Can these type of joists be used structurally instead of metal beams
for internal work, where only timber is involved (notwithstanding
their size) ?
i.e. Could I use these instead of a steel in a loft conversion, in
order to support floor joists and a purlin wall ?
And to support the ridge if required (aka a ridge beam).

http://www.trussform.co.uk/content.php/info_id/326

Simon.

I've seen them used structurally (in fact I can't see another purpose
for them) - more than one company was promoting them at Interbuild
this year - that you get structural strength, without blocking pipe
and cable routing.

The downside may be the design effort - I'll read the docs on that
website when I have time.

On Thu, 18 Dec 2008 02:56:27 -0800 (PST) Sm_jamieson wrote :
Can these type of joists be used structurally instead of metal
beams for internal work, where only timber is involved
(notwithstanding their size) ?
i.e. Could I use these instead of a steel in a loft conversion,
in order to support floor joists and a purlin wall ?
And to support the ridge if required (aka a ridge beam).

http://www.trussform.co.uk/content.php/info_id/326

In very simple terms, timber sizing is generally governed by
deflection considerations. Stiffness is proportional to breadth x
depth cubed. So taking the smallest size, 35mm edge sections
separated by 125mm, the stiffness is proportional to
72x(195^3-125^3)=393246016, which is the same stiffness as a piece
of solid timber 47mm wide x 203mm. The manufactured joist does the
same job as a solid joist using nearly twice the timber (with added
advantages re straightness, service installation etc).

For this size the solid timber is 20% stronger, and has a
significantly higher shear capacity. Meanwhile (for a 3m span) a
little 127x76 steel will take more than three times the load, a
similarly sized 203x102 more than nine times the load.

So whilst these joists make a good replacement for traditional
solid joists, they are not dissimilar structurally to a solid piece
of timber, so won't be an adequate replacement for steel beams
(assuming a steel beam is needed in the first place.

--
Tony Bryer, 'Software to build on' from Greentram
www.superbeam.co.uk www.superbeam.com www.greentram.com

On 18 Dec, 11:48, Tony Bryer wrote:
On Thu, 18 Dec 2008 02:56:27 -0800 (PST) Sm_jamieson wrote :

Can these type of joists be used structurally instead of metal
beams for internal work, where only timber is involved
(notwithstanding their size) ?
i.e. Could I use these instead of a steel in a loft conversion,
in order to support floor joists and a purlin wall ?
And to support the ridge if required (aka a ridge beam).

http://www.trussform.co.uk/content.php/info_id/326

In very simple terms, timber sizing is generally governed by
deflection considerations. Stiffness is proportional to breadth x
depth cubed. So taking the smallest size, 35mm edge sections
separated by 125mm, the stiffness is proportional to
72x(195^3-125^3)=393246016, which is the same stiffness as a piece
of solid timber 47mm wide x 203mm. The manufactured joist does the
same job as a solid joist using nearly twice the timber (with added
advantages re straightness, service installation etc).

For this size the solid timber is 20% stronger, and has a
significantly higher shear capacity. Meanwhile (for a 3m span) a
little 127x76 steel will take more than three times the load, a
similarly sized 203x102 more than nine times the load.

So whilst these joists make a good replacement for traditional
solid joists, they are not dissimilar structurally to a solid piece
of timber, so won't be an adequate replacement for steel beams
(assuming a steel beam is needed in the first place.

--
OK. However, these are basically trussed structures. Presumably you
could make a trussed structure to replace a steel joist if it was tall
enough. For example, say you had 500mm height available under a roof
ridge (say 6000mm length, clay tiled, cut roof) that needed
supporting, and the usual solution was a ridge steel. What type of
timber structure would be needed to replace the ridge steel ?
Thanks,
Simon.

"sm_jamieson" wrote in message
...

OK. However, these are basically trussed structures. Presumably you
could make a trussed structure to replace a steel joist if it was tall
enough. For example, say you had 500mm height available under a roof
ridge (say 6000mm length, clay tiled, cut roof) that needed
supporting, and the usual solution was a ridge steel. What type of
timber structure would be needed to replace the ridge steel ?

Possibly several cut sheets of wbp glued together, BUT you need to do the
maths and be damned sure the glue is done properly. I wouldn't want to do it
on site, I would use steel.

sm_jamieson wrote:
Can these type of joists be used structurally instead of metal beams
for internal work, where only timber is involved (notwithstanding
their size) ?
i.e. Could I use these instead of a steel in a loft conversion, in
order to support floor joists and a purlin wall ?
And to support the ridge if required (aka a ridge beam).

http://www.trussform.co.uk/content.php/info_id/326

Simon.
yes, with caveats that you still need a structural engineer to specify them.

Tony Bryer wrote:
On Thu, 18 Dec 2008 02:56:27 -0800 (PST) Sm_jamieson wrote :
Can these type of joists be used structurally instead of metal
beams for internal work, where only timber is involved
(notwithstanding their size) ?
i.e. Could I use these instead of a steel in a loft conversion,
in order to support floor joists and a purlin wall ?
And to support the ridge if required (aka a ridge beam).

http://www.trussform.co.uk/content.php/info_id/326

In very simple terms, timber sizing is generally governed by
deflection considerations. Stiffness is proportional to breadth x
depth cubed. So taking the smallest size, 35mm edge sections
separated by 125mm, the stiffness is proportional to
72x(195^3-125^3)=393246016, which is the same stiffness as a piece
of solid timber 47mm wide x 203mm. The manufactured joist does the
same job as a solid joist using nearly twice the timber (with added
advantages re straightness, service installation etc).

For this size the solid timber is 20% stronger, and has a
significantly higher shear capacity. Meanwhile (for a 3m span) a
little 127x76 steel will take more than three times the load, a
similarly sized 203x102 more than nine times the load.

So whilst these joists make a good replacement for traditional
solid joists, they are not dissimilar structurally to a solid piece
of timber, so won't be an adequate replacement for steel beams
(assuming a steel beam is needed in the first place.

the issue being of curse that although you might need to go much larger
for the stiffness, it might still be lighter and cheaper than the steel.

On 18 Dec, 13:55, The Natural Philosopher wrote:
sm_jamieson wrote:
Can these type of joists be used structurally instead of metal beams
for internal work, where only timber is involved (notwithstanding
their size) ?
i.e. Could I use these instead of a steel in a loft conversion, in
order to support floor joists and a purlin wall ?
And to support the ridge if required (aka a ridge beam).

http://www.trussform.co.uk/content.php/info_id/326

Simon.

yes, with caveats that you still need a structural engineer to specify them.

Yes, of course, the point is it may be cheaper and easier for a DIYer,
as you mention in your other post.

Simon.

In an earlier contribution to this discussion,
sm_jamieson wrote:

Can these type of joists be used structurally instead of metal beams
for internal work, where only timber is involved (notwithstanding
their size) ?
i.e. Could I use these instead of a steel in a loft conversion, in
order to support floor joists and a purlin wall ?
And to support the ridge if required (aka a ridge beam).

http://www.trussform.co.uk/content.php/info_id/326

Simon.

I need to support the ends of some joists, where the joists change direction
in the middle of a room. They are currently supported by a transverse lintel
which runs below ceiling level, but I want to get rid of that and support
them from within the ceiling space, so as to have a flat ceiling.

An RSJ would do, with the ends of the joists cut into the webspace *except*
that I need to get some pipes (up to 22mm)and cables through it.

Anyone know whether you can get something which looks like an RSJ, but with
the web in the form of a lattice rather than solid?
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

"Roger Mills" wrote:

I need to support the ends of some joists, where the joists change direction
in the middle of a room. They are currently supported by a transverse lintel
which runs below ceiling level, but I want to get rid of that and support
them from within the ceiling space, so as to have a flat ceiling.

An RSJ would do, with the ends of the joists cut into the webspace *except*
that I need to get some pipes (up to 22mm)and cables through it.

Anyone know whether you can get something which looks like an RSJ, but with
the web in the form of a lattice rather than solid?


In larger sizes, "Universal Beams" rather than RSJs, it is possible to
have them "castellated". This involves cutting a regular zigzag
through the web, and re-welding the two halves together peak to peak
rather than peak to trough, making the beam taller. But this would be
quite expensive.

For your purpose, it would be cheaper and easier to cut some holes in
the web of an RSJ to allow pipes and cables to pass through. A
structural engineer will specify the beam to suit your loadings and
the number/size of holes you need.

"Roger Mills" wrote in message
...
In an earlier contribution to this discussion,
sm_jamieson wrote:

Can these type of joists be used structurally instead of metal beams
for internal work, where only timber is involved (notwithstanding
their size) ?
i.e. Could I use these instead of a steel in a loft conversion, in
order to support floor joists and a purlin wall ?
And to support the ridge if required (aka a ridge beam).

http://www.trussform.co.uk/content.php/info_id/326

Simon.

I need to support the ends of some joists, where the joists change
direction in the middle of a room. They are currently supported by a
transverse lintel which runs below ceiling level, but I want to get rid of
that and support them from within the ceiling space, so as to have a flat
ceiling.

An RSJ would do, with the ends of the joists cut into the webspace
*except* that I need to get some pipes (up to 22mm)and cables through it.

Anyone know whether you can get something which looks like an RSJ, but
with the web in the form of a lattice rather than solid?

Its quite common to cut an I beam and weld it back together so it is full of
holes.
It makes it stiffer while only using the same amount of steel.
A steel fabricator will do it if they don't have a stock of them.

You could DIY it but you would need something like a plasma cutter and a
welder and be good at welding.

The Natural Philosopher wrote:

Tony Bryer wrote:
On Thu, 18 Dec 2008 02:56:27 -0800 (PST) Sm_jamieson wrote :
Can these type of joists be used structurally instead of metal
beams for internal work, where only timber is involved
(notwithstanding their size) ?
i.e. Could I use these instead of a steel in a loft conversion,
in order to support floor joists and a purlin wall ?
And to support the ridge if required (aka a ridge beam).

http://www.trussform.co.uk/content.php/info_id/326

In very simple terms, timber sizing is generally governed by
deflection considerations. Stiffness is proportional to breadth x
depth cubed. So taking the smallest size, 35mm edge sections
separated by 125mm, the stiffness is proportional to
72x(195^3-125^3)=393246016, which is the same stiffness as a piece
of solid timber 47mm wide x 203mm. The manufactured joist does the
same job as a solid joist using nearly twice the timber (with added
advantages re straightness, service installation etc).

For this size the solid timber is 20% stronger, and has a
significantly higher shear capacity. Meanwhile (for a 3m span) a
little 127x76 steel will take more than three times the load, a
similarly sized 203x102 more than nine times the load.

So whilst these joists make a good replacement for traditional
solid joists, they are not dissimilar structurally to a solid piece
of timber, so won't be an adequate replacement for steel beams
(assuming a steel beam is needed in the first place.

the issue being of curse that although you might need to go much larger
for the stiffness, it might still be lighter and cheaper than the steel.


I don't think enough emphasis is being given in this discussion to
these trusses' comparative lack of shear capacity. While they would
appear to be potentially highly efficient in bending, and offer strong
resistance to torsion, shear forces are normally taken by the web of a
steel or timber beam, and no web means little or no shear capacity.

I find it difficult to imagine a situation where these trusses would
not be subjected to significant shear forces. I do wonder whether
some of the end details shown in the Technical Guide would offer
sufficient shear capacity for larger spans.

This is a product that is best suited to new build (where close
adherence to the original design and to the required standards of
workmanship may be more easily checked) rather than conversions, where
engineered I-joists may be a more logical choice, given their
intrinsically greater shear capacity.

On Thu, 18 Dec 2008 22:04:49 -0000 Roger Mills wrote :
An RSJ would do, with the ends of the joists cut into the webspace
*except* that I need to get some pipes (up to 22mm)and cables
through it.

Anyone know whether you can get something which looks like an RSJ,
but with the web in the form of a lattice rather than solid?

http://www.fabsec.co.uk/

though may not be economic for a one-off.

You can cut holes in the web of any I-beam as long as the effect of
doing so is checked. Our ProSteel software includes this option, but
I'm not suggesting DIY calculations. Background info (which may make
your eyes glaze over) at http://www.superbeam.co.uk/webopen.htm

So decide where you want the openings, get an engineer to design
accordingly, and fabricator to cut openings before beam is delivered.

--
Tony Bryer, 'Software to build on' from Greentram
www.superbeam.co.uk www.superbeam.com www.greentram.com

Bruce wrote:
The Natural Philosopher wrote:

Tony Bryer wrote:
On Thu, 18 Dec 2008 02:56:27 -0800 (PST) Sm_jamieson wrote :
Can these type of joists be used structurally instead of metal
beams for internal work, where only timber is involved
(notwithstanding their size) ?
i.e. Could I use these instead of a steel in a loft conversion,
in order to support floor joists and a purlin wall ?
And to support the ridge if required (aka a ridge beam).

http://www.trussform.co.uk/content.php/info_id/326
In very simple terms, timber sizing is generally governed by
deflection considerations. Stiffness is proportional to breadth x
depth cubed. So taking the smallest size, 35mm edge sections
separated by 125mm, the stiffness is proportional to
72x(195^3-125^3)=393246016, which is the same stiffness as a piece
of solid timber 47mm wide x 203mm. The manufactured joist does the
same job as a solid joist using nearly twice the timber (with added
advantages re straightness, service installation etc).

For this size the solid timber is 20% stronger, and has a
significantly higher shear capacity. Meanwhile (for a 3m span) a
little 127x76 steel will take more than three times the load, a
similarly sized 203x102 more than nine times the load.

So whilst these joists make a good replacement for traditional
solid joists, they are not dissimilar structurally to a solid piece
of timber, so won't be an adequate replacement for steel beams
(assuming a steel beam is needed in the first place.

the issue being of curse that although you might need to go much larger
for the stiffness, it might still be lighter and cheaper than the steel.


I don't think enough emphasis is being given in this discussion to
these trusses' comparative lack of shear capacity. While they would
appear to be potentially highly efficient in bending, and offer strong
resistance to torsion, shear forces are normally taken by the web of a
steel or timber beam, and no web means little or no shear capacity.


What shear forces? In what plane?

I beams are designed to resist bending loads. Period. The only shear
forces as such, that I can envisage, are at the beam ends where its
supported, and, provided these are less at its maximum loading than the
strength of the structure, which it usually is by a huge margin, there
is no problem.

The other shear forces encountered by the web are to resist
parallelogram type movement: but that is exactly what the composite
warren truss type structure does efficiently anyway. Neiher they, bo the
copmpresive/stretching type fores in teh wen to resist the Euler bucklng
modes are in anyway large. It is veryty easy to design a lattce braced
structire such that hwat keeps the webs part ois of much lighter
constrctio than the top and bottom sections: in essence they are where
teh strength resides: the function of the web is purely to prevent them
moving relative to one another. In this the steel I beam is relatvely
wasteful.




I find it difficult to imagine a situation where these trusses would
not be subjected to significant shear forces. I do wonder whether
some of the end details shown in the Technical Guide would offer
sufficient shear capacity for larger spans.

This is a product that is best suited to new build (where close
adherence to the original design and to the required standards of
workmanship may be more easily checked) rather than conversions, where
engineered I-joists may be a more logical choice, given their
intrinsically greater shear capacity.

The Natural Philosopher wrote:

What shear forces? In what plane?

I beams are designed to resist bending loads. Period. The only shear
forces as such, that I can envisage, are at the beam ends where its
supported, and, provided these are less at its maximum loading than the
strength of the structure, which it usually is by a huge margin, there
is no problem.


Before answering the questions I posed, it might have been a good idea
for you to study the theory of structures.

Your use of terms such as "bending loads" and your lack of
understanding both of the term "shear force" and of the relationship
between bending and shear, means that you really don't have the
faintest idea what you are talking about.

"Bruce" wrote in message
...
The Natural Philosopher wrote:

What shear forces? In what plane?

I beams are designed to resist bending loads. Period. The only shear
forces as such, that I can envisage, are at the beam ends where its
supported, and, provided these are less at its maximum loading than the
strength of the structure, which it usually is by a huge margin, there
is no problem.


Before answering the questions I posed, it might have been a good idea
for you to study the theory of structures.

Your use of terms such as "bending loads" and your lack of
understanding both of the term "shear force" and of the relationship
between bending and shear, means that you really don't have the
faintest idea what you are talking about.

See http://en.wikipedia.org/wiki/Shear_stress#Beam_shear

There is a difference between the external shear forces and the internal
shear forces generated to resist bending

Some shear stupidity being shown here. Although I'm not going to be
bending anyone's ear about it.

I've already got my coat!

Bruce wrote:
The Natural Philosopher wrote:

What shear forces? In what plane?

I beams are designed to resist bending loads. Period. The only shear
forces as such, that I can envisage, are at the beam ends where its
supported, and, provided these are less at its maximum loading than
the strength of the structure, which it usually is by a huge margin,
there is no problem.


Before answering the questions I posed, it might have been a good idea
for you to study the theory of structures.

Your use of terms such as "bending loads" and your lack of
understanding both of the term "shear force" and of the relationship
between bending and shear, means that you really don't have the
faintest idea what you are talking about.

There's really no need to be discourteous, you utter ****sock.

Steve Walker wrote:
Bruce wrote:
The Natural Philosopher wrote:
What shear forces? In what plane?

I beams are designed to resist bending loads. Period. The only shear
forces as such, that I can envisage, are at the beam ends where its
supported, and, provided these are less at its maximum loading than
the strength of the structure, which it usually is by a huge margin,
there is no problem.

Before answering the questions I posed, it might have been a good idea
for you to study the theory of structures.

Your use of terms such as "bending loads" and your lack of
understanding both of the term "shear force" and of the relationship
between bending and shear, means that you really don't have the
faintest idea what you are talking about.

There's really no need to be discourteous, you utter ****sock.


and if you look at the struts, you see they are perfectly placed to
withstand shear loads induced by bending, In fact that is their only
purpose. Which is why I said what i said. I assumed the ****wit had
some other shear force - like the beam ends - in mind.

The Natural Philosopher wrote:

and if you look at the struts, you see they are perfectly placed to
withstand shear loads induced by bending, In fact that is their only
purpose.


Nice of you to display your profound ignorance of structural
engineering again. God help us if you are ever in a position to
design a structure to withstand shear forces, because you clearly have
not the faintest idea what they are, or where they come from!

Thank you - it's a long time since I laughed as much as this! ;-)

Bruce wrote:
The Natural Philosopher wrote:

and if you look at the struts, you see they are perfectly placed to
withstand shear loads induced by bending, In fact that is their only
purpose.


Nice of you to display your profound ignorance of structural
engineering again. God help us if you are ever in a position to
design a structure to withstand shear forces, because you clearly have
not the faintest idea what they are, or where they come from!


Good thing I actually have a degree in it, and indeed do design
structures routinely.

Though not for buildings

If anyone else is reading this, don't take my word for it, look up beams
on wiki for full and reasonable explanations.


Thank you - it's a long time since I laughed as much as this! ;-)


Laugh away, but then ask yourself why if these beams are so unsuitable
as beams, why they are allowed to be used at all.

Then look at the innumerable Warren trusses in use round the world, also
used as low deflection load carrying beams. and ask yourself why they
don't flex and bow.

The only reason to use a solid steel I beam is that it is simple to
make. It uses far more material in the web than it needs to. The web
forces are extremely low compared with the top and bottom elements.

A truss design is far lighter and more efficient use of material. It
just is more complex to fabricate.

You will note that in a warren truss structure, there can be no 'shear
stresses' within the elements. since here are no elements big enough to
have substantial shear applied across them. That analysis is reserved
for beams with solid webs. The truss elements go into tension and
compression alone.

I.e. the tendency of the beam under load to have lateral shifting
between the top and bottom element is not constrained by a solid web
resisting 'shear' but by a series of struts connecting the upper and
lower elements with diagonal braces. If you HAD done proper beam
analysis, you would know that these lateral forces are relatively low
compared with the actual tensile and compressive forces in the beam
upper an lower elements, and in the limit the failure mode is usually -
with wood anyway - the failure of the top element under compression. Or
due to Euler buckling if its not constrained in the lateral plane. Or if
the trussing is TOO sparse. However that is, whilst not trivial, a
fairly simple calculation to do, if the elasticity of the member under
compression is known, there being a critical length for a given size and
elasticity below which failure by buckling simply will not occur before
the material itself undergoes plastic deformation.

So I suugest you actually instead of trying to apply solid beam or solid
web ideas to a truss, go back and get the NEXT chapter of the book you
haven't understood, and try and understand that.


TNP. MA, Eng.

On Tue, 23 Dec 2008 09:14:23 +0000 The Natural Philosopher wrote :
Laugh away, but then ask yourself why if these beams are so
unsuitable as beams, why they are allowed to be used at all.

Then look at the innumerable Warren trusses in use round the
world, also used as low deflection load carrying beams. and ask
yourself why they don't flex and bow.

Yes, long span structures carrying relatively light loads, where the
sizing of a solid member would be determined by deflection
considerations.

The only reason to use a solid steel I beam is that it is simple
to make. It uses far more material in the web than it needs to.
The web forces are extremely low compared with the top and bottom
elements.

It doesn't perhaps occur to you that the proportions of a steel UB
are not an accident. Not only are there considerations of shear, web
bucking and web crushing, but if the web is considered slender
according to BS5950 the section is unlikely to be a good choice as a
column ( http://www.superbeam.co.uk/columncap.htm - not for the
faint of heart) and you don't then have a lot of metal for the
normal angle cleat, end plate and fin plate connections.

--
Tony Bryer, 'Software to build on' from Greentram
www.superbeam.co.uk www.superbeam.com www.greentram.com

Tony Bryer wrote:
On Tue, 23 Dec 2008 09:14:23 +0000 The Natural Philosopher wrote :
Laugh away, but then ask yourself why if these beams are so
unsuitable as beams, why they are allowed to be used at all.

Then look at the innumerable Warren trusses in use round the
world, also used as low deflection load carrying beams. and ask
yourself why they don't flex and bow.

Yes, long span structures carrying relatively light loads, where the
sizing of a solid member would be determined by deflection
considerations.

The only reason to use a solid steel I beam is that it is simple
to make. It uses far more material in the web than it needs to.
The web forces are extremely low compared with the top and bottom
elements.

It doesn't perhaps occur to you that the proportions of a steel UB
are not an accident.

No, of course its no accident. Its a pragmatic compromiose between
simplicity and other prperties like witgh, stuffness, etc etc.


Not only are there considerations of shear, web
bucking and web crushing, but if the web is considered slender
according to BS5950 the section is unlikely to be a good choice as a
column ( http://www.superbeam.co.uk/columncap.htm - not for the
faint of heart) and you don't then have a lot of metal for the
normal angle cleat, end plate and fin plate connections.

Exactly. Its a compromise to fabricate a general purpose element.

Elements in compression must be below Euler limits in length, or they
will buckle before they actually crush:

For any specific function, you can do much better though, if you are
prepared to fabricate more complex shapes than a simply rolled I beam..

Try examining the structure of an aircraft wing, to see how MUCH
material can be extracted from an I beam like element without
compromising its strength and stiffness.

Tony Bryer wrote:
On Tue, 23 Dec 2008 09:14:23 +0000 The Natural Philosopher wrote :
Laugh away, but then ask yourself why if these beams are so
unsuitable as beams, why they are allowed to be used at all.

Then look at the innumerable Warren trusses in use round the
world, also used as low deflection load carrying beams. and ask
yourself why they don't flex and bow.

Yes, long span structures carrying relatively light loads, where the
sizing of a solid member would be determined by deflection
considerations.

Not at all.

Trusses carry very heavy loads. Over any span.

They are no more than I beams with the low or zero stress bits replaced
with fresh air, ultimately.

"The Natural Philosopher" wrote in message
...
Tony Bryer wrote:
On Tue, 23 Dec 2008 09:14:23 +0000 The Natural Philosopher wrote :
Laugh away, but then ask yourself why if these beams are so unsuitable
as beams, why they are allowed to be used at all.

Then look at the innumerable Warren trusses in use round the world, also
used as low deflection load carrying beams. and ask yourself why they
don't flex and bow.

Yes, long span structures carrying relatively light loads, where the
sizing of a solid member would be determined by deflection
considerations.

Not at all.

Trusses carry very heavy loads. Over any span.

They are no more than I beams with the low or zero stress bits replaced
with fresh air, ultimately.

Ah, Brunel's Tamar bridge, the top part of the truss carrying the load and
that all in compression. Simply supported, like a beam so no sideways force
on the pillars and it looks the part and it's got his name on - perfick!


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)