Expansion Joints 

© Wojciech Remisz, M.Sc., P.Eng.

In Ontario, problems with the wrong expansion joint selection will show up on bridge decks longer than approx. 60 m between the joints. On the shorter decks the joints selected 'by default' may have some reserve in movements. But this may vary a lot with bridge location, and corresponding ambient temperature changes: winter low-summer high. Joint seal failure means leaking joints. And leaking joints let the water drip on the beam-ends, deck, ends, ballast walls, accumulate dirt around the bearings and ultimately damage concrete. In some more drastic cases, this can lead to jamming of joints, bridge jumping out of bearings, and even structural catastrophes. Undersized expansion joints will hence lead to a very costly maintenance, which is public nuisance and affects safety.

Therefore, we emphasize that the selection of the joints and the bearings directly below them be an extremely careful process, performed with due diligence, respect for the forces of nature, bridge articulation, and with an allowance for an additional 10-50 mm in movement.

The arguments are that:

1. Loads are represented by vectors.
2. Vectors have point of application, magnitude, and a specific direction.
3. Loads are representations of potential energy, the results of gravity.
4. For the structural analysis and design purposes, loads are static.
5. For the structural analysis and design purposes, loads create mechanical strains and mechanical stresses.
6. In static, we can superimpose loads, like vectors, because they are of the same form of potential energy.
7. Thermal elongation is the result of heat energy. It does not have a specific point of application. It does not have a specific uni-direction, unless restrained in all other directions. Hence, it is not a vector.
8. Unconstrained thermal strains do not result in mechanical strains and mechanical stresses.
9. In static, we can superimpose (make load combinations) only the same forms of loads, coming from the same form of energy; i.e. potential + potential, or heat + heat.
10. In static, we cannot directly superimpose / combine various forms of energy; i.e. heat + potential. We must first convert heat energy into potential energy, by restraining free movement.

This elaborate and logical introduction of engineering principles is important to understand the difference between LOADS and free MOVEMENTS as dealt with in the Ontario Highway Bridge Design Code (OHBDC) or the new Canadian Highway Bridge Design Code (CHBDC), which effectively replaced OHBDC but retained almost the same wording in many sections.

Misunderstanding of the above Code requirements by designers can lead to numerous construction problems, very dangerous situations, jamming of joints or bearings, and endangering public welfare and safety as a result. We design steel armour components of the joints for factored LOADS, but we select the type of joint for free and unfactored actual MOVEMENTS.

Please note that the definition of Serviceability Limit States (SLS) criteria is different in different sections of the Code, as it is clearly spelled out in OHBDC paragraph 2-6.2.1, and re-affirmed in 4-4.1 for expansion joints, 5-6.3, 5-7.2 for barriers, 6-4.2 for foundations, 8-5.2 for concrete, 9-4.2 for wood, 10-5.2 for steel. Similarly, it is in also spelled out in various sections of the Canadian Highway Bridge Design Code.

For example SLS condition for concrete is that it will not crack, and for expansion joints that they will not leak and will not exceed particular gap width.

If one is to use the philosophy of calculating the amount of free thermal expansion based on any SLS factors, one would get completely different gaps using the next step, any of the ULS combinations. This would be a serious engineering and logical error in judgment. The gap would be the same, under the same temperature, material, deck length, deck depth, articulation of supports, and thermal coefficient regardless of the Code being used, future changes in the design Codes, or changes in the intended use of the structure. It is strictly physical, volumetric change due to forces of nature, over which we have almost no control but are subject to.

Undersized expansion joints can lead to a disaster, especially if it happens to be a longer span structure on a main highway, obviously being used by more vehicles each day, and with a much higher probability of an accident. Misinterpretation of the Code requirements of water tightness and maximum gap width creates a situation that may cause nuisance, physical discomfort, and safety hazard. Additionally, significant damage to substructures, bearings and girder ends can result from leaking expansion joints.

Design Codes around the world are making a very clear distinction between the mechanical DESIGN of the expansion joints (armouring, can be to any SLS or ULS combination) and the proper SELECTION of the joints to span a particular air gap (no loads here, created by the environmental effects, not designed by us, only measured according to the laws of physics).

Free elongation of a bridge deck under ambient temperature change is the result of heat energy, and not of any external load. Unrestrained elongation does not create or change any loads or mechanical stresses, and it cannot be used in linear superposition of loads.

However, if the elongation is not free, but restrained by fixed bearings or jamming against the abutments, tremendous reaction forces will occur and then these forces can be used in the load superposition / combinations.

So, a JOINT SELECTION flow chart looks like this:

sun

ê

heat energy

ê

restrained movement ç volume changes è free joint movement

It is recommended that no depth reduction factors be used for steel or steel+concrete superstructures, which react fast to temperature changes. Depth reduction factor may be used only for deep and solid concrete sections, due to their larger thermal inertia.

The structure movements are very seldom exactly symmetrical, due to the bridge articulation, stiffness of supports, plan and elevation geometry, or direction of traffic which affects breaking forces. Attention should be given to a proper estimate of the 'contributing length' which should be taken in between the free end and the 'zero movement point' of the deck system, which is equivalent to the 'center of mass' of the deck system. For higher accuracy, stiffness of supporting slender or stiff piers and framing elements, shear stiffness of bearings, end rotation of deep beams, an allowance for earthqake movements may be considered in a more refined analysis. However, the final selection must be on the safe side, with larger rated movement than theoretical calculation would suggest to allow for construction tolerances, tilt or settlement of supports, end rotation of deep beams. The longitudinal movement of deck joint elements must be consistent with that provided by bearings at that location. Together, bearings and joints must be a part of a complete and effective system of bridge articulation. All joints shall be selected to accommodate total movement with reference to the 'zero movement point'.

èsave this illustration in Acrobat pdf format: sls_uls.pdf

The environmental effects like global temperature change affecting free shortening or free elongation of the bridge decks or free concrete shrinkage should be un-factored, the way they happen, or if one wants to use a word 'factored' than the factor should be '1.00'. For ease of selection, one can add additional 15%-20% to this previously calculated free movement and then select the joint type from manufacturer's list of materials and rated movements. This 15%-20% extra will allow for construction tolerances, tilt or settlement of supports, accuracy of calculations and variability of material properties and bridge articulation. This may be treated as 1.20 performance or safety factor to be applied to the calculated free movement, but obviously has nothing to do with loads, SLS or ULS. Any of the SLS or ULS load factors will apply only to the physical piece of metal armour, edge of concrete dam when these elements are subject to wheel loads. The joint assembly should be selected in such a way, that the seals will be allways in a little bit of pre-compression and never in tension. Otherwise, they will not perform the function for which they were selected in the first place. (watertightness is the serviceability limit state for joints). And tension in the rubber seal is a 'no-no', it will simply fell off the assembly creating a safety hazard.

Expansion Joints Policies by Various Bridge Codes and Departments

selected by Mohan Sharma, Ph.D., P.Eng.

Bridge Design Manual- Washington State Department of Transportation

(M 23-50 – August 1998)

Extracted portion of design criteria for Expansion Joints (Section 8.4.1) from Bridge Design Manual of Washington State Department of Transportation.

Seals must be precompressed at all times, generally 40% to 85% of the uncompressed width.

1. Types of Movement Joints:

2. Temperature Ranges:

When more exact temperature data is not available, use the following design temperature range:

3. Total Movement (Mt) along the Bridge Centreline:

Mt = Temp + Shrink + Other Movement

Mt(large) = Mt + additional 15 % factor of safety

Where

Temp = 12 L a D T

Shrink = 12 L b m

Other Movement includes all other factors which affect movement.

In the case of Large Movement Joints, consider additional 15 % factor of safety, which allows for unpredictable non-seismic movements.

4. Design and Selection of Expansion Joint Structures:

Generally, the compression range for bridge compression seals is 40 to 85 percent of the uncompressed width. All movement of the joint must be within this range. Detail of design and selection of expansion joint structures for different types of Movement Joints is given in the Bridge Design Manual.

Temperature Movement Policy- Nevada State Department of Transportation

(January 2000)

Extracted portion of Temperature Movement Policy (Section 2.2.7) and Expansion Device Policy (Section 3.1.6) of Nevada State Department of Transportation

1. Temperature Movement Calculation:

(Temperature Movement Policy: Section 2.2.7)

Temperature Movement (MT) = D T a L

Where

2. Movement Rating (MR) for Bridge Deck Expansion Devices:

(Expansion Device Policy: Section 3.1.6)

MR = TM + 25 mm for steel structures

MR = TM + 40 mm for concrete structures

Bridge Design Manual- Colorado State Department of Transportation

(June 1998)

Extracted portion of Bridge Deck Expansion Joints (Subsection 15.2 and 15.3) from Bridge Design Manual of Colorado State Department of Transportation.

1. Horizontal Movement (HM) Calculation:

HM = L (TR) (ct) (sin skew) (Tn)

Where

2. Design Aid for Expansion Devices:

Design aid for different types of expansion joints devices is given in Subsections 15.2 and 15.3 of Bridge Design Manual of Colorado Department of Transportation.

Expansion Joints : Composan Construction

According to Composan Construction, the following formulas are used for the calculation of total movement of the deck/girder of a bridge.

1. Concrete retraction:

Regardless of the factors that should be taken into account, such as degree of ambient humidity, piece thickness, concrete composition, typical strength, number of reinforcements and diameters, etc., an approximate value of 0.25 mm per metre is taken, with the correction corresponding to the elapsed time since the piece concreting and joint installation (100% in 2.5 years).

D 1_R⁺ = 0.25 x L x K_TR K_TR = (1-T/30) , T=elapsed months

2. Concrete Creep:

With the same considerations as above, an approximate value of 0.20 mm per metre is taken, with the corresponding correction (10% in 10 years).

D 1_F^- = 0.20 x L x K_TF K_TF = (1-T/120) , T=elapsed months

3. Thermal Expansion/Contraction:

Maximum and minimum temperatures of the structure location area have to be considered, as well as the assembling temperature and its structure and thickness. The approximate average value is 0.01 mm per metre and degree of centesimal (Celsius) temperature.

D 1_T⁺ = (Tmax – Tinst) x L x 0.01 x K_H

D 1_T^- = (Tmin – Tinst) x L x 0.01 x K_H

Where K_H ranges from 0.95 to 1.15 for different types of girders.

4. Breaking/Starting:

Let us consider a maximum horizontal force of 18,000 kg, which deforms all the neoprene supports upon which the structure rests, and instantaneous deflection factor G = 14 Kg/cm²

D 1_N^+/- = (F x t)/(Ginst x a x b x n)

F = 18,000 Kg.

t = average net thickness in mm.

Ginst = 0.14 Kg/mm²

a x b = average size in mm.

n = total number of supports

5. Total Movements:

Total maximum opening movement of joint:

S D 1^- = D 1_R^- + D 1_F^- + D 1_T^- + D 1_N^-

Total maximum closing movement of joint:

S D 1⁺ = D 1_T⁺ + D 1_N⁺

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We welcome any comments on the above, as well as reports of premature joints failures, leaks, jamming, distorted bearings, or bridges jumping out of bearings.