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I notice that quite a number of deep excavation projects adopt double I-section walers to form a box beam since standard rectangular hollow sections don't come with such large sizes. The I-sections are usually factory butt-welded together at the flanges.

Braced Excavation: Struts and Walers

How are such members spliced together? Unlike a single I-section, the inner flanges of the double I-section would not be accessible. There would also be problem connecting the webs due to the same accessibility issue.

From the pictures I found on the internet, it isn't clear how this can be done to achieve effective moment transfer.

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Bolts

Flanges can be added to the members as Wasabi mentioned in his answer. Another option would be to cut a small hole in the web of the members to allow for an iron worker to insert his hand to insert a bolt or hold a wrench. The reduction in strength from the access holes is minimal. This type of access is also used in the bolted connections of truss members. With this method, typical flat splice plates can be used.

Welds

Single sided complete joint penetration (CJP) welds are not a problem. They are done frequently. The joint will require a backer rod that will be left in place.

In the case of these walers, fatigue won't be an issue, so partial joint penetration (PJP) welds would even work. As long as the splices were located in good locations, the full strength of the member wouldn't be required at the splice location.

Weld examples from AWS:

enter image description here enter image description here

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  • $\begingroup$ AWS also allows single sided CJP welds with an open root, if they are specifically qualified as such (and the welder who welds them is too.) My understanding is that they are possible, although don't have very good fatigue characteristics. Are they explicitly forbidden in AISC or other steel design codes for structural work, or just very expensive and unusual? $\endgroup$ – Ethan48 Aug 17 '15 at 1:33
  • $\begingroup$ @Ethan48 I think that the important point is that they have to be specifically qualified. Technically any configuration of weld can be used as long as it it properly tested. The ones shown are "prequalified". Or did I miss a prequalified weld without a backer? $\endgroup$ – hazzey Aug 17 '15 at 2:28
  • $\begingroup$ OK, but if they did qualify it, it would be acceptable. You're correct that there are not prequalified open root welds in D1.1 - with the exception of PJP square groove welds on very thin material (B-P1a & c) $\endgroup$ – Ethan48 Aug 17 '15 at 2:33
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Double-I beams cannot be spliced like single-I beams for the reasons you mentioned. Indeed, they must be spliced like box beams, which suffer from the same problems (lack of access to internal face). To deal with this, it is common to weld plates at the ends of the beams to be spliced and then bolting the plates together, as seen below (source):

enter image description here

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