Lattice or solid-web girder: which girder goes on your hall and why
For the same hall you get two quotations, and one says lattice girder, the other solid-web. The price is similar, so the decision comes down to which quotation looks nicer. Yet the difference between those two solutions is not one of style: it changes how much steel goes into the hall, how many hours go on welding, how much surface has to be protected against corrosion, how high the wall is and how much clear height is left to you beneath the roof. Here is how that decision is actually made.
Two completely different load-carrying principles
A solid-web girder — a rolled IPE or HEA, or a welded I-section — carries load with its top flange in compression and its bottom flange in tension, while the web transfers the shear force. The material is packed into a single piece and the greater part of the web works far below its capability. That is why a solid-web girder is heavy, but simple: one section, two cuts, short fabrication.
A lattice girder solves the same task by separation. The chords are spread as far apart as they can be, and between them go only the members that transfer the force — diagonals and verticals. The material is thereby moved to where it is of most use. The result is a lighter structure, but with far more connections: every node has to be cut, fitted, welded and later tested.
Kilograms of steel and the price of the structure are not the same thing. A lattice girder regularly uses less steel, but pays for that with work in the plant and with a larger surface area for corrosion protection. That is why comparing two quotations “by weight” almost always leads to the wrong conclusion.
The first question is how deep it is, not how heavy
A lattice girder has to be deep in order to be light — its whole advantage comes from the spacing of the chords. In practice, for a lattice girder with parallel chords a depth of the order of one twelfth to one fifteenth of the span is taken, and for a solid-web girder one twentieth to one twenty-fifth. At a span of 24 m that is the difference between a girder about a metre deep and one two metres deep.
That difference does not stay in the structural design. If you need six metres of clear height beneath the structure, with a lattice girder the wall is higher by that whole metre — around the entire perimeter of the hall. That is more panels, more flashings, more volume to heat and, on plots with a prescribed maximum building height, sometimes the difference between permitted and not permitted.
How much height the girder itself takes up
Span of the hall
24 m
The clear height you need beneath the structure
6.0 m
Lattice girder
Parallel chords, infill of diagonals
Girder depth
1.60 – 2.00 m
Eaves height
7.60 – 8.00 m
L/15 to L/12
Solid-web girder
Rolled or welded I-section
Girder depth
0.96 – 1.20 m
Eaves height
6.96 – 7.20 m
L/25 to L/20
Largest difference in wall height
+1.04 m
That is how much higher the wall is if you go for the deepest lattice girder instead of the shallowest solid-web girder — on every metre of the perimeter of the hall. That is additional envelope, additional area to heat and, in the spatial plan, an additional 1.04 m against the permitted building height.
* The ratios L/12 to L/15 for the lattice girder and L/20 to L/25 for the solid-web girder are indicative ranges from design practice, and not values from a standard. The actual depth follows from the calculation of load capacity and deflection for the specific load — snow, wind, equipment hung from the roof and a possible crane all change it. The illustration applies to a girder of constant depth; on a duopitch lattice girder or a girder with haunches the depth varies along the length.
Where the limit lies in practice
There is no span at which a standard says “a lattice girder from here on”. There is a range in which the economics tip over, and it depends on the price of steel, on the price of an hour in the plant and on whether the fabricator has automated node fabrication. What repeats itself in practice looks like this:
up to ~18 m
Almost always a solid-web girder
The rolled section is ordered, cut and installed. A lattice girder would be lighter, but the difference in weight would be too small to cover the work on the nodes.
~18 to ~30 m
The range in which the decision is really made
Here both solutions make sense and the decision falls on other things: the available height, the passage of the services, the crane, the fabrication time and what the fabricator does better.
over ~30 m
The lattice girder takes over
A solid-web girder would call for a welded section of great depth, which uses a lot of steel and is hard to transport. The saving in weight becomes large enough to cover all the additional hours.
Those limits come from experience, not from a standard. A fabricator with good work preparation will make lattice girders economically even at 20 m; one who makes them by hand will not even at 35 m. So ask for a justification of the choice, not for a rule.
Four items that turn the decision, and are not in the structural design
Corrosion protection
A lattice girder has a considerably larger surface area per tonne of steel than a solid-web girder — every member has four sides and every node its own edges. The coating system is paid for by the square metre, not by the tonne, so part of what is saved on the steel goes on the protection. In corrosivity categories C4 and C5 this item becomes serious.
Weld inspection
More nodes means more welds, and the execution class prescribes the percentage of additional testing. For EXC3 that is a considerably greater extent than for EXC2 — and time and cost grow with the number of connections, not with the weight.
Services
Ventilation, sprinklers, cable routes and lighting pass through a lattice girder without a single penetration. Through a solid web, openings are drilled, and every opening in the web is an item that the designer has to verify. On halls with a lot of services this is the decisive advantage of the lattice girder.
Transport and erection
A standard semi-trailer takes about 13.6 m of usable length. Anything longer travels in segments and is spliced on site — which means site connections, the crane longer on site and protection of the connections in situ. A lattice girder is often transported dismantled, a solid-web girder in one piece as long as it fits.
What is most easily overlooked: the compression chord
The top chord of a lattice girder is in compression and, like every compression member, it can buckle sideways before the material reaches its yield strength. What prevents it from doing so are the purlins — and their spacing is a figure that determines the section of the chord just as much as the force in it. The same applies to the column of the hall, about which we have written separately.
The trap comes later. On roofs with heavy wind suction, or on halls in which tension from uplift occurs, the bottom chord can temporarily go into compression— and it usually has no purlins that would restrain it. That is why special lateral restraints to the bottom chord are fitted in such cases. This is an item that cheaper versions are known to “save” on, and there is no one to notice it until a strong wind comes.
The same applies to subsequent alterations. Hanging a crane, ventilation or solar panels from an existing lattice girder is not a matter to be agreed with the erector — it is a new load on members that were sized for something else. Every subsequent load on the roof calls for a check by the designer.
What to ask when you receive two different quotations
- What is the clear height beneath the structure in each quotation — not the eaves height, but what actually remains?
- Is the same weight compared with the same protection in both quotations, or does one rely on fewer square metres of coating?
- What is the execution class, and how many welds go for additional testing under it?
- How do the services run — through the structure or beneath it, and how much height do I lose that way?
- Is lateral restraint of the bottom chord provided for the case of wind suction?
- How many site splices are there on the building site, and who protects those connections after erection?
- Is a reserve for future loads provided — a crane, solar, ventilation — or is the structure at its limit?
A lattice girder and a solid-web girder are not a better and a worse solution, but two different places at which the same task is paid for: one in steel, the other in hours. A quotation that gives the reasons for it — why exactly that system for your span, your height and your services — is worth more than a quotation that is merely cheaper, because it shows that someone thought about your hall at all. The details of how all that surface is protected are in the article on corrosion protection, those on the connections that hold it all together in the article on bolted connections, and those on the columns beneath them in the article on the column of a hall.
We make steel structures and prefabricated halls from our own production in Đakovo, with delivery throughout Croatia. If you have the span, the required clear height and the use, we can start from that: about steel structures and an enquiry with the dimensions.
Frequently asked questions
In practice, up to a span of about 18 m a solid rolled section is almost always chosen, because the saving in weight would be too small to cover the work on the lattice nodes. Between about 18 and 30 m both solutions make sense and the decision comes down to the available height, the passage of services, the crane and the programme. Above about 30 m the lattice girder regularly takes over. These limits come from experience, not from a standard — they depend on the price of steel and of an hour in the plant, and on whether the fabricator has automated node fabrication.
English version of the Croatian original: Rešetkasti ili puni nosač: koji nosač ide na vašu halu i zašto.