Expansion Joints and Horizontal Drift.

[IngDod]

Greetings to everyone,

I am having a bit of a dilemma, I have a structure that is L shaped.. the long dimension is 200 feet and the short is 115 feet. Its steel deck and steel moment frames, with the assumption of pinned bases. I am trying to ascertain the necessity of an expansion joint due to thermal effects, temperature here is around 104F on very hot days and 68F on very cool days. I have read the Federal Construction Council Federal Report No. 65, and it seems to me that this particular structure could be built without expansion joints. However it is common practice here to add expansion joints when the structure is long, the common method is to use a double line of columns. My doubt arises from the fact that this is a seismic area, after modeling the structure as two separate buildings I am getting drifts on the top floor of around 4cm; it would seem to me that then the expansion joint would have to be 8 or 10cm wide in order to accommodate movement of the structures during an earthquake. Is this correct? an 8cm expansion joint seems very wide.

Thanks.

 

[JAE]

8 cm (3 inches?) sounds like an appropriate amount. Sometimes I've seen/heard of joints much wider.
The later US code (IBC / ASCE 7) now allows use of a square root of the sum of squares of the two buildings when determining joint width I believe.

 

[IngDod]

The code here allows the same. And what about the foundation system for this double columns? I believe sharing the same footing is allowed as the foundations wont undergo thermal expansion (and the report says its possible) however I would like to hear from a practical point of view. The baseplates for this double columns are 38x38 cm, leading to pedestals of 40x40 cm... I have seen details of two pedestals side by side on on spread footing, and the footing designed as if it received a single pedestal of that perimeter, would it not be possible to do one large pedestal to fit both base plates? The other solution I can think of are eccentric footings.. but it seems impractical to do this for a 10cm separation between pedestals.

 

[JAE]

In all building expansion joints I've ever done I've not taken the joint through the foundation unless I was using two different foundation types (i.e. shallow vs. deep).

I had that condition only once with a site that had a fault through it and we switched foundation types across it.

 

[IngDod]

Thanks, I suspected as much.. If the movement occurs due to thermal expansion I dont see how they could affect the footings (no freezing or snow here). How would you design the footing? I am leaning towards using a single footing, but I am conflicted as to the pedestal... If I were to use two pedestals it could lead to pounding of the pedestals as they would be much closer than the 10cm of the columns above the ground.

 

[wannabeSE]

You need to decide were the seismic base is located. If the pedestals are above the seismic base, then you need to be concerned with their movement during the design earthquake. If the seismic base is above the pedestals, then the movement is not a concern. It is common for the building separation to increase at each story level.

I don not know what code or method is being used for the analysis. With ASCE 7, the elastic drift needs to be multiplied by a deflection amplification factor when sizing building separations. A building is expected to undergo inelastic deformation during the design earthquake. The deflection amplification factor is a simple way to account for this.

The architect needs to know the separation dimensions early in the design process. Also, the drift needs to be coordinated with the cladding. For instance, many curtain walls can take sizeable in plane deflections (drift). But the maximum drifts allowed by the code may require special connection detailing between the cladding and the structure. Often times, it is better to stiffen the building to reduce the drift, than deal with all of the consequences with cladding and other architectural elements.