Thermal Expansion

[stuctural101]

With regards to the change in temperature parameter, I believe that the upper value is the maximum temperature that the steel will experience in operation/field. What does the lower value represent? I would assume that this is the temperature of the steel during cutting of the steel to the length specified on the design drawings. Is there some rule of thumb value for this lower temperature?

 

[ishvaaag]

Spanish building code CTE specifies a typified (or to be identified) construction temperature, and then upper and lower temperatures (to be evaluated or identified) to be considered in the analysis; hence the excursions the structure is typically expected to see in both elongation and shortening.

Things remain to be clarified, like the transition from the specified temperatures along the faces of the building towards the soil, that for rigid setups near the base may turn very high stresses if kept constant.

Bridge code is more specific as usual on gradient temperature issues, but the idea is always the same, to identify the ranges to explore in analysis and proper placement of the temperature excursions in the models.

 

[csd72]

structural101,

I would say the minimum temp would be the air temp in the middle of winter of the plant was shut down. Unless you know for certain which time of year the steelwork is to be installed.

 

[Teguci]

It has been my experience that construction is "locked in" during high temperature seasons when concrete can be cast without heaters. Therefore, I would suggest considering the high temperature as the start temperature and the low temperature as the worst case in-service condition. However, it would not hurt to consider the construction as completed during cold weather and expanding during summer if your structure is steel only.

Also consider, Concrete shrinks over time. If you have a supported concrete deck, this will probably be the controlling factor.

The steel is "locked in" when the bolts are tightened and field welds completed.

 

[umrce]

I try not to think about think thermal design in terms of high and low temperatures and what they are being evaluated for, but more in terms of the change it temperature. If you think about in this perspective you are essentially evaluating the structural member for the change in temperature from the point of fabrication. Typically, it is the responsibility of the contractor to make sure that at the time of erection there are not fitup issue due to expansion or contraction of the steel.

For example, one of AASHTO's methods provides min/max values for steel of 0 and 120 deg. F but if you evaluate delta T for 120 you are essentially determining the horizontal loads applied due to expansion or contraction of the steel after the steel is "locked in"