Restraint ties on steel portal joists.

[David R]

Hi Folks,

 

Has anyone ever had to use restraint ties to tie in first floor joists to the steel frame?  or any part of a steel portal frame.

 

My design plans show that every joist should have a 600 mm 2.5 mm x 30 mm strap each end of the joist , shot fired into the steel rsj's / beams of the frame?  

 

I know that this is the engineers detail and should be carried out. However, the engineering spec so far, is very cautious, to say the least.

 

Has anyone used steel portal frame construction and had similar level of detail?  

 

Thanks in advance.

 

                                                                                                                                                                                                                                                                                                                 

[Mr Punter]

This is one for your engineer to comment on.  Do you have a simple / cheaper proposal?  Does the joist hanger not provide restraint?

 

You can hire the Hilti / Spit gun to fasten to the steel it is simple to operate.  How many joists are we talking?

[David R]

Mr Punter Thanks for the comment.  I would have thought the same. Why the extra restraints?  I have the spit gun. There are about 100 joists if you include the balcony which they need restraint straps on as well. It is a bit tricky, firing a strap to a web of an rsj then fitting 2 packer battens then hoping the joist is in the right place. A bit of forethought is requires. The dwangs on the the flat roof will need notched for ventilation as the strapping is required over 3 joists as the joists don't run the right way, then does the flooring sit well on the straps? Plus the furring strips for the flat roof will need rebated to sit over the straps ( perhaps ). all a bit of a head ache.

 

I wish I just could understand the science behind it.  Wind loading? BS standards?  The spec for the joist hangers is SAE, I don't understand that either when jiffy hangers are the norm, I could even shot fire them to the steel.

 

Why do engineers go so far over the top and don't give a hoot about the practicalities?

 

 

[SimonD]

17 minutes ago, David R said:

Why do engineers go so far over the top and don't give a hoot about the practicalities?

 

Unfortunately, the image is too small for me to read the detail. But basically it's because their *rse is on the line if something goes wrong ?. It also depends on the predilection of the engineer. Some might have a x 1.6 margin, with some it's only x1.2.

 

As per @Mr Punter I've found it most helpful to call my SE to ask the questions and to understand their point of view and reasoning. This can then open up a discussion about potentially simpler alternatives.

[David R]

SimonD, Mrpunter Thanks for the replies.  I had a talk to my architect and he explained it was due to the SE having to issuer the SER certificate and their liability insurance having to cover potential structural issues.

 

He also said that this particular detail is not unusual.  He did comment on the change over the last few years from timber beams to over sized steel beams.  

 

 

 

 

 

 

[Gus Potter]

28 minutes ago, David R said:

SimonD, Mrpunter Thanks for the replies.  I had a talk to my architect and he explained it was due to the SE having to issuer the SER certificate and their liability insurance having to cover potential structural issues.

 

He also said that this particular detail is not unusual.  He did comment on the change over the last few years from timber beams to over sized steel beams.  

 

 

 

 

 

 

Hello David.

 

Hope this is of some help and gives you a bit of an insight as to what you have. I can't expand the drawing enough to show the detail so the following is an overview. I have written a bit but also attached a very quick sketch.

 

A starting point would be to first look at the steel beam that is I assume forming the top of the portal. You have the floor joists framing in on one side, a bit of masonry, what looks like the timber frame and maybe a bit of sloping roof framing in, but this framing is intended for restraint rather than load bearing.

 

Each of these elements will impose different loads on the beam at different times. For example you could have a party (with lots of refreshment) and no snow or wind.. thus the floor joists will load the beam more at that time. You may also get a uniform snow load on a night where there is no wind.  You may also get a windy night and here again this will cause the beam to be loaded differently. Call these different load cases.

 

You can see that all the different bits of the structure don't rest over the middle of the beam.. they are offset from the centre of gravity. When you offset a load on a beam it can cause the beam to twist.. torsion.

 

Now when you design a beam like this you need to consider all the different load cases.. and you can easily see how you can get a few combinations.

 

Designing these portal type frames can be a bit of work when you get torsional effects. A lot of SE's won't be that keen to do it (include for torsion) unless you pay a bit extra or are a client that appreciates this finer point.

 

You have options but two of these are to either design the beam for torsion, and the associated portal connections or to use the joists that are framing in to resist the torsion. In other words for the latter you transfer the twisting effect back into the floor joists. If you do the latter you can often reduce the sizes of the steels and the connection sizes. But there is no free lunch! The forces need go somewhere.

 

You also need to make sure that all the bits framing into the beam are tied together. This tying is very important as when say the wind blows it wants to pull the building apart at times. That may be partly why you have the straps.

 

I can see the straps on the drawing, but they seem to be side fixed to the floor joists, practically this can lead to splitting of the timber, especially if the joiner etc does not get them right first time and is not familiar with the minimum timber edge distance for a nail. We know if you put a nail too close to the edge of a timber then the timber can split, splinter etc.

 

Also, the straps seem to have an extra crank in them as they need to bend again back down to the top flange of the steel. Best to confirm what manufacturer the SE ( I can't see enough of the drawing) is proposing for the straps and whether this type of strap cranking is ok. Also as the timber shrinks in the vertical dorection will the strap not start to force the joist away from the beam? If so by how much?

 

One option I would look at is to get the fabricator to weld fin plates on to web of the left hand side of the beam. This would allow you to bolt through the fin plate to tie the floor joists to the beam. You could also then explore whether you can reduce the size of the beam by extending the fin plates a bit. You would need to check the floor joists again though as they would be loaded differently. You are now using them to take the torsion out of the beam.

 

On the right hand side you could get the the fabricator to provide drill holes in the web of the beam at 1200mm centres. Budget roughly that a drill hole is about a £1.00 a go for standard domestic type beams by the fabricator.

 

You would then insert threaded rod through the web and extending mid depth within the roof joists. You dwang adjacent to the tie rods. Now you have simplified to some extent the buildability issues, removed a safety risk in terms of playing about with what can be an unfriendly tool (shot firing) in the wrong hands. Generally, what you do is to get the fabricator to do all the hard measuring by way of controlled drawings and remove as much complex work as you can for the folk on site.

 

Yes it may on first glance appear that we are adding the extra cost of the fin plates but the labour savings could well pay dividends. Make it simple stupid! Also,  you can get a better job as you are controlling these difficult details in a comfy office, rather than the poor builder who often has to do this with the rain running down the back of their necks and up their shirt sleaves.

 

One good thing about the tie rod / fin plate appraoch is that it lets you form up the roof ventilation more easily as you have moved all the steel connection design out of your ventilation zone.

 

Have attached a very quick sketch to assist.

 

I hope I have given you enough info to enable you to ask your Architect and SE if this may be some kind of option worth developing up. Remember that you can ask ten different SE's the same question and get ten different answers as there are many ways of skinning a cat. One key is to explain what suits your method of approach, how you want to finance, programme the works. You can hopefully now say that you have read that there may be other ways of doing this.. often once an SE sees that you have put in a bit of extra work so will they, sometimes for free too!

 

 

 

 

 

 

Beam sketch 24-03-21.pdf

 

 

 

 

[David R]

Gus Potter.

 

Gus, 

 

Many thanks for your detailed explanation and drawings. I now have a far deeper understanding of the straps intentions and will ask my SE about the impact of timber shrinkage. They have allowed 12 mm for joist shrinkage in height above the steel but nothing for shrinkage horizontally.

 

Apologies for not putting sufficient detail in my original post.

 

Unfortunately, the steel is in place on site, with most of the kit in place.  I know what you are saying about joiners being aware of nailing too close to joist edges, the speed of construction on my kit did not allow for a lot of thought time.

 

The joiner missed the straps out but luckily we can retrofit most of them, apart from under one panel on the gable.  We had the opportunity to rebuild the panel, and fit the straps, as the joiner built it not on the steel beam but on top of the floor joists. He said that the steel was in the wrong place not his panel and the joists hangers would support the panel and my roof.  The kindest thing that was said was by my architect  "what an incredible lack of common sense".  Not my words when the brickie pointed it out. His company are billing themselves as timber frame specialist, go figure.