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Expansion Joints in Buildings

Expansion Joints in Buildings

The number and location of roof expansion joints or building expansion joints is a design issue not fully treated in technical literature. The National Roofing Contractors Association (NRCA, 2015) gives the following recommendations for the location of roof expansion joints.

1.   Where expansion or contraction joints are provided in the structural system

2.   Where steel framing, structural steel, or deck material change direction

3.   Where separate wings of L, U, T or similar configurations exist

4.   Where the type of deck material changes; for example, where a precast concrete deck and a steel deck abut

5.   Where additions are connected to existing buildings

6.   At junctions where interior heating conditions change, such as a heated office abutting unheated warehouse, canopies, etc. A two-piece reglet and counterflashing assemblies can accommodate the movement at main building-to-canopy intersections

7.   Wherever differential movement between vertical walls and the roof deck may occur.

NRCA standard details show that the roof structure under roof expansion joints is intended to be discontinuous.

In 1974 the Building Research Advisory Board of the National Academy of Sciences (NAS) published Federal Construction Council Technical Report No. 65 “Expansion Joints in Buildings” (NAS, 1974). It cites recommendations of the Brick Institute of America and the National Concrete Masonry Association, that buildings supported by continuous exterior unreinforced masonry walls, be expansion jointed at intervals not exceeding 200 feet.

Tech. Report 65 also presents the figure shown below as a guide for spacing expansion joints in beam-and-column frame buildings based on design temperature change.  We have included for download (on the right edge of this page nearer the top) a table of design temperatures for major municipalities in the US for use with the figure below.  This table of design temperatures was derived from temperature data presented in the appendix of Tech. Report 65.  Utilizing the design temperature in this table for the major municipality that most nearly represents your jobsite should be sufficient.  Also, the (NAS) figure gives five modifying factors which should be applied to the Allowable Building Length, as appropriate.

In equation form, the NAS requirements given above can be shown in the figure as follows:

Lmax = Lallow + (R1 – R2 – R3 – R4)Lallow

Where:
Lmax= maximum length of a building with no expansion joints or between expansion joints
R1 = 0.15, if the building is heated and air-conditioned
R2 = 0.33, if the building is unheated
R3 = 0.25, if columns are fixed base
R4 = 0.25, if the building has substantially greater stiffness at one end
Lallow = allowable length from the figure below

AISC provides additional information about the design and use of expansion joints it the article Expansion Joints: Where, When and How By James M. Fisher, S.E. which can be viewed Here.

Maximum allowable building length without use of expansion joints for various design temperature changes. These curves are directly applicable to buildings of beam and column construction, hinged at the base and with heated interiors. When other conditions prevail, the following rules are applicable.

1.     If the building will be heated only and will have hinged column bases, use the allowable length as specified

2.     If the building will be air conditioned as well as heated, increase the allowable length by 15 percent (provided the environmental control system will run continuously)

3.     If the building will be unheated, decrease the allowable length by 33 percent

4.     If the building will have fixed column bases, decrease the allowable length by 15 percent

5.     If the building will have substantially greater stiffness against lateral displacement at one end of the plan dimension, decrease the allowable length by 25 percent

When more than one of these design conditions prevail in a building, the percentile factor to be applied should be the algebraic sum of the adjustment factors of all the various applicable conditions.

 

 

Reports

Federal Construction Council Technical Report No. 65 “Expansion Joints in Buildings”

City Temperature Data