STRUCTURAL DESIGN STRUCTURAL DESIGN STRUCTURAL DESIGN STRUCTURAL DESIGN

12.1 DESIGN CONCEPTS

The isolation system design and evaluation procedures produce the maximum base shears, displacements and structural forces for each level of earthquake, usually the DBE and MCE. These represent the maximum elastic earthquake forces that will be transmitted through the isolation system to the structure above. Even though isolated buildings have lower seismic loads than non-isolated buildings, it is still not generally cost effective to design for elastic performance at the MCE level and sometimes yielding may be permitted at the DBE level.

Building projects to date in New Zealand have generally been designed elastically to the DBE level of loading with some ductility demand at the MCE level. This is because of the nature of buildings isolated so far, which have been either older buildings with limited ductility or buildings providing essential services where a low probability of damage is required.

For new buildings in the ordinary category, design forces are usually based on the DBE level of load reduced to account for ductility in the structural system. This is the approach taken by the UBC for new buildings. An isolated building, if designed elastically to the DBE, will likely have higher design forces than a ductile, non-isolated building which would be designed for forces reduced by ductility factors of 6 or more. These higher forces, plus the cost of the isolation system, will impose a significant first cost penalty on the isolated building.

Total life cycle costs, incorporating costs of earthquake damage over the life of the building, will usually favor the isolated building in high seismic regions. However, life cycle cost analysis is rare for non-essential buildings and few owners are prepared to pay the added first cost.

The UBC addresses this issue by permitting the structural system of an isolated building to be designed as ductile, although the ductility factor is less than one-half that specified for a non- isolated building. This provides some added measure of protection while generally reducing design forces compared to an equivalent non-isolated building. This approach would seem to be permitted by NZS3101 for New Zealand buildings and so it is recommended that the UBC approach be modified for local conditions for projects outside the U.S.

12.2 UBC REQUIREMENTS

The UBC requirements for the design of base isolated buildings differ from those for non- isolated buildings in three main respects:

1. The importance factor, I, for a seismic isolated buildings is taken as 1.0 regardless of occupancy. For non-isolated buildings I = 1.25 for essential and hazardous facilities. As

discussed later, a limitation on structural design forces to the fixed base values does indirectly include I in the derivation of design forces.

2. The numerical coefficient, R, which represents global ductility is different for isolated and non-isolated buildings.

3. For isolated buildings there are different design force levels for elements above and elements below the isolation interface.

12.2.1 ELEMENTS BELOW THE ISOLATION SYSTEM

The isolation system, the foundation and all structural elements below the isolation system are to be designed for a force equal to:

D D B k D

V

=

Where kDmax is the maximum effective stiffness of the isolation system at the design displacement

at the center of mass, DD. All provisions for non-isolated structures are used to design for this

force.

In simple terms, this requires all elements below the isolators to be designed elastically for the maximum force that is transmitted through the isolation system at the design level earthquake. One of the more critical elements governed by elastic design is the total moment generated by the shear force in the isolation system plus the P-∆ moment. As discussed in the connection design chapter, the moment at the top and bottom of an elastomeric type isolation bearing is:

) ( 2 1 D BH PD V M = +

where H is the total height of the bearing and P the vertical load concurrent with VB. The

structure below and above the bearing must be designed for this moment. For some types of isolators, for example sliders, the moment at the location of the slider plate will be PDD and the

moment at the fixed end will be VH.

12.2.2 ELEMENTS ABOVE THE ISOLATION SYSTEM

The structure above the isolators is designed for a minimum shear force, VS, using all the

provisions for non-isolated structures where:

I D D S R D k V

₌

This is the elastic force in the isolation system, as used for elements below the isolators, reduced by a factor RI that accounts for ductility in the structure.

Table 12-1 lists values of RI for some of the structures included in UBC. For comparison the

equivalent ductility factor used for a non-isolated building, R, is also listed in Table 12-1. UBC includes other structural types not included in this table so consult the code if your structural system does not fit those listed in Table 12-1. All systems included in Table 12-1 are permitted in all seismic zones.

The values of RI are always less than R, sometimes by a large margin. The reason for this is to

avoid high ductility in the structure above the isolation system as the period of the yielded structure may degrade and interact with that of the isolation system.

TABLE 12-1STRUCTURAL SYSTEMS ABOVE THE ISOLATION INTERFACE

Structural System Lateral Force Resisting System Fixed

Base R

Isolated RI

Bearing Wall

System Concrete Shear WallsMasonry Shear Walls 4.54.5 2.02.0 Building Frame

System Steel Eccentrically Braced Frame (EBF)Concrete Shear Walls Masonry Shear Walls

Ordinary Steel Braced Frame

Special Steel Concentric Braced Frame

7.0 5.5 5.5 5.6 6.4 2.0 2.0 2.0 1.6 2.0 Moment Resisting

Frame System Special Moment Resisting Frame (SMRF) Steel Concrete

Intermediate Moment Resisting Frame (IMRF) Concrete

Ordinary Moment Resisting Frame (OMRF) Steel 8.5 8.5 5.5 4.5 2.0 2.0 2.0 2.0 Dual Systems Shear Walls

Concrete with SMRF Concrete with steel OMRF Masonry with SMRF Masonry with Steel OMRF Steel EBF

With Steel SMRF With Steel OMRF Ordinary braced frames Steel with steel SMRF Steel with steel OMRF Special Concentric Braced Frame Steel with steel SMRF

Steel with steel OMRF

8.5 4.2 5.5 4.2 8.4 4.2 6.5 4.2 7.5 4.2 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 Cantilever column

Building systems Cantilevered column elements 2.2 1.4

There are design economies to be gained by selecting the appropriate structural system. For example, for a non-isolated building the design forces for a special moment resisting steel frame

are only about 53% of the design forces for an ordinary steel moment resisting frame. However, for an isolated moment frame the design force is the same regardless of type. In the latter case, there is no benefit for incurring the extra costs for a special frame and so an ordinary frame could be used. Be careful with this because, as discussed later, there may be some penalties in structural design forces if the ratio of R/RI is low.

Table 12-1 also shows that some types of building are more suited to isolation, in terms of reduction in design forces, than others. For bearing wall systems the isolation system only needs to reduce response by a factor of 4.5 / 2 = 2.25 or more to provide a net benefit in design forces. On the other hand, for an eccentrically braced frame the isolation system needs to provide a reduction by a factor of 7.0 / 2.0 = 3.5 before any benefits are obtained, a 55% higher reduction. The value of VS calculated as above is not to be taken as lower than any of:

1. The lateral seismic force for a fixed base structure of the same weight, W, and a period equal to the isolated period, TD.

2. The base shear corresponding to the design wind load.

3. The lateral force required to fully activate the isolation system factored by 1.5 (e.g. 1½ times the yield level of a softening system or static friction level of a sliding system).

In many systems one of these lower limits on VS may apply and this will influence the design of

the isolation system. Fixed Base Structure Shear

In general terms, the base shear coefficient for a fixed base structure is:

RT I C

C ₌ V

and for an isolated structure

BT R C C I VD I

=

There is a change in nomenclature in the two sections and in fact CV = CVD and

so to meet the requirements of Criterion 1 above, CI ≥ C, the two equations can

be combined to provide: I R R B I ≤

Therefore, the limit on forces to be not lower than the fixed base shear for a building of similar period effectively

FIGURE 12-1LIMITATION ON B 0% 10% 20% 30% 40% 50% 60% 1.50 1.75 2.00 2.25 2.50 2.75 3.00 Ratio R / RI M ax im um Da mp in g I=1 I=1.25