## Diaphragm discontinuities

Home Owner Builder Mistakes And How To Avoid Them

Get Instant Access

In the ideal world of the structural engineer, diaphragms in buildings are not penetrated by anything larger than say a 300 mm diameter pipe. Diaphragms are also planar and level over the whole floor plan. However, the real world of architecture is quite different, because in most buildings quite large penetrations are required for vertical circulation such as stairways and elevators. Building services, including air ducts and pipes also need to pass through floor slabs and in the process introduce potential weaknesses into diaphragms.

Chapter 4 outlines the roles and requirements of diaphragms. It likens diaphragms to horizontal beams resisting and transferring horizontal inertia forces to their supports which, in this case, consist of vertical structural systems such as shear walls or moment frames. It explains how penetrations are acceptable structurally, provided they respect the shear force and bending moment diagrams of a diaphragm. Recall that the web of a diaphragm resists shear forces, while perimeter diaphragm chords acting in tension or compression, resist bending moments.

The size of a penetration can be large enough to ruin the structural integrity of a diaphragm altogether. Consider the case of a simple rectangular diaphragm spanning between two shear walls that act in the y direction (Fig. 8.14). What are the structural options if a full-width slot is required? The slot destroys the ability of the diaphragm to span to the right-hand wall. If the purpose of the slot is to introduce light or services through the diaphragm one option is to bridge the slot by introducing a section of steel bracing (Fig. 8.15(a)). If designed and connected strongly enough it restores the original spanning capability of the diaphragm. Alternatively, if the geometry of diagonal members isn't acceptable aesthetically a horizontal vierendeel frame, with its far larger member sizes, can be inserted to restore structural function (Fig. 8.15(b)) . In both solutions, light and services can pass between structural members.

If the intention of the penetration in Fig. 8.14 is to provide a staircase, then both previous options are unacceptable. It is now impossible for the diaphragm to transfer forces to the right-hand shear wall. The only option is to no longer consider that wall as a shear wall but to provide a new shear wall to the left of the penetration. Now a shortened diaphragm spans satisfactorily between shear walls. The force path has been restored. All that remains to complete the design is to stabilize the right-hand wall for x direction forces by tying it back to the newly y x y x

Non-structural wall —

Tie connecting wall to diaphragm

Non-structural wall —

Tie connecting wall to diaphragm

Plan

▲ 8.16 A new shear wall enables the right-hand wall to become non-structural.

Plan

▲ 8.16 A new shear wall enables the right-hand wall to become non-structural.

Slot in diaphragm

Shear wall

Slot in diaphragm

Shear wall

Moment frame down-sized diaphragm (Fig. 8.16). The two new ties may also have to act as horizontal cantilever beams or members of a vierendeel frame. This will transfer seismic forces from the now non-structural wall to the diaphragm if there is insufficient bracing in the wall to deal with its own inertia forces.

Figure 8.17 considers a more difficult scenario. Now a penetration is required near the middle of a diaphragm, also spanning between two walls. If the insertion of any horizontal structure like the diagonal bracing of Fig. 8.15(a) is impossible due to architectural requirements the only option is to physically separate the two portions of the building. Although perhaps perceived as one building with penetrated diaphragms, each section now becomes an independent structure. The end shear walls need to be replaced by moment frames to minimize torsion ( Fig. 8.17(b)). All non-structural connections bridging the gap, such as walls and roof, are detailed to accommodate the relative seismic movements between the two structures.

Another equally serious diaphragm discontinuity occurs where a potential floor diaphragm consists of more than one level. If a relatively small area is raised or lowered it can be treated, as far as seismic behaviour is concerned, as if it were a penetration. But consider the situation where a step is introduced across a diaphragm near the middle

Moment frame x

Step in diaphragm

Shear wall

▲ 8.17 A diaphragm slotted near the middle leads to the formation of two separated structures (a). To avoid serious torsional eccentricities, the shear walls are substituted by moment frames (b). The torsional configuration of each structure can be improved if the inner two frames are moved closer to the gap. (X direction structure not shown.)

Step in diaphragm

Shear wall

Plan

 Step Plan Section ▲ 8.19 A kinked beam showing internal compression and tension forces that can not be achieved. The beam is structurally unsound. ▲ 8.21 Two examples of non-parallel systems. Gravity-only structure not shown. of its span (Fig. 8.18). The diaphragm is now kinked, and just as a beam kinked in plan is unable to transfer force neither can a kinked diaphragm (Fig. 8.19). If you are skeptical, model a simple straight beam from cardboard. Note how it withstands reasonable force where spanning a short distance. Now introduce a kink. Observe how you have destroyed the integrity of the beam. The other problem caused by the step is to prevent x direction inertia forces from the right-hand end of the building being transferred into the two shear walls acting in that direction (Fig. 8.20(a)). Two ways to overcome these problems are; firstly, to fully separate the building into two structures as discussed previously; or secondly, to introduce a shear wall or frame along the line of the step (Fig. 8.20(b)) and provide x direction shear walls at each end of the building. Now there are two diaphragms. Both span independently between their original perimeter lines now braced by moment frames and a new frame along the line of the step. Frames have replaced the walls to allow for circulation between both halves of the floor plan. If the step is higher than several hundred millimetres, one diaphragm will apply y direction forces directly to the columns of the centre frame. This could lead to their premature failure and so the best approach would be to separate the diaphragms and their supporting members into two independent structures. Step prevents inertia forces from right-hand side being transferred to the x direction shear walls Inertia Force Step ▲ 8.21 Two examples of non-parallel systems. Gravity-only structure not shown. ▲ 8.20 The structural difficulty posed by the diaphragm step (a) is solved by increasing the number of shear walls effective in the x direction to four and connecting two to each diaphragm section (b). Moment frames replace y direction shear walls to avoid a mixed system once a moment frame is introduced along the step. Had a shear wall been introduced along the step, the original shear walls in the y direction could have remained.

0 0

## Greener Homes for You

Get All The Support And Guidance You Need To Be A Success At Living Green. This Book Is One Of The Most Valuable Resources In The World When It Comes To Great Tips on Buying, Designing and Building an Eco-friendly Home.

Get My Free Ebook

### Responses

• LONGO
What is diaphragm discontinuity?
3 years ago