Connection Of Truss To Supporting Beam Column

[Hasan Tariq]

Hello,

 

I am designing the mosque in zone 4 . The roof of the mosque is a Truss.I have following questions 

 

1. Do truss rest on the beam ? if yes .? do i have to assign any support conditions ? or etab will selelct by default.

2. what will be the grade of steel for the truss ? yield or tensile strength. some one told me go for A 36 ( 36,58)

[UmarMakhzumi]

1) Truss can rest on any support type: Columns directly or beams. However,  It is very likely that it would be supported on columns as trusses are generally provided to cover extra long spans. If you are supporting it of a beam, make sure you do your due diligence with beam design. You have to specify the boundary condition based on the kind of connection you would provide to connect the truss with your supporting member. Oversized slotted holes in base plate can act as rollers. 

2) Grade depends upon what the market has to offer. Confirm with the rolling mills about what ASTM Standard they roll.

 

Thanks.

[Hasan Tariq]

Dear Admin Thanks:)

 

I have assumed one side as a roller and other as a pin and practically they are going to be bolted into the beam .and for the grade of steel i am using A 36 ( 36 ksi yield and 58 tensile ) correct me if i am wrong .

[UmarMakhzumi]

Please also detail one side of the truss to behave as a roller by providing slotted hole on the plate that would be welded or bolted to plate embedded in beam. 

 

You should specify the grade of steel on your drawings so that the contractor is bound to use what you picked. 

 

Thanks.

[Hasan Tariq]

I have assumed 40 psf snow load and apply on the model as a gravity load and in the load case consider snow as a dead gravity.Is that ok 

[Ayesha]

I have assumed 40 psf snow load and apply on the model as a gravity load and in the load case consider snow as a dead gravity.Is that ok 

 

This depends upon what does your building code says and how do you establish your design criteria. Follow your firm's practise unless you have very good reasons not to do so.

[Hasan Tariq]

I have assumed the max depth of snow fall and multiply it with the unit weight of snow .and calculated the load as i have mentioned earlier 

[Syed Umair Haider]

40 psf is a high value for snow load that could result in an uneconomical truss design.You could check for snow load reduction provision (UBC97 Eqn 14-1) if not applied.

 

Moreover structural beams that will be used to support truss will necessarily required to be cater for torsional design as they will be subjected to high torsional moments in two ways,

 

1, Axial forces in bott & top chord of truss will create a couple acting about the longitudinal axis of beam i.e creating torsion equivalent to axial force times depth of truss.

 

2, While providing slotted holes (roller supp) to eliminate thermal stresses you have to embed a bracket plate into beam that will be supporting truss and correspondingly will be subjected to vertically directed pressure (from truss end shear) acting at an eccentricity equals to 0.5xbeam width + distance of beam edge to centroid of loaded plate area i.e creating additional torsion moment in beams equals to truss's end shear times eccen.

[Hasan Tariq]

I have designed using ASCE in which 40 psf has been reduced to 22 psf by incorporating different factors and yes torsion is the additional knowledge for me i will definitely look in to it JAZAKALLAH

[Ayesha]

What is your wind load? Wind load would be more critical for your truss design than snow load provided you don't assume a very high snow load. Umair has mentioned good points here. Do note that the beam end supporting pinned end of truss would have torsion in it . The end with slotted hole will create no torsion in the supporting beam. 

[Badar (BAZ)]

40 psf is a high value for snow load that could result in an uneconomical truss design.You could check for snow load reduction provision (UBC97 Eqn 14-1) if not applied.

 

1, Axial forces in bott & top chord of truss will create a couple acting about the longitudinal axis of beam i.e creating torsion equivalent to axial force times depth of truss.

 

2, While providing slotted holes (roller supp) to eliminate thermal stresses you have to embed a bracket plate into beam that will be supporting truss and correspondingly will be subjected to vertically directed pressure (from truss end shear) acting at an eccentricity equals to 0.5xbeam width + distance of beam edge to centroid of loaded plate area i.e creating additional torsion moment in beams equals to truss's end shear times eccen.

I have few points on this comment.

1) How can you say that 40Psf is high value for snow load without asking him the location of building. The building, in question, is located in northern areas where there can be at-least 2 feet snow.

2) Your point number 1 is only valid for a situation where both top and bottom chords of truss are connected to beam. The truss in question is pratt, or Howe; consequently, only bottom chord is connected to beam.

[Ayesha]

I have few points on this comment.

1) How can you say that 40Psf is high value for snow load without asking him the location of building. The building, in question, is located in northern areas where there can be at-least 2 feet snow.

2) Your point number 1 is only valid for a situation where both top and bottom chords of truss are connected to beam. The truss in question is pratt, or Howe, consequently, only bottom chord is connected to beam.

Good points being raised. Let me also clarify that my answer was with the assumption that only bottom cord of truss is supported on beam. If there is a horizontal reaction at pinned end then that would result in some torsion(reaction *distance to top of beam) in supporting beam. The action of wind + snow can result in uneven snow distribution of one side of the structure(if the area is considerable enough) resulting in substantial snow load. There is a good reference in book called "Why buildings fall down."

[Badar (BAZ)]

The action of wind + snow can result in uneven snow distribution of one side of the structure(if the area is considerable enough) resulting in substantial snow load. There is a good reference in book called "Why buildings fall down."

Hassan should have applied snow loads according to ASCE/SEI 7-05 standards, and they have given guidelines, indicating the procedure to apply even and uneven snow loads. ASE/SEI calls it as: BALANCED and UNBALANCED .

[Syed Umair Haider]

I have few points on this comment.

1) How can you say that 40Psf is high value for snow load without asking him the location of building. The building, in question, is located in northern areas where there can be at-least 2 feet snow.

2) Your point number 1 is only valid for a situation where both top and bottom chords of truss are connected to beam. The truss in question is pratt, or Howe; consequently, only bottom chord is connected to beam.

1, 40 psf can be said a high value with regard to my experienced design snow load for similar nature projects in various localities of Pakistan and it is confirmed by Hasan as he has revised it to 22 psf after incorporating several factors.

 

Moreoever,in Hasan's case usage of MBMA 1996 (Metal Building Manufacturer's Association) provisions for snow load calculation,reduction & application in contrast to truss system, should be more appropriate rather than ASCE as MBMA adresses specifically such types of structures.

 

Moreover,as you recommended to assess snow loads in Pakistan using ASCE/SEI,how can you calculate Ground snow load "Pg" for your area as specified in section 7.2 which is based on American localities only?.

 

Conclusively,if you intend to use snow loads in Pakistan you have to go for several assumptions due to unavailability of precise information and in hasan's case (steel structure) appropriate assumptions could be made using MBMA 1996 that will differentiate ASCE mainly in terms of application of load.

 

2, Yes it is evident that couple could only exist in case of bott & top chord connected to side face of beam only.In case of single chord connection torsion could only exist if the chord is provided with a moment connection in supporting beam that is generally avoided for truss structures.

[Badar (BAZ)]

 

Moreover,as you recommended to assess snow loads in Pakistan using ASCE/SEI,how can you calculate Ground snow load "Pg" for your area as specified in section 7.2 which is based on American localities only?.

 

Pakistan building code does not provide the value of ground snow for our areas. Hence, we have to make a guess, or do  statistical analysis of extreme values; both of them is based on snow data of that site.

 

When I mentioned 40PSF, I meant ground snow load, not the design snow load.

 

You might be right about MBMA, I don't have awareness to comment on that.

[UmarMakhzumi]

I have posted ground snow load values for different cities in Pakistan based on IBC 06, International Building Supplement.

 

Have a look here:

http://www.sepakistan.com/topic/1503-design-snow-load/?p=3477

 

Thanks.