Thermal Movements in Large Format Structures

[strucuturesareus]

How would you deal with thermal movements in large building structures? Would you reduce the 'design' movement for the moderated temperature of an enclosed building? Would you attempt to calculate the net temperature of the structure considering the daily thermal lag (ie, peak daily temperature is greater than peak structural temperature)?
Would you allow movements of the structure during construction and then lock in the connections after the building is heated?

We are working through the analysis of a large format structure for thermal movements. It is about 600 ft square to be built as a single unit - steel upper portions and concrete lower. There are internal parking decks that are exposed to the exterior. Temperature variations amount to about 95 degrees in this location.

To handle the total potential thermal movement, this building would have troubles. However, thermal lag between overnight and daytime temperatures at both cold and warm days could reduce thermal variations significantly.

Once the building temperature is stabilize during its service, the real movements due to external temperature swings will be controlled. Is it common in design to account for reduced thermal temperature swings as a result of controlled internal temperature of the building?

If so, then we need to determine the net average temperature within the structure. A thermal variation within a structure between exposed (or insulated exterior) structure and the internal surface temperature results in a net average structural temperature. This variation could substantially reduce the actual movement that the structure must undergo. Has anyone a reference to methods of analyzing this temperature variation?

Thanks for your help!

 

[BryanStein]

There are lots of good guidelines to use for predicting volume change movements in large structures. Once a structure reaches a certain size, expansion joints are required becuase the stresses to restrain the thermal (creep, shrinkage) movements become excessive.

This is a major consideration in concrete structures (creep, and shrinkage). Expansion joints are typically installed less than 300 feet apart to reduce thermal stresses. This number can go up or down depending upon the interior and exterior environments. You really have to run some numbers to determine your optimum expansion joint spacing. These numbers will be approximate but should get you into the ball park as to how the structure will behave.

I've seen some bad structural damage result when volume changes are not properly designed into the system.

 

[JedClampett]

There is a publication called "Expansion Joints in Buildings" by the Building Research Advisory Board/Federal Construction Council Technical Report No. 65. It addresses in a somewhat disorganized manner, the need and spacing of expansion joints.
It's not terribly long. You should be able to find a copy at a larger universlty library and make a copy.

 

[ishvaaag]

Certainly I agree with what Bryan says ... and still see the challenge of what struc tries to make. It is very common for even meter lintels or some meters steel rail inserts spall brick. From there and above bad control of expansion is no doubt assured problem.

This said, in the hypothesis of that some degree of control of the temperature variation is assumed first and then ensured to be in place, one shouldn't have any problem in reducing the range of temperatures mandated to be contemplated for design of typical structures where no other thant the typical efforts (say expansion joints in buildings) are being made to put in place such control of the variation of the temperature.

Then a construction scheme can be devised that manages to reduce the effective range of temperatures once your whole plan is made solidary. You may for example cut your building to pieces (leaving joints to be later concreted) and then build an envelope (sliding at the joints) that ensures either by its mere presence or HVAC-controlled inner environment the range of temperatures is kept under some target value.

Of course if HVAC dependent, a general failure of the HVAC service would make the building run over the target range with HVAC running, and if this is not properly accounted the effects of the actual range of temperatures will be seen with the corresponding consequences, that may be of any level for a structure of your size if not properly accounted for.

An alternative design is to build the whole structure of steel box shapes infilled with water, both columns and beams, and from the start ensuring a uniform HVAC controlled temperature for the whole building, that can be then also from the start be made solidary. Of course floor and façade attachments need then have proper expansion devices built in to not distroy through the action of the supported parts the benefits of constant temperature attained through HVAC action. Inner pressure and proper control may cause to part the structure in independent zones... and the stability of the thing stays dependent of continuous control of the temperature. Ultra precise continuous redundant control of the expansions and control of temperatures may be needed to be installed with this structure to deliver water at the proper temperature where demanded.