices. Another alternative is installation of energy-absorbing devices at key points in the structural framing, such as at the bearings of bottom columns or the intersections of cross bracing.

Designers usually utilize floors and roofs, acting as horizontal diaphragms, to transmit lateral forces to the resisting structural members. Horizontal bracing, however, may be used instead.

Where openings occur in floors and roofs, for example for floors and elevators, structural framing should be provided around the openings to bypass the lateral forces.

As for wind loads, the weight of the building and of earth adjoining foundations are the only forces available to prevent the horizontal loads from overturning the building. Also, as for wind loads, the roof should be firmly anchored to the superstructure framing, which, in turn, should be securely attached to the foundations.

Furthermore, individual footings, especially pile and caisson footings, should be tied to each other to prevent relative movement. Building codes often limit the drift per story under the equivalent static seismic load.

Connections and intersections of curtain walls and partitions with each other or with the structural framing should allow for a relative movement of at least twice the calculated drift in each story. Such allowances for displacement may be larger than those normally required for dimensional changes caused by temperature variations.

(N. M. Newmark and E. Rosenblueth, ‘‘Fundamentals of Earthquake Engineering,’’ and J. S. Stratta, ‘‘Manual of Seismic Design,’’ Prentice-Hall, Englewood Cliffs, N.J.; ‘‘Standard Building Code,’’ Southern Building Code Congress International, Inc., 900 Montclair Road, Birmingham, AL 35213-1206; ‘‘Uniform Building Code,’’ International Conference of Building Officials, Inc., 5360 South Workman Mill Road, Whittier, CA 90601.)