Where Is a Needle Beam Used?
This has nothing to do with needles and nothing to do with light. A needle beam is a term used by builders and architects to describe the supports used when the foundations of a wall or a column need attention. If a wall needs underpinning to enable the foundations to be strengthened, steel needle beams are inserted through slots cut into the wall a, foot or so from the bottom.
The ends of the beams are supported by screw jacks which can be moved along the beams according to the extent of the foundation area needing attention. Once the work has been done and the foundations have been restored, the needle beams are removed and the holes in the wall filled. When a needle beam is used to shore up a column it is usually bolted on. The needle beam is so called because it is “threaded” through the wall it has to support. Underpinning may be necessary for a variety of reasons:
- The original foundation is simply not strong or stable enough.
- The usage of the structure has changed.
- The properties of the soil supporting the foundation may have changed (possibly through subsidence) or were mischaracterized during design.
- The construction of nearby structures necessitates the excavation of soil supporting existing foundations.
- To increase the depth or load capacity of existing foundations to support the addition of another storey to the building (above or below grade).
- It is more economical, due to land price or otherwise, to work on the present structure’s foundation than to build a new one.
- Earthquake, flood, drought or other natural causes have caused the structure to move, thereby requiring stabilization of foundation soils and/or footings.
Underpinning may be accomplished by extending the foundation in depth or in breadth so it either rests on a more supportive soil stratum or distributes its load across a greater area. Use of micropiles and jet grouting are common methods in underpinning. An alternative to underpinning is the strengthening of the soil by the introduction of a grout, including expanding urethane-based engineered structural resins. Underpinning may be necessary where P class (problem) soils in certain areas of the site are encountered.
Through semantic change the word underpinning has evolved to encompass all abstract concepts that serve as a foundation. The decision of underpinning requirement can be made based on observations. When an already existing structures start to show certain change through settlement or any kind of distress, it is necessary to establish vertical level readings as well as at the offset level, on a timely basis. The time period depends upon the how severe is the settlement.
Now, before the excavation for a new project, professionals have to closely examine and determine the soil capability to resist the structure that is coming over it. Based on that report the need for underpinning is decided. Sometimes such test would avoid underpinning to be done after the whole structure is constructed. Whatever be the types of underpinning method selected for strengthening the foundation, all of them follow a similar idea of extending the existing foundation either lengthwise or breadth wise and to be laid over a stronger soil stratum. This enables distribution of load over a greater area.