Wall Forces
- Shear walls (in-plane lateral forces):
- The shear walls support the diaphragm by receiving the diaphragm
reaction, R, through proper connections at the diaphragm boundary.
- See lateral load path for a review of the basic building's behavior.
- If you have a masonry or concrete building supported by a
flexible diaphragm, the shear wall load (e.g. the diaphragm
reaction) could be recalculated to reflect the lower design
requirement.
- This comes from the R = 4.5 for vertical LFRS elements vs. R = 4.0 for the horizontal LFRS.
- In addition, the wall itself develops inertial forces that also
act parallel to the wall and must be accounted for.
- This additional seismic force is assumed to be generated from the weight of the top half of the wall
- The shear wall is then usually evaluated for shear stress capacity
at mid-height.
- According to '97 UBC 2107.1.7 if in seismic zones 3 or 4, the shear wall must be designed to resist 1.5 times v.
- The shear walls support the diaphragm by receiving the diaphragm
reaction, R, through proper connections at the diaphragm boundary.
- Out-of-plane bending of walls (lateral forces perpendicular to wall):
- Refer to '97 UBC 1632 - Lateral forces on elements of structures.
- Applied forces on elements, (vs. the LFRS), may be larger in magnitude because these elements respond dynamically to the motion of the structure instead of the ground.
- The strength-level design equation for elements and components
has changed considerably from earlier (pre-1997) versions of the
code. It now is:
Where:
- the subscript "p" refers to elements or components (e.g. parts) of the structure.
- ap = in-structure component amplification factor found
in '97 UBC Table 16-O
- £ ap £ 2.5
- Ca = seismic response spectrum value found in '97 UBC Table 16-Q.
- Rp = component response modification factor form '97 UBC Table 16-O
- hx = the location (elevation) of the attachment point of the part taken with respect to grade
- hr = the structure's roof elevation with respect to grade
- wp = the weight of the element or component under consideration
- Fp is at strength-level and must be adjusted by 1.4 to reduce it to ASD level.
- r = 1.0 for elements and components.
- A common calculation that makes use of this element's provision
is to determine the seismic force normal to a wall as shown in the
following figure. A sample calcualtion is given in Example
2.
- Please note that there appears to be a height mistake and
a code interpretation problem in Breyer's Example 2.17
- Please note that there appears to be a height mistake and
a code interpretation problem in Breyer's Example 2.17
- Refer to '97 UBC 1632 - Lateral forces on elements of structures.
- Diaphragm anchorage:
- Lateral forces acting perpendicular to the wall will tend to separate
the wall from the horizontal diaphragm.
- Must provide a positive anchorage system connecting masonry walls to diaphragms, shown above as the "specially designed anchor".
- This anchorage must resist:
- Wind forces on wall element.
- Seismic force normal to the wall using UBC '97 Eqn 32-2. According
to '97 UBC 1633.2.81:
- In seismic zones 3 and 4 with a flexible diaphragm, Rp = 3.0 and ap = 1.5. This ap factor essentially increases the design forces at the wall to diaphragm by 50%.
- In seismic zone 4, the Fp for wall anchorage ³ 420 lb/ft.
- Regardless of governing lateral force (wind vs. seismic) the code specifies a minimum, strength-level, anchorage force of 280 lb/ft for concrete and masonry walls ('97 UBC 1605.2.3 and 1611.4)
- Requirements about anchorage detailing for concrete or masonry
walls:
- In seismic zones 3 and 4 ('97 UBC 1605.2.3)
- Use embedded straps that attach or hook around reinforcing steel or ensure effective transfer of forces to steel.
- Limit anchor spacing to £ 4' unless wass are designed to resist bending between anchors.
- In seismic zones 2, 3, and 4('97 UBC 1633.2.9.5)
- Anchorage shall not use nails in withdrawal or toe nails.
- Ledgers or framing shall not be used in cross-grain bending/tension,
which is shown in the following figure.
- To avoid these problems, use specially designed seismic ties that are known as purlin anchors by Simpson Strong-Tie, a manufacturer of prefabricated, light-gauge, steel connectors for wood construction.
- In seismic zones 3 and 4 ('97 UBC 1605.2.3)
- Lateral forces acting perpendicular to the wall will tend to separate
the wall from the horizontal diaphragm.
