How stresses affect structural members (struts, ties, beams, columns, walls frames), particularly in commercial buildings
There are several members that make up a building frame. A few main members are struts, ties, beams, columns and walls that make up the frame of the building. It’s important to understand how stresses affects these, especially in commercial building.
Struts are present in the frame in order to withstand compressive stress. This stress comes from the force of each level of the structure going down to the foundations, and there will always be a reaction to that force. This creates a present longitudinal stress, and struts are positioned in the frame to withstand this by strengthening other structural members and making the structure overall more rigid.
Compressive stress can change the shape of structural members.
This would mean that the strut would be under constant compression, and this can affect this member in a number of ways depending on its properties. Also, as all parts of a frame are interlinked this can cause other problems amongst the structure.
Compressive stress can change the shape of structural members and in struts if it reaches a level of stress that is too much for the member, which is normally an effect called buckling. Buckling can affect a member in a way that it will make it unstable and deform in shape. This happens when the stress from the load causes the member to move laterally and shorten.
A similar member that also is built to hold compressive stress is a column. Columns are a lot less slender than struts, and their purpose is to transmit the weight of the above structures down to the foundation. These provide support for the whole structure.
These compressive stresses can affect columns the same way as struts, however, columns are a lot thicker and can bear a higher compressive stress resulting in the member taking a lot more to buckle, as they are thicker and expected to support a higher load than struts.
Beams are generally made of steel and are designed so they can withstand stress
Ties are structural members that are designed to withstand tensile stress – making it an opposite to a strut. These generally connect the bottom of the rafters on the opposite sides of a roof structure. These will need to be made of a material that is non-ductile and a strong tensile strength in order to withstand the stress that will be constant. If ties don’t have these properties the roof will sag as a result. Tensile stress affects structural members such as ties in a way that it can cause a deformity such as a bend. This, in simpler terms, would be the member being stretched resulting in a dip.
However, most of the time in a structure it isn’t noticeable to the eye and there will always be tensile stress present in a tie – this stress only starts to noticeably affect a member in that way if the tensile stress exceeds the tensile strength. This constant force on the object will eventually weaken the tie and deform its properties if the member’s properties haven’t been calculated properly in order to withstand these forces.
Another important member that is affected by stresses in a structure would be the beam. When loads are applied to beams, this creates stress. Beams in structures are expected to carry loads that create forces perpendicular to the beam, as it is a longitudinal member supporting structures above.
Beams are generally made of steel, and they are designed in a way that they can withstand stress, and prevent that stress affecting the integrity of the member. However, if the loads have been miscalculated and the force distributed over the area of the beam is too high, it can be subjected to bending moments. This causes the beam to bend – not always visible.
The role of stresses in failure
Structural failure is when a structure loses its structural integrity, which is the ability of an object to remain intact under a load. A structure or a member in a structure will have a certain amount of strength depending on its properties, which it is how much stress it can withstand caused by loads and the forces resultant from those loads. When that member or whole structure meets it’s maximum amount of stress it can withhold, the object can begin to deform which is strain.
Too much strain and the object will eventually fail. As stated before, structural members in a structure are all interlinked, yet sometimes stress can cause a localised failure rather than a total structure failure. When designing a structure, engineers prevent failure further by selecting materials that have a high factor of safety for the load they are expecting to carry.
Stress-based failure is mainly caused due to yield and fracture.
This means that the structural members can carry a lot more than is usually expected from the current use of the building. All materials that are used in construction are tested to see their properties, so they can be deemed suitable for the task that they are being assigned. Failure will happen in an object or structure when the stress state of that object is equal to that of the tested material that caused it to fail.
Stress-based failure is mainly caused due to yield and fracture. A material can be classified as either ductile or brittle, and objects that are ductile fail due to yield and those that are brittle to fracture. Upon assessment, the engineers can establish a stress-strain curve, which establishes the relationship between the stress that the material is suspected to experience and the strain that is the result. In other words, the object either surpasses its yield point in ductile materials or the ultimate tensile strength in brittle materials.
Typical ductile materials used in constructions would be steels and many types of alloys. These have very a very linear stress-strain curve up to a defined yield point – which means that the amount of stress and resultant strain coincides up the point where the object yields – and then fails. Brittle materials, such as concrete, will not have a yield point and will fail if the object experiences a deformity that is elastic.