May 2008

Write short notes on the following:

1. Difference between live and dead loads

Dead loads (permanent force acting on structure)(generally do not change over time)

Dead loads are the self weight of components put on the structure like plasterboard or carpet and the structure itself. Typically they are relatively constant throughout the structure's life, and so they are also known as Permanent loads. The designer can also be relatively sure of the magnitude of the load as it is closely linked to density of the material, which has a low variance, and is normally responsible for the specification of the component (e.g. plasterboard thickness) and can check it on-site.

Dead loads also include forces set up by irreversible changes in a structure's constraints. For example, loads due to settlement, the effects of pre-stress or due to shrinkage and creep in concrete.

Live loads (changing, non-permanent force acting on structure)(changes over time)

Live loads, sometimes referred to as probabilistic loads include all the forces that are variable within the object's normal operation cycle not including construction or environmental loads. Using the staircase example the live load would be considered to be –

Pressure of feet on the stair treads (variable depending on usage and size) Wind load (if the staircase happens to be outside)

Live loads (roof) produced (1) during maintenance by workers, equipment and materials; and (2) during the life of the structure by movable objects such as planters and by people.]

Live Load (Bridge) produced by vehicles traveling over the deck of the bridge.

2. Timber trusses

Prefabricated trusses provide support to the roof claddingand ceiling material and generally span from external walls.Trusses can be manufactured to match almost any profile and are designed to withstand a range of loadings fromsnow loads to cyclonic winds.

Trusses are typically formed from a frame made from triangles. The strength of the truss is based on triangles which are the strongest geometic shape. When connected together, the triangles form a strong and lightweight frame.

A strong, stable structural frame results from the truss chords providing stiffness (similar to a deep beam) as well as webs that act in compression and tension.

Benefits of using timber roof trusses1. Timber trusses are cheaper than conventionally pitched or steel roofs.2. Using timber roof trusses results in reduced construction time and therefore earlier weather protection for the walling and flooring.3. Trusses rarely need internal support as they generally span from outside wall to outside wall. This allows for larger open areas in a building and makes a trussed roof ideally suitable for medium-scale commercial work. The omission of internal supports also saves the cost of load-bearing walls and footings.4. Timber trusses are lightweight, helping to speed up their construction.5. Timber roof trusses are ecologically sound. They use smaller lengths and cross-section timbers than conventionally pitched roofs - this is actually a key reason for their reduced cost as it reduces the total timber volume in a roof.

3. Uses of ground beams

1. Ground beams are used in situations where foundation walls may not be achievable. Situations where foundations are not achievable is in swampy ares, loose soils, or where the soil bearing capacity is very low. The beams are erected over columns or piers taken into the soil.

2. In home construction, they are used to carry boundary walls or sheds built outside on landscaped soils. They can be erected over drains, sewers or other services in the ground.

3. Ground beams save on the costs of very deep excavations for foundation and walls. They can carry greater loads in loose soils since they do not rest on the soils. They carry their on weight from column to column.

4. Ground beams improve construction programmes by reducing poor ground conditions and eliminating wet concrete trade on site. 

5. Ground beams are designed to support brick/block work or to form a permanent shutter to the edge of in-situ concrete floor slab. 

4. Relevance of building bye laws

The building byelaws are defined as the standards and specifications designed to 1. grant minimum safeguards to the workers during construction, 2. to the health and comfort of the users  and to3. provide enough safety to the public in general. 4. The regulation set out the basic requirements to be observed in the of  design and construction of buildings. They are applied to new building and also to extensions, material alterations, and certain changes of use of existing buildings.

5. A building byelaw is a local law framed by a subordinate authority. It channelizes to achieve the concepts & policies outlined in the master plans of the city, in order to give a particular ARCHITECTURAL CHARACTER to the city.

5. Difference between manhole and inspection chambers(inspection chambers: no man access,provide access only to equipements Shallow depth Diameter smaller -600mm Usually made of plastic normally occur where branches join the main run used when direction of flow changesor where different sewer streams converge

(manhole: man access Deeper depth Larger diameter Usually made of brickwork or sectional concrete ringsThe dimensions of manholes given make allowances for man-entry and are large enough to allow for rodding and clearance while standing on the benching, and for the operative to carry breathing apparatus if necessary.

6. Raft foundations

Raft foundations are used to spread the load from a structure over a large area, normally the entire area of the structure. They are used when column loads or other structural loads are close together and individual pad foundations would interact.

A raft foundation normally consists of a concrete slab which extends over the entire loaded area. It may be stiffened by ribs or beams incorporated into the foundation.

Raft foundations have the advantage of reducing differential settlements as the concrete slab resists differential movements between loading positions. They are often needed on soft or loose soils with low bearing capacity as they can spread the loads over a larger area.

7. Load bearing structures and framed structures

Load bearing structures are structures where the load is transferred to the foundations via load bearing internal and external walls e.g. masonry houses, pyramids in Egypt. They are generally characterized by having a small window to wall ratio (i.e. more structural wall area than window openings) and internal walls. Due to the large stresses within the brick or stone walls the height of load bearing structures is limited.

A framed structure is composed of beams and columns. With the load being transferred into the beams via the floor and then down to foundations through the columns. These types of structures can have open internal spaces and clad in glass (curtain walling) or have large windows. Think of modern day skyscrapers. The first framed building was the Home Insurance Building in Chicago built in 1884-85.

8. Differential settlement

Differential settlement refers to the the unequal settling of a building's piers or foundation that can result in damage to the structure. The damage occurs when the foundation sinks in different areas at different times.

Causes: Differential settlement is primarily due to the condition of the soil upon which the structure sits. Soil has the capacity to expand or contract based upon the temperature or weather conditions. It can also shift or wash away due to poor drainage, heavy rainfall, soil drying unevenly, or changes in the water table.

1. Improperly compacted fill 2. The presence of organic material in the soil that over time decomposes 3. Naturally occurring, isolated weak soil zone. For example one corner of a house may

be built over a saturated clay lens while the rest is on sandy loam.