Fennell Engineering
Call 212-639-9300
M-F 9-5 or by Appointment
Home Structural Engineering Site Inspections Masonry Walls About Us Testimonials Contact Us

VERTICAL ANNALYSIS

LIVE LOADS

Vertical analysis of building occupancies need to be addressed, even when individuals are not possibly present. These "Live Loads" are to be listed and addressed by an authority with proper jurisdiction. The owner of the building should also take into consideration, occupancy loads not originally contemplated. Floor loads measured in a live load survey are usually well below present design values. However, buildings must be designed to withstand the maximum loads they are likely to be subjected to during some reference period, usually around 50 years. The floor load normally taken into consideration for a given occupancy is referred to as the sustained load. This load is constant until a change in tenant or occupancy type occurs.

Before and after

PARTIAL LOADING

The full intensity of the appropriately reduced live load, over portions of the building need to be considered, as well as a live load of significant intensity over the full length of the structure. Loads on either side of a specific area will produce greater negative weight distribution. Where a uniform roof live load is considered an occupancy, partial or patterned loading should be considered regardless of the magnitude of the uniform load.

HEAVY LIVE LOADS

Heavy live loads occupying large warehouse or storage buildings, may have several adjacent floors fully loaded as far as spacing allows. Rarely is any story loaded to more than 80% of its capacity. Requirements state that live loads should be reduced by 20% for the design of supports for more than one floor.

LOADS IN EXCESS OF THE DESIGN VALUE

A designated value that reduces risk of a snow load to a minimum. Excess loads should be taken into account for weight, melting and relative safety issues.

FLAT-ROOF SNOW LOADS

The minimum generated loads by a snowstorm may exceed values as far as snow loads are concerned. These variables may be modified by region, the structural design itself, or by obstructions both attached and surrounding the roof. Prevalent amounts of snow may exist on colder roofs than warm. Conditions should also take into account the lifespan of the building. Heated roofs occasionally lose power where thawed snow may now be refreezing in certain sections. Drains can be hampered by ice and may build up as extra loads. These situations should be observed regularly. Fixed roof items may present similar conditions due to their own heating or cooling features. Sloped roofs have their own inherent qualities where ice dams may become clogged and frozen. Proper ventilation rules have been established for attics and cathedral ceilings. Glass, plastic, and fabric roofs, due to their specific heat qualities, should cause snow to melt and slide off. These unique roofs may need proper professional inspections for such cases.

SLOPED-ROOF SNOW LOADS

The more intense the slope of a roof, less snow loads are to be expected. This mainly accounts for wind as well as sloping. Roof surfaces that are considered "slippery" have been broken down by surface qualities and documented. Sloped roofs that have stopped receiving heat may freeze and ice may hold to the roof. Obstructions such as other roofs, or accumulation from the ground may obstruct eaves and prevent snow loads from sliding off. Less snow loads atop a sloped roof may increase near the lower ends. Lateral and vertical loads should be taken into consideration for these roofs especially A-Frame roofs.

DRIFTS ON LOWER ROOFS

Snow loads may increase due to shade from adjacent roofs or other nearby obstructions. Drift size accounts for driftable snow, upwind roof length, and snow load on the ground. Drift height relationship is based on snow from a higher roof and it’s effects on a lower roof below. These changes in snow loads where a drift forms is called a "leeward step." Or "windward steps" where wind is the main factor. Drift load provisions cover most situations. For unusually shaped roofs, wind tunnel or water-flume tests may be used to establish drift loads.

ROOF PROJECTIONS

Drifts of the "windward step" variety are applied to structures where a roof may be absent or too small, such as in a penthouse, buttresses, or solar paneled areas, to accommodate snow loads. Several rows of solar panels may accumulate additional loads. A roof with solar collectors is somewhat covered, since the roof is moderately exposed. By raising solar collectors by several feet, the snow loads drift and lower adverse factors. The solar panels should be designed to handle a snow load abiding by the "unobstructed slippery surfaces" curve. This final load should not be considered in the roof design because the load of sliding snow from the panels has already been anticipated.

RAIN-ON-SNOW SURCHARGE LOAD

The roof load affected by rain, upon snow loads, may be sizeable. The seriousness of this is measured by the overall strength of the rainstorm, the drainage of snow, roof shape, and type of drainage installed. Rain on snow loads are more extreme for large, wide, low-sloped roofs.

PONDING INSTABILITY

Ponding loads occur when there is significant rain and/or melting of ice or snow. This excess water produces ponding in low sections of roof tops. As rain or melted snow water flows to these low areas, ponding instability occurs. If the building does not retain enough strength to withstand these loads, ponding instability may be the consequence. Ponding instability has been the cause of roof instability due to rain and snow loads. Proper drainage and sloping will prohibit such results.

OTHER ROOFS AND SITES

Wind tunnel models, and similar evaluations utilizing liquids, other than air, successfully established methods examining snow loads for unique roof designs and structural sites. These trials must replicate the chaotic characteristics of wind and how snow particles accumulate upon roofs then affected by wind variables. These studies are more reliable when tested in a similarly large scale environment.

REQUEST A QUOTE

Name:

Phone:

Email:
Comments/Questions:
 
Agree to terms & conditions


Read the privacy statement

Call Today 212-639-9300!