Architecture Tips
Read these 13 Architecture Tips tips to make your life smarter, better, faster and wiser. Each tip is approved by our Editors and created by expert writers so great we call them Gurus. LifeTips is the place to go when you need to know about Earthquake tips and hundreds of other topics.
In-Plane Failure
In-Plane failure occurs when the ground motion during a quake is in plane parallel to the wall. A telltale sign of in-plane failure is the presence of cracks in the walls that form an X-shape. In-plane failures are caused by quake-induced shear forces which exceed the strength of masonry material. Generally, an in-plane failure alone will not cause a building to collapse. Although a wall may have cracks present due to this type of failure, it usually still has enough strength to support the weight of the building.
To prevent in-plane failures in existing URM buildings, the lateral strength of the load-bearing masonry walls must be improved. It's not done easily on masonry buildings. Sheathing cannot simply be screwed into masonry as it can be with timber. The traditional remedy is to build additional walls that will adequately resist shear loads and attach them to the interior of the URM bearing walls. It's a very costly procedure.
Timber Buildings - Structure Failure
Buildings constructed from timber perform well in earthquakes due to wood's ductile nature. When a timber structure is subjected to lateral loads produced by an earthquake, it tends to absorb the seismic energy by distorting the waves, then quickly returns to its original shape unharmed. Despite the ability to cushion this energy, there are 4 risk areas commonly associated with wood homes and buildings:
1. Foundation Anchorage
2. Cripple Walls
3. Soft Stories
4. Chimneys
Chimneys
Masonry Chimneys easily collapse or fall over in a quake. These chimneys can also crumble, causing damage to objects or people when the pieces crash to the ground.
There are 2 common mitigation options for this problem. The easiest is to place a metal brace around the chimney and then connect it to the trusses of the roof. Masonry chimneys may also be replaced by a wood frame covered by a thin brick veneer. When replacing the chimney just remeber that the height of the new chimney needs to be restricted to 2 ft (60 cm) above the roof line. If possible, patios, children's play areas, and parking spaces should be located away from the fall line of a chimney too.
Soft Stories
A soft story is defined as a building level that has a lateral stiffness less than 70% of the stiffness of the story above it. An apartment complex with a row of garages below the first level is a fine example of this scenario. Another example is a house supported by stilts on a mountain slope.
Mitigation of the soft story weakness involves the addition of more bracing or sheathing on the soft level in a manner similar to that of cripple walls or unreinforced masonry columns. Though it is more difficult to sufficiently reinforce a soft story, it can be done, in turn reducing damage to structures and the lives within them.
Diaphragm Failures
Another type of failure is known as a diaphragm failure. Masonry walls tend to behave as rigid members, while the floors and interior walls, which are made of timber, behave as flexible diaphragms. When a building is subjected to quake loads, the flexible diaphragms are moved within their planes. Problems are caused where the diaphragms meet the exterior masonry walls. The movement of these diaphragms exert considerable force on the walls, causing them to be pushed outward or even collapse. Once again, reinforcements can be installed at the connections of the joints and walls to mitigate this problem.
Masonry Anchorage
The anchors connecting the joists to the masonry walls in older buildings are often inadequate. Joists merely resting on ledges built for their support are common. If metal anchor brackets were installed, there are usually not a sufficient number of them to resist the forces created by a quake. When a building is shaken during a quake, insufficiently reinforced joists not only fail to lend support to the walls against out-of-plane failure, but can also fall off their ledge or break free from their connection. If enough anchors fail, entire stories can fall to the levels below, not only destroying the building, but also potentially crushing poor souls inside. In order to strengthen the connections between the joists and walls, the existing brackets can be replaced with newer ones, or of no brackets are in use, they can be installed.
Out-of-Plane Failure
Out-of Plane Failure in URM walls is usually much more destructive and spectactular than in-plane failures. As the name implies, out-of-plane failure occurs when the shockwaves in the ground move in any direction other than the plane of the wall. Unsupported masonry walls are far more susceptible to failure caused by ground motion perpendicular to the wall than from motion parallel to the wall. Out-of-Plane failure can cause a wall to literally explode. Because the ground motion caused by an earthquake is usually not merely in one direction, a wall will often fail due to the combination of in-plane and out-of-plane forces. Cracking caused by an in-plane failure may occur first, considerably weakening the wall. The wall could still support the building, but would be very susceptible to a devastating out-of-plane failure, which often occurs moments later when the ground begins shaking in a different direction.
The risk of an out-of-plane failure is greatly reduced if the distances between wall supports are reduced. The floor and ceiling joists, if properly attached to the masonry walls, can provide protection against out-of-plane failure. On older buildings, the attachments between the joists and the walls are often inadequate and provide little support for the walls. If these connections are reinforced, the spans between masonry wall supports are effectively reduced, significantly strengthening the building.
Cripple Walls
Homes built without a basement often use cripple walls, which are short (2ft - 60cm) stud walls that help support the main floor and rest on the foundation. Failure of the cripple wall can cause a building to collapse. These walls need to be braced horizontally to be able to resist the destructive quake forces.
Sheathing strengthens cripple walls by providing this lateral reinforcement. The size and nailing requirements are determined by the size of the building and the expected forces acting upon it. Specifications can be obtained through a local contractor or a handbook of construction codes which can be found in a local library. This installation is inexpensive and easy to do, and can prevent homes from complete collapse.
Checking Structure
Once an earthquake has stopped, you'll need to have a thorough check of your home. Repair any deep cracks in ceilings or foundations and get expert advice if there are any signs of structural damage.
Masonry Structures
Many of America's older buildings were built with unreinforced masonry (URM). These buildings are extremely vulnerable to quake-induced failures because of the nature of the material and the manner in which they were constructed. The inability of URM to stand up to seismic stresses stems from the fact that masonry is a brittle material. When a building is subjected to lateral accelerations caused by a quake, large forces are put on structural members of the buildings. Unlike buildings made of ductile materials like steel or timber which are able to deform to these forces, masonry can only deform to a very small degree unharmed. If too large a force is imposed upon these buildings, the masonry will crack and fail. The 4 most common areas of danger are:
1. In-Plane Failure
2. Out-of-Plane Failure
3. Anchorage Failure
4. Diaphragm Failure
Masonry-Concrete Columns
To reinforce masonry or concrete columns against in-plane failures, prestressed jacketing can be installed. Prestressed jacketing is like wrapping a metal blanket around a column. This jacket strengthens concrete and masonry beams by holding them together and keeping them from easily cracking. It also helps prevent the columns from buckling and bending.
Liquefaction
There are many hazards caused by earthquakes that put structures in danger. The most obvious is the ground shaking but, another danger is liquefaction.
This is caused from the ground shaking which in turn causes the soil to lose its strength and behave as a liquid-much like quicksand.
Foundation Anchorage
Structures are supported by their foundations. During an earthquake, buildings may be picked up and moved off their foundations either by being pushed in the horizontal direction of the quake or vertically perpendicular to the tremor, resulting in damage to the walls floors, windows, and ceilings.
Mitigation to this problem is both simple and inexpensive. Anchor bolts may be used to fix the home to the foundation. The bolts may be placed in newly poured concrete or drilled into an existing foundation. Anchor bolts are then connected to the sill plate with a nut. One bolt should be placed for every 269 ft (25 m.) of floor area above the foundation. This procedure will help the building remain connected to its foundation during an earthquake.
| Guru Spotlight |
| Jennifer Mathes, Ph.D. |
