Common Deficiencies And Upgrade Methods

Seismic upgrade of buildings typically involves strengthening of their horizontal and vertical lateral-load-resisting elements. These can be reinforced in-place, or new elements can be added to them. If the existing lateral-load-resisting structure is grossly deficient, it can be replaced. Whenever buildings are upgraded to resist a larger seismic load, their foundations must be checked for the new loading, and be reinforced if necessary.

Prime candidates for renovation and strengthening are

• Buildings with irregular configurations, such as those with abrupt changes in stiffness, large floor openings, very large floor heights, reentrant corners in plan, and soft stories.

• Buildings with walls of unreinforced masonry, which tend to crack and crumble under severe ground motions.

• Buildings with inadequate diaphragms lacking ties between walls and floors or roofs.

• Buildings with nonductile concrete frames, in which shear failures at beam-column joints and column failures are common.

• Concrete buildings with insufficient lengths of bar anchorage and splices.

• Concrete buildings with flat-slab framing, which can be severely affected by large story drifts.

• Buildings with open storefronts.

• Buildings with clear-story conditions.

• Buildings with elements that tend to fail during ground shaking: Examples are unreinforced masonry parapets and chimneys, and nonstructural building elements, which may fall, blocking exits and injuring people.

6.5.1. Diaphragms

A floor deck must act as a diaphragm—a deep horizontal beam capable of lateral-load transfer among the vertical rigid elements. To do so effectively it must have

1. The ability to resist horizontal shear forces, meaning that it must possess a certain degree of strength and rigidity in its plane. This also means that decking elements must be attached to each other and to the supporting floor structures with fasteners capable of transmitting these shear forces. In other words, the decking must be able to function as a web of the beam that does not break and does not deflect excessively under load.

2. Flanges at opposite ends of the diaphragm perpendicular to the applied forces. These flanges, called chords, must be attached to the diaphragm's web with connections capable of transmitting the seismic forces.

3. Drag struts, also called collector elements, to deliver the seismic load from the diaphragm to the vertical lateral-load-resisting elements.

The horizontal distribution of load among the walls or frames depends on the types of floor and roof diaphragms in the building. Flexible systems such as plywood or thingauge metal deck diaphragms without structural concrete topping are assumed to distribute lateral loads to the walls or frames in proportion to their tributary areas. In contrast, rigid diaphragms, such as those made of concrete, and concrete topping on composite metal deck distribute lateral loads to the walls or frames in proportion to their relative rigidities. Rigid diaphragms can distribute horizontal forces by developing torsional resistance. This is helpful in buildings with irregular wall layout. Flexible diaphragms are considered too supple to work in torsion. The majority of real-life floor structures fall between the two categories; engineering judgment is required to predict the behavior of these semirigid or semiflexible diaphragms. However, prevailing practice allows the assumption of rigid diaphragms for concrete slabs and concrete-topped composite metal decks, unless diaphragm spans are very large.

The type and function of existing diaphragms must be evaluated prior to making a decision about how to strengthen the vertical lateral-load-resisting elements of the building. For example, it is unwise to add shear walls or braced frames in an asymmetric manner if this introduces torsion into the existing diaphragm and leads to its possible distress. If shear walls or braced frames are placed in the interior of the building, collector elements must be present in the diaphragm to carry the inertial forces to them.

Methods of strengthening diaphragms depend on their composition and the nature of their weaknesses. Deficiencies of existing diaphragms typically fall into two categories: insufficient strength or stiffness, and the absence of chords and collectors or proper connections to them. Replacing a diaphragm, which involves taking out the building floor, is reserved for the most critical condition.

6.5.1.1. Steel Deck Diaphragms

Inadequate diaphragm shear and chord capacities and excessive diaphragm stresses at openings or plan irregularities are common deficiencies in steel deck diaphragms. Steel deck diaphragm shear capacity is limited by the shear capacity of the corrugated sheet steel and the fastener capacity connecting adjacent deck sheets (typically through crimping of the seams or seam welding). Capacity is also controlled by the spacing of deck-to-beam connections, which prevent out-of-plane buckling of the deck.

A modest increase in shear capacity can be achieved by additional welding at sheet seams. This, however, requires the removal of insulation fill on roof decks to provide access for the welding. Should added welding be insufficient or impractical, reducing the demand to below the shear capacity of the diaphragm can be accomplished by adding supplemental vertical lateral-force-resisting elements. New steel braced frames or shear walls can be added to cut down the diaphragm span. Drag struts connecting to the new braced frame or shear wall will be required to distribute the loads into the diaphragm.

Inadequate flexural capacity of steel deck diaphragms may occur due to incomplete or inadequate chord members. Perimeter steel beams or ledgers need to be continuous to act as chords. Beam-to-column connections at the perimeter may have inadequate stiffness or strength in the axial direction of the beams to adequately act as chords.

The following measures may be effective in rehabilitating bare metal diaphragms:

1. Adding shear connectors for transfer of load to chord or collector elements.

2. Strengthening existing chords or connectors by the addition of steel plates to existing frame components.

3. Adding puddle welds or other shear connectors.

4. Adding diagonal steel bracing to form a horizontal truss to supplement diaphragm strength.

5. Replacing nonstructural fill with structural concrete.

6. Adding connections between the deck and supporting members.

6.5.1.2. Metal Deck Diaphragms with Nonstructural Topping

Metal deck diaphragms with nonstructural concrete topping are typically evaluated as bare metal deck diaphragms, unless the strength and stiffness of the nonstructural topping is substantiated by approved test data. These diaphragms are commonly used on roofs of buildings where the gravity loads are small. The concrete fill, such as lightweight insulating concrete, usually does not have usable structural properties and is most often unreinforced. Consideration of any composite action must be done with caution after extensive investigation of field conditions. Material properties, force transfer mechanisms, and other factors must be verified in order to include composite action. Typically, decks are composed of corrugated light-gauge sheet steel, with rib depth varying from 9/16 to 3 in. in most cases.

The following measures may be effective in rehabilitating metal deck diaphragms with nonstructural concrete topping:

1. Adding shear connectors to transfer forces to chord or collector elements.

2. Strengthening existing chords or collectors by the addition of steel plates to existing frame components, or attaching plates directly to the slab by embedded bolts or epoxy.

3. Adding puddle welds at the perimeter of diaphragms.

4. Adding diagonal steel bracing to supplement diaphragm strength.

5. Replacing nonstructural fill with structural concrete.

6.5.1.3. Metal Deck Diaphragms with Structural Concrete Topping

This system consists of metal deck diaphragms with structural concrete topping, consisting of either a composite deck with indentations or a noncomposite form deck and a concrete topping slab with reinforcement acting together to resist diaphragm loads.

The concrete fill is either normal or lightweight structural concrete, with reinforcing composed of wire mesh or reinforcing steel. Decking units are attached to each other and to structural steel supports by welds or by mechanical fasteners. The steel frame elements to which the topped metal deck diaphragm boundaries are attached are considered to be the chord and collector elements. These types of diaphragms are frequently used on floors and roofs of buildings where typical floor gravity loads are on the order of 100 psf. The resulting concrete slab has structural properties that significantly add to diaphragm stiffness and strength. Concrete reinforcing ranges from light mesh reinforcement to a regular grid of #3 or #4 reinforcing bars. Metal decking is typically composed of corrugated sheet steel from 16 gauge down to 22 gauge. Rib depths vary from iy2 to 3 in. Attachment of the metal deck to the steel frame is usually accomplished using puddle welds at 1 to 2 ft on center. For composite behavior, shear studs are welded to the frame before the concrete is cast.

A relatively recent innovation is to attach the deck to supports with pneumatic shot fasteners. In some cases, self-drilling screws have also been used in these connections. Diaphragms made of concrete fill on steel deck typically fall into the semirigid and semiflexible categories. The flexibility characteristics and shear resistance of a steel deck diaphragm depend on the depth and gauge of the deck, the length of the span between supports, and the method of attachment to the supports.

When the existing steel-deck diaphragm lacks proper attachments to chords or to intermediate beams, attachments can be upgraded. Additional plug welding requires removal of the floor or roof finishes, and a better course of action may be to add overhead fillet welds from below. Attachments to the chords and collectors are usually made in the same manner.

The following measures are effective in rehabilitating metal deck diaphragms with structural concrete topping:

1. Adding shear connectors to transfer forces to chord or collector elements.

2. Strengthening existing chords or collectors by the addition of steel plates to existing frame components, or attaching plates directly to the slab using embedded bolts or epoxy.

3. Adding diagonal steel bracing to supplement diaphragm strength.

6.5.1.4. Cast-in-Place Concrete Diaphragms

Cast-in-place diaphragms are sturdy elements that rarely require major upgrade except at their connections to the chord. However, common deficiencies at diaphragm openings or plan irregularities include inadequate shear capacity, inadequate chord capacity, and excessive shear stresses.

Two alternatives may be effective in correcting the deficiencies: either improve strength and ductility, or reduce demand. Providing additional reinforcement and encasement may be an effective measure to strengthen or improve individual components. Increasing the diaphragm thickness may also be effective, but the added weight may overload the footings and increase the seismic loads. Lowering seismic demand by providing additional lateral-force-resisting elements, introducing additional damping, or base isolating the structure may also be effective rehabilitation measures.

Inadequate shear capacity of concrete diaphragms may be mitigated by reducing the shear demand on the diaphragm by providing additional vertical lateral-force-resisting elements or by increasing the diaphragm capacity by adding a concrete overlay. The addition of a concrete overlay is usually quite expensive, since this requires the removal of existing partitions and floor finishes and may require the strengthening of existing beams and columns to carry the added dead load. Adding supplemental vertical lateral-force-resisting elements will provide additional benefits by reducing demand on other elements that have deficiencies.

Figure 6.2. Superimposed diaphragm slab at an existing concrete wall.

Increasing the chord capacity of existing concrete diaphragms can be realized by adding new concrete or steel members or by improving the continuity of existing members. A common method for increasing the chord capacity of a concrete diaphragm with the addition of a new concrete member is shown in Figs. 6.2 and 6.3. This member can be placed above or below the diaphragm. Locating the chord below the diaphragm will typically have less impact on floor space. A common method of increasing the strength and stiffness of an existing simple connection of a steel beam to provide adequate chord capacity is shown in Fig. 6.4. Strengthening of existing concrete and steel deck diaphragms is shown in Figs. 6.5 and 6.6, respectively. Figure 6.7 shows addition of collectors at reentrant corners of a diaphragm.

Figure 6.3. Diaphragm chord for existing concrete slab.
Shore spandrel beam, as necessary. Weld connection
Modified connection

Existing

Existing

Figure 6.4. Modification of existing steel framing for diaphragm chord forces.

The following measures may be effective in rehabilitating chord and collector elements:

1. Strengthening the connection between diaphragms and chords and collectors.

2. Strengthening steel chords or collectors with steel plates attached directly to the slab with embedded bolts or epoxy, and strengthening slab chord or collectors with added reinforcing bars.

3. Adding chord members.

Figure 6.5. Strengthening of openings in a superimposed diaphragm.
Figure 6.6. Strengthening of existing steel deck diaphragms.
Figure 6.7. New chords at reentrant corners.

6.5.1.5. Precast Concrete Diaphragms

Common deficiencies of precast concrete diaphragms include inadequate shear capacity, inadequate chord capacity, and excessive shear stresses at diaphragm openings or plan irregularities. Existing precast concrete slabs constructed using precast tees or cored planks commonly have inadequate shear capacity. Frequently, limited shear connectors are provided between adjacent units, and a minimal topping slab with steel mesh reinforcement is placed over the planks to provide an even surface to compensate for irregularities in the precast elements. The composite diaphragm may have limited shear capacity.

Strengthening the existing diaphragm is generally not cost-effective. Adding a reinforced topping slab is generally not feasible because of the added weight. Adding mechanical connectors between units is generally not practical, because the added connectors are unlikely to have sufficient stiffness, compared to the topping slab, to resist an appreciable load. The connectors would therefore need to be designed for the entire shear load assuming the topping slab fails. The number of fasteners, combined with edge distance concerns, typically makes this impractical. The most cost-effective approach is generally to reduce the diaphragm shear forces through the addition of supplemental shear walls or braced frames.

Inadequate chord capacity in a precast concrete deck can be mitigated by adding new concrete or steel members, as discussed earlier for a cast-in-place concrete diaphragm. A new chord member can be added above or below the precast concrete deck. Excessive stresses at diaphragm openings or plan irregularities in precast concrete diaphragms can also be mitigated by introducing drag struts, as described earlier for cast-in-place concrete diaphragms.

6.5.1.6. Horizontal Steel Bracing

Horizontal steel bracing, commonly referred to as steel-truss diaphragms, may be designed to act as dia phragms independently or in conjunction with bare metal deck roofs. Where structural concrete fill is provided over the metal decking, relative rigidities between the steel-truss and concrete systems must be considered in the analysis.

Figure 6.8. Strengthening of an existing steel frame building with horizontal bracing.

Floor framing plan

Figure 6.8. Strengthening of an existing steel frame building with horizontal bracing.

Where horizontal steel-truss diaphragms are added as part of a rehabilitation plan, interaction of the new and existing elements in the strengthened diaphragm systems should be evaluated for stiffness compatibility. Also, the load transfer mechanisms between new and existing diaphragm elements must be considered in determining the flexibility of the strengthened diaphragm. Shown in Figs. 6.8 and 6.9 are some common methods of upgrading steel deck diaphragms.

Greener Homes for You

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


Post a comment