Figure 2-1. Wood light frame three-story structure elevation
Foreword Foreword Foreword Foreword
After careful consideration and extensive discussion, SEAOC is recommending that large wood frame structures, such as the three-story building in this design example, be designed for seismic forces considering both rigid and flexible diaphragm assumptions. This method represents a significant change from current practice. At present, California practice has almost exclusively used the flexible diaphragm assumption for determining distribution of story shears to shear walls.
There are two principal reasons for considering both rigid and flexible diaphragms.
First, since adoption of the 1988 UBC, there has been a definition of diaphragm flexibility in the code (§1630.6 of the 1997 UBC). Arguably, when introduced in 1988, this definition may not have been intended to apply to wood framed diaphragms. After considerable discussion and re-evaluation, it is now the joint opinion of the SEAOC Code and Seismology Committees that this definition should be considered in wood framed diaphragms. The application of this definition in wood construction often requires the use of the rigid diaphragm assumption, and subsequent calculation of shear wall rigidities, for distribution of story shears to shear walls. In fact, this definition results in many, if not most, diaphragms in wood frame construction being considered rigid.
Many engineers feel that exclusive use of the flexible diaphragm assumption results in underestimation of forces on some shear walls. For example, a rigid
88 SEAOC Seismic Design Manual, Vol. II (1997 UBC)
diaphragm analysis is judged more appropriate when the shear walls are more flexible compared to the diaphragm, particularly where one or more lines of shear walls (or other vertical resisting elements) are more flexible than the others are.
Second, in some instances, the use of flexible diaphragm assumptions can actually force the engineer to provide a more favorable lateral force resisting system than would occur by only using rigid diaphragm assumptions. Flexible diaphragm assumptions encourage the placement of shear walls around the perimeter of the floor and roof area, therefore minimizing the need to have wood diaphragms to resist torsional forces.
In this design example, the floor diaphragms are constructed using screw shank nails, sheathing is glued to the framing members (to reduce floor squeaks), and lightweight concrete fill is placed over the floor sheathing (for sound insulation).
Additionally, gyp board is applied to the framing underside for ceiling finish.
These materials in combination provide significantly stiffer diaphragms than those represented by the diaphragm deflection equation of UBC standard 23-2.
For the part of the analysis that assumes a rigid diaphragm, the engineer must also select a method to estimate shear wall rigidities (and rigidities of other vertical resisting elements). This also requires use of judgment because at the present time there is no consensus method for estimating rigidities. In the commentary of Design Example 1, several alternatives are discussed.
Prior to starting design of a wood light frame structure, users of this document should check with the local jurisdiction regarding both the level of analysis required and acceptable methodologies.
Overview Overview Overview Overview
This design example illustrates the seismic design of a three-story 30-unit hotel structure. The light frame structure, shown in Figures 2-1, 2-2, 2-3, and 2-4, has wood structural panel shear walls, and roof and floor diaphragms. The roofs have composite shingles and are framed with plated trusses. The floors have a 1½-inch lightweight concrete topping framed with engineered I joists. The primary
tiedowns for the shear walls use a continuous tiedown system.
This structure cannot be built using conventional construction methods for reasons shown in Part 6 of this design example. The following sections illustrate a detailed analysis for some of the important seismic requirements of the 1997 UBC. This design example is not a complete building design, and many aspects of a complete design, including wind design (see UBC §606 ), are not included. Only selected items of the seismic design are illustrated.
In general, the UBC recognizes only two diaphragm categories: flexible and rigid.
However, the diaphragms in this design example are considered to be semi-rigid.
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Hence, the analysis will use the envelope method, which considers the worst loading condition from the flexible and rigid diaphragm analyses for each vertical shear resisting element. It should be noted that the envelope method, although not explicitly required by code, is deemed necessary and good engineering practice for this design example.
Initially, the shear wall nailing and tiedown requirements are determined using the flexible diaphragm assumption. Secondly, use these shear wall forces to determine shear wall rigidities for the rigid diaphragm analysis. Finally, further iterations may be required with significant stiffness redistributions.
The method of determining shear wall rigidities used in this design example is by far more rigorous than normal practice but is not the only method available to determine shear wall rigidities. The commentary following Design Example 1 illustrates two other simplified approaches that would also be appropriate for this design example.
Outline Outline Outline Outline
This example will illustrate the following parts of the design process:
1111. Design base shear and vertical distributions of seismic forces.
2222. Lateral forces on the shear walls and required nailing assuming flexible diaphragms.
3333. Rigidities of shear walls.
4444. Distribution of lateral forces to the shear walls.
5555. Reliability/redundancy factor ρρρρ.
6666. Does structure meet requirements of conventional construction provisions?
7777. Diaphragm deflections to determine if the diaphragm is flexible or rigid.
8888. Tiedown forces for shear wall on line C.
9999. Tiedown connection at the third floor for the shear wall on line C.
10101010. Tiedown connection at the second floor for the shear wall on line C.
11111111. Anchor bolt spacing and tiedown anchor embedment for shear wall on line C.
90 SEAOC Seismic Design Manual, Vol. II (1997 UBC) 12121212. Detail of tiedown connection at the third floor for shear wall on line C
(Figure 2-9).
13131313. Detail of tiedown connection at the second floor for shear wall at line C.
(Figure 2-10).
14141414. Detail of wall intersection at exterior shear walls (Figure 2-11).
15151515. Detail of tiedown connection at foundation (Figure 2-12).
16161616. Detail of shear transfer at interior shear wall at roof (Figure 2-13).
17171717. Detail of shear transfer at interior shear walls at floors (Figure 2-14).
18181818. Detail of shear transfer at interior shear walls at foundation (Figure 2-15).
19191919. Detail of sill plate at foundation edge (Figure 2-16).
20202020. Detail of shear transfer at exterior wall at roof (Figure 2-17).
21212121. Detail of shear transfer at exterior wall at floor (Figure 2-18).